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The McMurdo Dry Valleys (MDV) in Antarctica harbor a diverse assemblage of mat-forming diazotrophic cyanobacteria that play a key role in nitrogen cycling. Prior research showed that heterotrophic diazotrophs also make a substantial contribution to nitrogen fixation in MDV. The goals of this study were to survey autotrophic and heterotrophic diazotrophs across the MDV to investigate factors that regulate the distribution and relative ecological roles of each group. Results indicated that diazotrophs were present only in samples with mats, suggesting a metabolic coupling between autotrophic and heterotrophic diazotrophs. Analysis of 16S rRNA and nifH gene sequences also showed that diazotrophs were significantly correlated to the broader bacterial community, while co-occurrence network analysis revealed potential interspecific interactions. Consistent with previous studies, heterotrophic diazotrophs in MDV were diverse, but largely limited to lakes and their outlet streams, or other environments protected from desiccation. Despite the limited distribution, heterotrophic diazotrophs may make a substantial contribution to the nitrogen budget of MDV due to larger surface area and longer residence times of lakes. This work contributes to our understanding of key drivers of bacterial community structure in polar deserts and informs future efforts to investigate the contribution of nitrogen fixation to MDV ecosystems., A survey of N-fixing autotrophic and heterotrophic bacteria in the McMurdo Dry Valleys suggests a substantial role for heterotrophic diazotrophs in N cycling and reveals distinct distributional patterns and potential associations with the broader microbial community.

The extensive loss of tidal marsh habitat surrounding the San Francisco Bay has led to numerous restoration projects to restore ecosystem function. Native cordgrass (Spartina foliosa) is important for sediment accumulation and nesting habitat for endangered California Ridgway’s Rail (Rallus obsoletus obsoletus), and because salt marshes are typically nitrogen-limited ecosystems, previous restoration studies in California have used nitrogen fertilizers to increase plant growth. This study compared the use of two low-nitrogen, high-carbon fertilizers (Macrosystis pyrifera (kelp) and sodium alginate) to stimulate N2-fixing bacteria inhabiting the S. foliosa rhizosphere in a historic and recently restored marsh in San Francisco Bay. Alginate increased N2-fixation, though there was no observed S. foliosa growth response. Kelp suppressed N2-fixation, and increased the foliar nitrogen content of Salicornia pacifica in mixed Spartina-Salicornia stands. The restored marsh had less than 0.5 μM phosphate in porewaters, suggesting phosphorus, not nitrogen, limits S. foliosa growth in this marsh. Alginate increased rates of N2-fixation and may promote plant growth in marshes exhibiting nitrogen limitation. Phosphate limitation was a surprise considering numerous previous studies indicate nitrogen as the primary limiting nutrient of tidal marsh plants. We recommend first assessing nutrient stoichiometry when considering manipulations to promote plant growth in restored marshes.

Phytoplankton are the oceans' principal source of polyunsaturated fatty acids that support the growth and reproduction of consumers such as copepods. Previous studies have demonstrated ocean acidification (OA) can change the availability of polyunsaturated fatty acids to consumer diets which may affect consumer reproduction. Two laboratory experiments were conducted to examine the effects of feeding high-pCO2-reared phytoplankton on copepod egg production, hatching success, and naupliar survival. Marine phytoplankton Rhodomonas salina, Skeletonema marinoi, Prorocentrum micans, and Isochrysis galbana were exponentially grown in semi-continuous cultures at present (control) (400 ppm CO2, pH~8.1) and future (1,000 ppm CO2, pH~7.8) conditions and provided to Acartia tonsa copepods over 4 consecutive days as either nitrogen-limited (Exp. I) or nitrogen-depleted (Exp. II) mixed assemblage of phytoplankton. The composition of FAs in the phytoplankton diet was affected by pCO2 concentration and nitrogen deficiency; the ratio of essential fatty acids to total polyunsaturated fatty acids decreased in phytoplankton grown under high pCO2 and the mass of total fatty acids increased under nitrogen depletion. Additionally, total concentrations of essential fatty acids and polyunsaturated fatty acids in the diet mixtures were less under the high-pCO2 compared to the control-pCO2 treatments. Median egg production, hatching success, and naupliar survival were 48-52%, 4-87%, and 9-100% lower, respectively, in females fed high-pCO2 than females fed low-pCO2 phytoplankton, but this decrease in reproductive success was less severe when fed N-depleted, but fatty acid-rich cells. This study demonstrates that the effects of OA on the nutritional quality of phytoplankton (i.e., their cellular fatty acid composition and quota) were modified by the level of nitrogen deficiency and the resulting negative reproductive response of marine primary consumers.

In aquatic environments, plastic debris accumulates chemical pollutants from the surrounding water, potentially altering the fate of xenobiotics in these ecosystems. The effects of biofouling on the potential for plastic to sorb environmental pollutants remain poorly understood. In this study, we test the hypothesis that concentrations of metals are directly related to biofilm accumulation on microplastics submerged in natural estuarine waters. Two types of pre-production plastic pellets (polylactic acid (PLA) and low-density polyethylene (LDPE)) and glass pellets, were suspended for up to 28 days in an urbanized estuary (San Francisco Bay, California) to investigate how biofilm affects the accumulation of metals on these materials. During the initial weeks of the experiment, biofilm growth differed between locations, but after 28 days, PLA and LDPE had similar amounts of biofilm at the two field sites. Biofilm was the only significant predictor variable for Ba, Cs, Fe, Ga, Ni and Rb, and simple regressions of these metals after one month of submersion predicted much of the variability in the data (respective adjusted R2 values: 0.46, 0.90, 0.86, 0.81, 0.87, 0.90; p

As part of the ANACONDAS program, we investigated the role of the Amazon plume in stimulating offshore nitrogen fixation and export production during the river's high-discharge period (May–June 2010). Using the shipboard underway system, we performed high-resolution sampling of over 450,000 km2 of surface waters, characterizing the distribution of nutrients, phytoplankton, particulate organic matter (POM), and stable carbon and nitrogen isotopes of POM in the offshore plume. We found distinct regional variations in diazotroph communities, with the Diatom-Diazotroph Associations (DDA) Hemiaulus hauckii – Richelia intracellularis dominating the low N : P mesohaline waters to the northwest of the plume axis and Trichodesmium spp. primarily occupying oceanic waters to the east. Nutrient availability broadly shaped diazotroph distributions along the salinity gradient, but habitat longevity may also play a role in the finer-scale distributions of communities, particularly of DDAs. H. hauckii and Trichodesmium spp. affected the nitrogen and carbon budgets in fundamentally different ways within the plume-influenced regions, with H. hauckii making much greater contributions to the particulate nitrogen pool and to CO2 drawdown than Trichodesmium spp., leading to much higher export fluxes. Our findings provide an important constraint on the role of the Amazon plume in creating distinct niches and roles for diazotrophs in the nutrient and carbon budgets of the western tropical North Atlantic.

Pelagic primary production (14C uptake) was measured 81 times between 1990 and 2013 at sites spanning the broad, shallow Northern Australian Shelf (NAS; 120−145°E) which borders the Australian continent. The mean of all areal production measurements was 1048±109 mg C m−2 d−1 (mean±95% CI). Estimates of areal primary production were correlated with integral upper-euphotic zone chlorophyll stocks (above the 50% and 20% light penetration depths) accessible to ocean color remote sensing and total water column chlorophyll standing crop, but not surface (0–2 m) chlorophyll concentrations. While the NAS is subject to a well characterized monsoonal climate regime (austral summer-NW monsoon -wet: austral winter- SE monsoon -dry), most seasonal differences in means of regional-scale chlorophyll standing crop (11–33 mg Chl m−2 for 12 of 15 season-region combinations) and areal primary production (700–1850 mg C m− day−1 for 12 of 15 season-region combinations) fell within a 3−fold range. Apart from the shallow waters of the Torres Strait and northern Great Barrier Reef, picoplankton ( 80%. While the range of our post-1990 areal production estimates overlaps the range of production estimates made in NAS waters during 1960–62, the mean of post-1990 estimates is over 2-fold greater. We regard the difference to be due to improvements in production measurement techniques, particularly regarding the reduction of potential metal toxicity and incubations in more realistic light regimes.

Riverine nutrients acting in concert with local hydrographic conditions create distinct ecological niches for phytoplankton communities across river-ocean continuums. Here we compare two of the world’s largest river-ocean systems, the Amazon River Plume (ARP) which outflows into the Western Tropical North Atlantic (WTNA) and the Changjiang Diluted Water (CDW) which empties into the East China Sea to show how distinctly different N: P ratios of their source waters, shape phytoplankton communities along the river-ocean continuum. Sampling in the relatively unpolluted surface waters of the ARP during peak river discharge revealed that phytoplankton communities along the river-ocean continuum were strongly controlled by Dissolved Inorganic Nitrogen (DIN, nitrate plus nitrite) concentrations which were low or beyond detectable, while Dissolved Inorganic Phosphorous (DIP, phosphate) was not limiting. Downstream of the plume, nutrient concentrations at the surface were far below the Redfield N:P ratio of 16 : 1, and in this severely nitrogen limited but silica replete mesohaline region, Diatom-Diazotroph Associations (DDA) such as Hemiaulus hauckii-Richelia proliferated, while offshore in nitrate poor oceanic waters, Trichodesmium spp. and Prochlorococcus thrived. In contrast, in the CDW, a system which has witnessed a substantial rise in anthropogenic nitrogen inputs from human pressures within the Changjiang River system, phytoplankton community structure was regulated primarily by DIP which was rapidly consumed in the inner shelf. Data from a series of summer-time cruises in the mid-shelf and offshore regions of the East China Sea, at peak discharge, established that when DIP limitation was ameliorated by on-shelf upwelling of DIP rich Kuroshio Intermediate Waters, diatoms and dinoflagellates dominated. Conversely, during years of heavy discharge, the westward flowing CDW plume was severely DIP limited, probably because water column stratification dampened upwelling of subsurface waters. The consequent phosphate limitation led to the proliferation of small phytoplankton such as Chlorophytes and Cyanobacteria. The absence of diazotrophs in the CDW, leads us to hypothesize that river-ocean continuums, whose source waters are heavily impacted by anthropogenic activities and with nutrient concentrations substantially in excess of Redfield ratios, may not support diatoms on account of DIP limitation nor DDAs and diazotrophs because of excess DIN.

The cold deserts of the McMurdo Dry Valleys (MDV), Antarctica, host a high level of microbial diversity. Microbial composition and biomass in arid vs. ephemerally wetted regions are distinctly different, with wetted communities representing hot spots of microbial activity that are important zones for biogeochemical cycling. While climatic change is likely to cause wetting in areas not historically subject to wetting events, the responses of microorganisms inhabiting arid soils to water addition is unknown. The purpose of this study was to observe how an associated, yet non-wetted microbial community responds to an extended addition of water. Water from a stream was diverted to an adjacent area of arid soil with changes in microbial composition and activities monitored via molecular and biochemical methods over seven weeks. The frequency of genetic signatures related to both prokaryotic and eukaryotic organisms adapted to MDV aquatic conditions increased during the limited seven week period, indicating that the soil community was transitioning into a typical “high-productivity” MDV community. This work is consistent with current predictions that MDV microbial communities in arid regions are highly sensitive to climate change, and further supports the notion that changes in community structure and associated biochemical cycling may occur much more rapidly than predicted.

Coastal oceans link terrestrial and marine carbon cycles. Yet, carbon sources and sinks in these biomes remain poorly understood. Here, we explore the dynamics of dissolved organic matter (DOM) along the Amazon River-to-ocean continuum from the lower mainstem at Obidos to the open ocean of the western tropical North Atlantic. We molecularly characterized DOM via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), determined DOM stable carbon isotopes, and interpreted the data in the context of bacterial abundance and production, phytoplankton biomass and composition. Multivariate analysis revealed that the DOM molecular variability in the plume was mainly influenced by the input of terrigenous DOM. Incubation experiments with water from close to the river mouth showed that photo- and bio-degradation preferentially removed 13C-depleted and 13C-enriched terrigenous DOM, respectively. However, there was no significant quantitative change in the total amount of dissolved organic carbon (DOC) over five days. This result suggests that most of the reactive DOM had already been bio-degraded upstream within the river and that photo-degradation was diminished in the turbid plume close to the river mouth. Terrigenous DOM therefore appeared to be relatively non-reactive nearshore. In the less turbid offshore plume, enhanced light penetration stimulated growth of phytoplankton and increased bacterial production. Although marine DOM compounds became relatively enriched, bulk DOC concentrations were 9 to 30% below levels expected from conservative mixing of river and ocean endmembers suggesting that quantitative removal of terrigenous DOM was not compensated by marine DOM production. We propose that removal of terrigenous DOM in the outer plume may be enhanced by (i) bio-degradation primed by reactive algal DOM, (ii) photo-degradation, which may further break down DOM into more bio-available forms, and possibly (iii) sorption of DOM to sinking particles.

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50–76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.

A large percentage of CO2 emitted into the atmosphere is absorbed by the oceans, causing chemical changes in surface waters known as ocean acidification (OA). Despite the high interest and increased pace of OA research to understand the effects of OA on marine organisms, many ecologically important organisms remain unstudied. Calcidiscus is a heavily calcified coccolithophore genus that is widespread and genetically and morphologically diverse. It contributes substantially to global calcium carbonate production, organic carbon production, oceanic carbon burial, and ocean–atmosphere CO2 exchange. Despite the importance of this genus, relatively little work has examined its responses to OA. We examined changes in growth, morphology, and carbon allocation in multiple strains of Calcidiscus leptoporus in response to ocean acidification. We also, for the first time, examined the OA response of Calcidiscus quadriperforatus, a larger and more heavily calcified Calcidiscus congener. All Calcidiscus coccolithophores responded negatively to OA with impaired coccolith morphology and a decreased ratio of particulate inorganic to organic carbon (PIC:POC). However, strains responded variably; C. quadriperforatus showed the most sensitivity, while the most lightly calcified strain of C. leptoporus showed little response to OA. Our findings suggest that calcium carbonate production relative to organic carbon production by Calcidiscus coccolithophores may decrease in future oceans and that Calcidiscus distributions may shift if more resilient strains and species become dominant in assemblages. This study demonstrates that variable responses to OA may be strain or species specific in a way that is closely linked to physiological traits, such as cellular calcite quota.

Many efforts to improve science teaching in higher education focus on a few faculty members at an institution at a time, with limited published evidence on attempts to engage faculty across entire departments. We created a long-term, department-wide collaborative professional development program, Biology Faculty Explorations in Scientific Teaching (Biology FEST). Across 3 years of Biology FEST, 89% of the department’s faculty completed a weeklong scientific teaching institute, and 83% of eligible instructors participated in additional semester-long follow-up programs. A semester after institute completion, the majority of Biology FEST alumni reported adding active learning to their courses. These instructor self-reports were corroborated by audio analysis of classroom noise and surveys of students in biology courses on the frequency of active-learning techniques used in classes taught by Biology FEST alumni and nonalumni. Three years after Biology FEST launched, faculty participants overwhelmingly reported that their teaching was positively affected. Unexpectedly, most respondents also believed that they had improved relationships with departmental colleagues and felt a greater sense of belonging to the department. Overall, our results indicate that biology department–wide collaborative efforts to develop scientific teaching skills can indeed attract large numbers of faculty, spark widespread change in teaching practices, and improve departmental relations.

It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.

Active-learning pedagogies have been repeatedly demonstrated to produce superior learning gains with large effect sizes compared with lecture-based pedagogies. Shifting large numbers of college science, technology, engineering, and mathematics (STEM) faculty to include any active learning in their teaching may retain and more effectively educate far more students than having a few faculty completely transform their teaching, but the extent to which STEM faculty are changing their teaching methods is unclear. Here, we describe the development and application of the machine-learning-derived algorithm Decibel Analysis for Research in Teaching (DART), which can analyze thousands of hours of STEM course audio recordings quickly, with minimal costs, and without need for human observers. DART analyzes the volume and variance of classroom recordings to predict the quantity of time spent on single voice (e.g., lecture), multiple voice (e.g., pair discussion), and no voice (e.g., clicker question thinking) activities. Applying DART to 1,486 recordings of class sessions from 67 courses, a total of 1,720 h of audio, revealed varied patterns of lecture (single voice) and nonlecture activity (multiple and no voice) use. We also found that there was significantly more use of multiple and no voice strategies in courses for STEM majors compared with courses for non-STEM majors, indicating that DART can be used to compare teaching strategies in different types of courses. Therefore, DART has the potential to systematically inventory the presence of active learning with ∼90% accuracy across thousands of courses in diverse settings with minimal effort.

Salt marsh restoration is hypothesized to provide shoreline stabilization, increased fish habitat, and organic carbon subsidies for estuarine food webs. Organic carbon comes from diverse primary producers that differ in carbon fixation rates and areal extent within wetland systems. This study was designed to obtain some of the first estimates of the relative contribution of different primary producers to total organic carbon production within open water and tidally flooded wetlands of the northern San Francisco Estuary (SFE). Carbon fixation rates of phytoplankton, microphytobenthos, and low marsh emergent vegetation were measured in two natural and four restoring wetlands in 2004. Areal (m2) rates of carbon fixation were greatest for low marsh vegetation, while phytoplankton and microphytobenthos rates were one and two orders of magnitude lower, respectively. However, when areal production rates were scaled to the amount of habitat available for each primary producer group, the relative importance of each group varied by location. Given that each primary producer group supports a different subset of estuarine consumers, the type of food subsidy desired should influence the amount open water channel, mudflat and low marsh area restored. Large-scale wetland restoration activities should consider the types of primary producers likely to occupy restored habitats when estimating future food web impacts.

An Advanced Laser Fluorometer (ALF) capable of discriminating several phytoplankton pigment types was utilized in conjunction with microscopic data to map the distribution of phytoplankton communities in the Amazon River plume in May–June-2010, when discharge from the river was at its peak. Cluster analysis and Non-metric Multi-Dimensional Scaling (NMDS) helped distinguish three distinct biological communities that separated largely on the basis of salinity gradients across the plume. These three communities included an ‘‘estuarine type’’ comprised of a high biomass mixed population of diatoms, cryptophytes and green-water Synechococcus spp. located upstream of the plume, a ‘‘mesohaline type’’ made up largely of communities of Diatom-Diazotroph Associations (DDAs) and located in the northwestern region of the plume and an ‘‘oceanic type’’ in the oligotrophic waters outside of the plume made up of Trichodesmium and Synechococcus spp. Although salinity appeared to have a substantial influence on the distribution of different phytoplankton groups, ALF and microscopic measurements examined in the context of the hydro-chemical environment of the river plume, helped establish that the phytoplankton community structure and distribution were strongly controlled by inorganic nitrate plus nitrite (NO3 +N O 2) availability whose concentrations were low throughout the plume. Towards the southern, low-salinity region of the plume, NO3 +N O 2 supplied by the onshore flow of subsurface (80 m depth) water, ensured the continuous sustenance of the mixed phytoplankton bloom. The large drawdown of SiO3 and PO4 associated with this ‘‘estuarine type’’ mixed bloom at a magnitude comparable to that observed for DDAs in the mesohaline waters, leads us to contend that, diatoms, cryptophytes and Synechococcus spp., fueled by the offshore influx of nutrients also play an important role in the cycling of nutrients in the Amazon River plume.

Instructor Talk—noncontent language used by instructors in classrooms—is a recently defined and promising variable for better understanding classroom dynamics. Having previously characterized the Instructor Talk framework within the context of a single course, we present here our results surrounding the applicability of the Instructor Talk framework to noncontent language used by instructors in novel course contexts. We analyzed Instructor Talk in eight additional biology courses in their entirety and in 61 biology courses using an emergent sampling strategy. We observed widespread use of Instructor Talk with variation in the amount and category type used. The vast majority of Instructor Talk could be characterized using the originally published Instructor Talk framework, suggesting the robustness of this framework. Additionally, a new form of Instructor Talk—Negatively Phrased Instructor Talk, language that may discourage students or distract from the learning process—was detected in these novel course contexts. Finally, the emergent sampling strategy described here may allow investigation of Instructor Talk in even larger numbers of courses across institutions and disciplines. Given its widespread use, potential influence on students in learning environments, and ability to be sampled, Instructor Talk may be a key variable to consider in future research on teaching and learning in higher education.

The last several decades have witnessed dramatic advances in unfolding the diversity and commonality of oceanic diazotrophs and their N2 -fixing potential. More recently, substantial progress in diazotrophic cell biology has provided a wealth of information on processes and mechanisms involved. The substantial contribution by the diazotrophic cyanobacterial genus Trichodesmium to the nitrogen influx of the global marine ecosystem is by now undisputable and of paramount ecological importance, while the underlying cellular and molecular regulatory physiology has only recently started to unfold. Here, we explore and summarize current knowledge, related to the optimization of its diazotrophic capacity, from genomics to ecophysiological processes, via, for example, cellular differentiation (diazocytes) and temporal regulations, and suggest cellular research avenues that now ought to be explored.

Coccolithophores are unicellular phytoplankton that produce calcium carbonate coccoliths as an exoskeleton. Emiliania huxleyi, the most abundant coccolithophore in the world’s ocean, plays a major role in the global carbon cycle by regulating the exchange of CO2 across the ocean-atmosphere interface through photosynthesis and calcium carbonate precipitation. As CO2 concentration is rising in the atmosphere, the ocean is acidifying and ammonium (NH4 + ) concentration of future ocean water is expected to rise. The latter is attributed to increasing anthropogenic nitrogen (N) deposition, increasing rates of cyanobacterial N2 fixation due to warmer and more stratified oceans, and decreased rates of nitrification due to ocean acidification. Thus, future global climate change will cause oceanic phytoplankton to experience changes in multiple environmental parameters including CO2, pH, temperature and nitrogen source. This study reports on the combined effect of elevated pCO2 and increased NH4 + to nitrate (NO3 ) ratio (NH4 + /NO3 ) on E. huxleyi, maintained in continuous cultures for more than 200 generations under two pCO2 levels and two different N sources. Herein, we show that NH4 + assimilation under N-replete conditions depresses calcification at both low and high pCO2, alters coccolith morphology, and increases primary production. We observed that N source and pCO2 synergistically drive growth rates, cell size, and the ratio of inorganic to organic carbon. These responses to N source suggest that, compared to increasing CO2 alone, a greater disruption of the organic carbon pump could be expected in response to the combined effect of increased NH4 + /NO3 ratio and CO2 level in the future acidified ocean. Additional experiments conducted under lower nutrient conditions are needed prior to extrapolating our findings to the global oceans. Nonetheless, our results emphasize the need to assess combined effects of multiple environmental parameters on phytoplankton biology to develop accurate predictions of phytoplankton responses to ocean acidification.

Low nutrient availability in salt marsh ecosystems can potentially limit primary productivity and subsequent carbon export to coastal waters. In temperate marshes with low external nitrogen inputs, nitrogen (N2) fixation may enhance availability of usable nitrogen to marsh plant communities and increase their growth. The effects of sub-surface soil amendment of polysaccharides on rhizosphere N2 fixation, and on plant growth and tissue characteristics of the California cordgrass, Spartina foliosa, were tested in microcosms in a tidal simulator. Polysaccharide was added as alginate extract, and rhizosphere N2 fixation and plant characteristics were measured over 9 weeks. N2 fixation rates increased 10-fold in the alginate treatment compared to controls after 1 week of exposure as a result of increased availability of a readily usable form of carbon. N2 fixation rates in amended sediment were significantly higher than rates in control sediments at weeks 1 and 3. Although plant characteristics did no...

We analyzed flavodoxin gene sequences and transcription patterns in the marine nitrogen-fixing cyanobacterium Trichodesmium Ehrenberg ex Gomont 1892. While a typical cyanobacterial flavodoxin ortholog ( fld1 ) was identified from cultured (strain IMS101) and environmental Trichodesmium by PCR and Southern blot hybridization, a second flavodoxin ( fld2 ) was identified in the genome sequence. BLAST and phylogenetic analyses indicated that the two fld genes were highly divergent: fld1 was most closely related to fld common in cyanobacteria, wheras fld2 formed a distinct cluster with a fld so far only found in the unicellular diazotrophic cyanobacteria Cyanothece and Crocosphaera (in which fld2 was the only fld ). The fld2 cluster was more closely allied with a fld in non-cyanobacterial diazotrophs such as Azotobacter and Azoarcus than fld1 in other cyanobacteria. This result suggests different evolutionary history of these two genes. Real-Time RT–PCR analysis on iron-replete and iron-deplete Trichodesmium cultures revealed remarkable diel dynamics in transcription of both fld s but surprisingly failed to show induction under iron stress. Addition of nitrate to these cultures depressed nifH and fld2 transcription while elevating the transcription of fld1 in iron-stressed cultures. A positive correlation between fld and nifH transcript abundances was found, which was stronger in the case of fld2 than fld1 . Our data suggest that these two flavodoxins in Trichodesmium may be functionally diversified and that use of flavodoxin as an iron stress indicator needs to be validated for each organism with consideration of diel dynamics of gene expression.

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N 2 fixation far from the mouth and provides important pathways for sequestration of atmospheric CO 2 in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr −1 of the new production supported by NO 3 delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2.5 Tmol of C to the atmosphere [Mikaloff-Fletcher SE, et al. (2007) Inverse estimates of the oceanic sources and sinks of natural CO 2 and the implied oceanic carbon transport. Global Biogeochem Cycles 21, doi:10.1029/2006GB002751]. The enhancement of N 2 fixation and consequent C sequestration by tropical rivers appears to be a global phenomenon that is likely to be influenced by anthropogenic activity and climate change.

The harpacticoid copepod Macrosetella gracilis uses colonies of the nitrogen-fixing cyanobacterium Trichodesmium spp. as a floating substrate and nursery. This association enables the copepod to lead a pelagic existence. Molecular and morphological data both indicate that M. gracilis is maintained as a single, circumglobal species across the tropical and subtropical Atlantic and Pacific. Mitochondrial DNA sequence data from cytochrome c oxidase subunit I (COI) showed mod- erate to high levels of diversity and limited phylogeographic structure in M. gracilis. Thirty-six differ- ent haplotypes were found among the 149 M. gracilis individuals sequenced; 4 haplotypes were shared between the Atlantic and Pacific. Samples from Japan showed the highest haplotypic and nucleotide diversity levels, suggesting that the Indo-West Pacific may be the center of diversity for M. gracilis. Wide distribution and large population size in M. gracilis may limit divergence through increased gene flow or slowing of the loss of ancestral diversity through lineage sorting. The pelagic existence of M. gracilis has led to less phylogenetic structuring than found in other species of harpacticoid copepods, illustrating the evolutionary significance of specialization on a floating sub- strate in this copepod.

Large areas of the western tropical Atlantic Ocean are influenced by the Amazon River plume, in which dissolved inorganic nitrogen (DIN) concentrations are low and nutrient N:P and N:Si ratios are lower than elemental ratios typical of phytoplankton. Accordingly, nutrient concentrations and ratios measured during winter (January to February 2001) and summer (July to August 2001) cruises in the area 2 to 14° N and 57 to 40° W suggest that DIN may limit phytoplankton growth at the majority of the 40 stations sampled. As diatoms are the dominant plankton in the Amazon plume- waters, short-term nutrient enrichment experiments were conducted to determine whether ambient N, P, or Si limited silica production rates (SiP). Results indicated that SiP were substrate limited both above and below the pycnocline at the majority of stations during both high (summer cruise) and low (winter cruise) discharge conditons. DIN concentrations also limited SiP in plumewater stations, but N additions depressed SiP at other stations, possibly using energy to take up the limiting nutrient. Finally, primary production rates were limited by ambient dissolved silicon concentrations at 9 of 13 summer stations. These results indicate the importance of dissolved silicon availability in controlling phytoplankton growth rates in a diatom-dominated river plume system.

The vertical and horizontal distributions of seven diazotrophic populations in the western tropical north Atlantic (WTNA) Ocean were examined using a nifH DNA quantitative polymerase chain reaction (QPCR) approach. The nifH phylotype abundances were highest near the surface and decreased with depth, with the exception of the cyanobacterial symbiont Calothrix, which was not detected at any station. Richelia associated with the diatoms Rhizosolenia clevei and Hemiaulus hauckii were distributed within the freshwater lens of the Amazon plume. Abundances of H. hauckii-Richelia nifH genes dominated all depths in 6 of 10 vertical profiles and 10 of 20 surface samples. In addition, estimates of Richelia associated with H. hauckii increased northwest (8– 12uN, 56–54uW) from the river mouth, where significantly ( p , 0.001) higher abundances (.105 copies L–1) were found in mesohaline waters (31–34.9). nifH copy abundance for surface populations of the H. hauckii-Richelia symbioses were positively correlated (r2 5 0.59) with salinity. Three unicellular cyanobacterial groups and Trichodesmium had similar horizontal distributions, where the highest nifH copy estimates were at stations with salinity $35 and northeast (6–10u N5 0uW) of the freshwater lens. The abundance of Trichodesmium spp. and unicellular Group B nifH gene copies co-varied (r2 5 0.60). The QPCR study showed the dominance of H. hauckii-Richelia symbioses in the Amazon plume waters, implying that these associations had an ecological advantage over the other diazotrophs. Outside of the plume nutrients were below detection, abundances of freeliving unicellular cyanobacterial phylotypes, including a novel group designated Group C, were abundant (.105 copies L21) and comparable to the abundances of Trichodesmium spp. Thus, there appeared to be a cascade of diazotrophic communities along gradients of salinity and nutrients in the WTNA.

The Amazon River has the largest discharge of all rivers on Earth, and its complex plume system fuels a wide array of biogeochemical processes, across a large area of the western tropical North Atlantic. The plume thus stimulates microbial processes affecting carbon sequestration and nutrient cycles at a global scale. Chromosomal gene expression patterns of the 2.0 to 156 μm size-fraction eukaryotic microbial community were investigated in the Amazon River Plume, generating a robust dataset (more than 100 million mRNA sequences) that depicts the metabolic capabilities and interactions among the eukaryotic microbes. Combining classical oceanographic field measurements with metatranscriptomics yielded characterization of the hydrographic conditions simultaneous with a quantification of transcriptional activity and identity of the community. We highlight the patterns of eukaryotic gene expression for 31 biogeochemically significant gene targets hypothesized to be valuable within forecasting models. An advantage to this targeted approach is that the database of reference sequences used to identify the target genes was selectively constructed and highly curated optimizing taxonomic coverage, throughput, and the accuracy of annotations. A coastal diatom bloom highly expressed nitrate transporters and carbonic anhydrase presumably to support high growth rates and enhance uptake of low levels of dissolved nitrate and CO2. Diatom-diazotroph association (DDA: diatoms with nitrogen fixing symbionts) blooms were common when surface salinity was mesohaline and dissolved nitrate concentrations were below detection, and hence did not show evidence of nitrate utilization, suggesting they relied on ammonium transporters to aquire recently fixed nitrogen. These DDA blooms in the outer plume had rapid turnover of the photosystem D1 protein presumably caused by photodegradation under increased light penetration in clearer waters, and increased expression of silicon transporters as silicon became limiting. Expression of these genes, including carbonic anhydrase and transporters for nitrate and phosphate, were found to reflect the physiological status and biogeochemistry of river plume environments. These relatively stable patterns of eukaryotic transcript abundance occurred over modest spatiotemporal scales, with similarity observed in sample duplicates collected up to 2.45 km in space and 120 minutes in time. These results confirm the use of metatranscriptomics as a valuable tool to understand and predict microbial community function., Gordon and Betty Moore Foundation (River Ocean Continuum of the Amazon Project. Grants GBMF 2293 and 2928), National Science Foundation (U.S.) (Grant NSF-OCE 0934095)

During the summer months, wet (hyporheic) soils associated with ephemeral streams and lake edges in the Antarctic Dry Valleys (DVs) become hotspots of biological activity and are hypothesized to be an important source of carbon and nitrogen for arid DV soils. Recent research in the DV has focused on the geochemistry and microbial ecology of lakes and arid soils, with substantially less information being available on hyporheic soils. Here, we determined the unique properties of hyporheic microbial communities, resolved their relationship to environmental parameters and compared them to archetypal arid DV soils. Generally, pH increased and chlorophyll a concentrations decreased along transects from wet to arid soils (9.0 to ~7.0 for pH and ~0.8 to ~5 μg/cm(3) for chlorophyll a, respectively). Soil water content decreased to below ~3% in the arid soils. Community fingerprinting-based principle component analyses revealed that bacterial communities formed distinct clusters specific to arid and wet soils; however, eukaryotic communities that clustered together did not have similar soil moisture content nor did they group together based on sampling location. Collectively, rRNA pyrosequencing indicated a considerably higher abundance of Cyanobacteria in wet soils and a higher abundance of Acidobacterial, Actinobacterial, Deinococcus/Thermus, Bacteroidetes, Firmicutes, Gemmatimonadetes, Nitrospira, and Planctomycetes in arid soils. The two most significant differences at the genus level were Gillisia signatures present in arid soils and chloroplast signatures related to Streptophyta that were common in wet soils. Fungal dominance was observed in arid soils and Viridiplantae were more common in wet soils. This research represents an in-depth characterization of microbial communities inhabiting wet DV soils. Results indicate that the repeated wetting of hyporheic zones has a profound impact on the bacterial and eukaryotic communities inhabiting in these areas.

We study nonlinear light propagation through suspensions of Synechococcus cells. Such cyanobacteria in aqueous solution enable self-focusing and -defocusing of a light beam, leading to controlled transmission despite extremely low absorption and weak polarizability.

Cyanobacterial symbionts (cyanobionts) have been identified forming associations with various open ocean eukaryotic host genera, including two dinophysoid genera, Histioneis sp. and Ornithocercus sp., two radiolarians, Spongastaurus and Dictyocoryne truncatum, sp., and a tintinnid, Codonella sp. The TEM analysis revealed that single individual hosts were closely associated with one to two different cyanobacterial morphotypes (cyanobionts) and two hosts had in addition to cyanobionts, one to two bacterial cell types. Eleven significantly (P 71 ± 2 8 gold particles·μm -2 ). Immunolabeling-TEM using nitrogenase antisera demonstrated a significant (P

A reverse transcription-polymerase chain reaction (RT-PCR) method was developed for amplifying prokaryotic symbiont rRNA sequences from individual marine planktonic eukaryotic host cells belonging to the tintinnid genus Codonella, the spongiose radiolarian genus Dictyocoryne, and several Dinophysoid genera (Amphisolenia, Citharistes, Dinophysis, Histioneis, Ornithocercus). Of the 93 sequences obtained from 67 clone libraries, 26% were most similar to a variety of heterotrophic bacteria and 4% were most similar to plastids. Most (70%) of the sequences had the greatest similarity to cyanobacterial 16S rRNA sequences. Several (10%) of these had low sequence identity ( 96% identical) to Synechococcus sp., and three were >99% identical to Prochlorococcus sp. Subgroups of sequences most similar to Synechococcus sp. were derived from multiple host types, indicating that the same symbiont was capable of forming an association with a variety of host organisms. Sixteen libraries yielded two or three divergent (from 2% to 25% different) sequence types, indicating the presence of mixed assemblages in single host cells. This study provides the first insight into the phylogenetic diversity of the symbiotic cyanobacteria of open ocean protists.

Unicellular N 2 -fixing cyanobacteria from tropical marine oligotrophic environments have been proposed to be major contributors to the global N cycle but still remain poorly characterized. These organisms are likely to be limited by phosphorus availability in situ. The aim of this study was to identify growth kinetics of isolates from the tropical North Atlantic and subtropical North Pacific in relation to phosphorus and temperature in continuous cultures. Cells from the Atlantic measured 2.5 μm in diameter (A-2.5). Genetically identical isolates from the Pacific showed 2 diameters depending on P-media concentrations (small: 3 μm, 1 pM PO 4 [P-3] and large: 7 pm, 4 pM PO 4 [P-7]). All 3 isolates were highly stenothermal, and optimal growth temperatures ranged between 26 and 30°C. Small cells (A-2.5 and P-3) had lower half-saturation constants (K s ) for PO 4 than large cells (P-7) (0.06 to 0.21 μM vs. 0.20 to 0.25 μM). Maximum growth rates and N:P ratios increased with temperature for all isolates; N:P ratios were close to Redfield ratios (N:P = 16) when isolates approached maximum growth rates. N 2 -fixation activity did not vary between growth rates, but did increase with temperature; rates were consistently lower than previously published rates for the same isolates under non-P-limiting conditions. From these studies, we conclude that both Atlantic and Pacific unicellular cyanobacteria that have the capacity to fix N 2 have a limited temperature range for growth and that smaller sized isolates could be better adapted for conditions of phosphorus limitation.

Nitrogen fixing unicellular marine cyanobacteria may have a major role in the global biogeochemistry of N; nevertheless, little is known about their phylogeny and morphology. We isolated N-2 fixing unicellular cyanobacteria from the tropical North Atlantic and subtropical North Pacific Oceans and examined ultrastructural dynamics during dark:light cycles when grown in incubators. The isolate from the subtropical North Pacific was larger and showed a size variation from 3 to 7 mum but had similar morphology and cell division-plane characteristics as the isolate from the North Atlantic (2.5 mum). Nitrogen fixation only occurred during the dark phase, and ultrastructural analysis demonstrated changes in the appearance and quantity of large carbohydrate-like granules present in the cells. To verify the composition of these carbohydrate-like granules, staining with periodic acid, thioacetic acid, and silver was carried out, and a positive reaction was obvious in all cells. The cells from the Atlantic seemed to empty their polysaccharide granules during the night, whereas those from the Pacific showed a decrease in the number of their granules. Our work suggests that phylogenetically related strains of unicellular N-2 fixing cyanobacteria from different oceans showed similar carbohydrate-like granules that could be used to fuel N-2 fixation during darkness.

Grazing and growth of Pfiesteria piscicida (Pfiest) were investigated using batch and cyclostat cultures with Rhodomonas sp. (Rhod) as prey. Observed maximum growth rates (1.4 d - 1 ) and population densities (2 × 10 5 cells . mL - 1 ) corresponded to values predicted by Monod functions (1.76 d - 1 ; 1.4 × 10 5 cells.mL - 1 ). In batch cultures under a range of prey-to-predator ratios (0.1:1 to 180:1) and prey concentrations (1000-71,000 cells . mL - 1 ), Rhodomonas sp. was always depleted rapidly and P. piscicida concentrations increased briefly. The rate of Rhodomonas sp. depletion and the magnitude of P. piscicida population maxima depended on the prey-to-predator ratio and prey concentration. Starvation resulted in cell cycle arrest at G1 and G2 + M and ultimately the demise of both P. piscicida and Rhodomonas sp. populations, demonstrating the dependence of P. piscicida on the supply of appropriate prey. The depletion of Rhodomonas sp. populations could be attributed directly to grazing, because P. piscicida did not exert detectable inhibitory effects on the growth of Rhodomonas sp. but grazed intensely, with maximum grazing rates > 10 Rhod . Pfiest - 1 . d - 1 and with no apparent threshold prey abundance for grazing. The results suggest that 1) the abundance of appropriate prey may be an important factor regulating P. piscicida abundance in nature, 2) P. piscicida may control prey population, and 3) high growth and grazing potentials of P. piscicida along with cell cycle arrest may confer survival advantages.

Primary production and standing crop, as chlorophyll- a (chl- a ), of Trichodesmium spp., and other phytoplankton as well as the abundance and depth distribution of Trichodesmium were measured on three cruises to the tropical North Atlantic Ocean. Trichodesmium abundance was greatest on a cruise in May–June 1994, with average surface densities of 2250 trichomes l −1 and depth integrated abundance of 91×10 6 trichomes m −2 . Average surface densities were 292 and 222 trichomes l −1 and depth integrated abundance 21 and 8.6×10 6 trichomes m −2 for the April 1996 and October 1996 cruises, respectively. Total (phytoplankton plus Trichodesmium ) chl- a standing crop and the percentage as Trichodesmium averaged 47 (62%), 22 (13%) and 30 (11%) mg chl- a m −2 for May–June 1994 and April and October 1996. On the May–June 1994 and April and October 1996 cruises 89%, 93% and 92% of the trichomes were in colonies, and the remainder occurred as free trichomes. Peak abundances of Trichodesmium were generally in the upper water column, with an average biomass maximum at 12 m on the May–June 94 and October 96 cruises and at 40 m during the April 96 cruise. The average C:N ratio (atomic) of Trichodesmium was 6.5. Mean rates of total primary production ( Trichodesmium and other phytoplankton together) for May–June 1994, and April and October 1996 were 1080, 932 and 804 mg C m −2 d −1 , and Trichodesmium accounted for an average of 47%, 7.9% and 11%, respectively, of the total primary production for each cruise. These primary production rates exceed those typically reported at oligotrophic open ocean sites. Trichodesmium C assimilation numbers (mg C fixed mg chl- a −1 h −1 ) were highest at the surface and were always lower than those of other phytoplankton. Average nitrogen demand, as calculated from the mean Trichodesmium C fixation from all three cruises, was about 40 mg N m −2 d −1 , about 10% of concurrently determined rates of N 2 fixation.

The planktonic cyanobacterium Trichodesmium spp. is a globally important diazotroph, fixing at least 80 Tg N yr -1 in tropical waters. Despite its biogeochemical importance, the mechanisms of Trichodesmium mortality, and the means by which its fixed N enters upper levels of the food web, are poorly understood. Potential virus-like particle (VLP) production by Trichodesmium spp. was observed in both culture (IMS101) and field samples from the tropical North Pacific Ocean, in oceanic waters around the Hawaiian Islands. VLP observed by TEM in IMS101 lysate were approximately 56 nm wide and untailed, and VLP with similar morphology were observed in tissue treated with mit- omycin C. A most-probable number cultivation technique (utilizing bacterized cultures of Tricho- desmium) detected moderate abundances of Trichodesmium-infecting cyanophage (605 to 9750 ml -1 infecting only 1 cultured strain) in 0.2 µm-filtered seawater samples from surface, subsurface and deep chlorophyll maximum samples. Estimation of mortality from virus production was not possible due to rapid VLP release in the first 6 h of dilution incubations. Rather, an indirect approach using burst size (determined from mitomycin C treatment of washed trichomes resuspended in virus-free seawater), decay rate of VLP (from latter part of virus production incubations) and average cyanophage titer was used to estimate mortality. These conservative calculations suggested that 0.3 to 6.5% d -1 (mean = 1.65 ± 0.99% d -1 ) of trichomes could potentially be lysed by viruses, represent- ing the release of approximately 3 to 64% fixed N d -1 . These estimates are based upon a steady-state maintenance of observed VLP abundance, which in nature could be from lysogeny, pseudolysogeny, carrier state, or successive lytic infection. Viral lysis therefore may represent a significant novel mechanism of N release from Trichodesmium spp., even in non-bloom conditions.

We quantified the relationships among the specific growth rate, intracellular iron content, and steady state iron uptake rate for cultures of the marine diazotrophic cyanobacterium Trichodesmium (IMS 101) grown under differing conditions of Fe and N availability. The Fe quotas necessary to support a moderately Fe-limited growth rate (70% mmax) of 0.1 d−1 under diazotrophy and ammonium were 38 and 8 µmol mol−1, indicating a fivefold increased Fe cost for diazotrophy. This increased demand reflects the influences of both the ninefold lower marginal use efficiency and the greater maintenance Fe :C requirement at zero growth rate under diazotrophy (13.5 versus 5.2 µmol mol−1). For diazotrophic growth at µ = 0.1 d−1, we estimate that 19%–53% of the cellular Fe is bound in nitrogenase. Trichodesmium is capable of luxury uptake of at least 13-fold greater amounts of Fe than needed for moderately Fe-limited growth. At least half of the populations sampled from the continental shelf of northern Australia had Fe :C ratios within the range of iron limitation of laboratory cultures. Bottle incubations showed that, after 3–4 d with added iron, a low Fe :C population (21 µmol mol−1) had higher N2 fixation rates than controls, whereas a high Fe :C population (78 µmol mol−1) showed no response. The cellular N: P quotas and N2 fixation rates were compared to those previously reported for the Atlantic Ocean. Empirically, these data suggest a critical N: P ratio ~40–50 for the onset of P limitation of growth. Only 10% of the Trichodesmium populations from coastal Australia had N: P ratios within the putative P-limitation range (>40), whereas 40% the Atlantic populations fell within this range. These comparisons, along with those for N fixation versus colony Fe :C ratio, suggest that Fe limitation is more prevalent in the Australia Trichodesmium populations, while P limitation predominates in the Atlantic populations.

While several DNA-based methods have been developed for the putatively toxic dinoflagellate Pfiesteria piscicida Burkholder et Steidinger, an independent detection method such as immunofluorescence can be a useful alternative. In this study, P. piscicida-specific antisera were developed, and an immunofluorescence (IF) procedure was optimized. A total of six antisera were raised using whole cells (WCA) and the insoluble cellular fraction (ICF) as antigens, respectively, and their titer and specificity were examined using dot blot analysis and whole cell IF. Results showed that the two antisera produced from the ICF antigen had a markedly higher titer (1500) than the other four yielded from the WCA (200). In addition, the two ICF-derived antisera exhibited much higher species specificity, showing no cross-reaction with P. shumwayae, Cryptoperidiniopsis sp., Karlodinium micrum, and other more distant algae tested, and very low background for field collected samples. In evaluation of the IF technique using a P. piscicida-specific polymerase chain reaction (PCR) technique, results from both methods generally agreed well for both field samples (from eastern Long Island Sound) spiked with cultured P. piscicida and those containing naturally occurring P. piscicida (from Chesapeake Bay tributaries).

Estimates of the iron use efficiency (IUE) for diazotrophic plant growth have been used to suggest iron limitation of marine N 2 fixation. However, in the course of these inferences, neither the physiological complexity of these estimates nor the specific physiological parameters of marine diazotrophs were evaluated. Here, a semiempirical prediction of the IUE of diazotrophic growth for Trichodesmium was computed from considerations of the Fe content and reaction rates of the nitrogenase complex and PSI:PSII ratios, as well as field measurements of Mehler activity, cellular Fe-superoxide dismutase activity, and diel variability in C and N 2 fixation. With a PSI: PSII ratio of 1 and 48% Mehler activity, the instantaneous IUE (0.33 mol C fixed.mol cellular Fe - 1 . s - 1 ) was only 4-fold lower than that calculated for a phytoplankter growing on reduced N. We computed a range of daily integrated IUE values from 2900 to 7700 mol C.mol Fe - 1 . d - 1 , accounting for the diel variability in C and N 2 fixation as well as the uncertainties in cyanobacterial nitrogenase biochemistry and PSI:II ratios of field-collected Trichodesmium. The lowest observed Fe-superoxide dismutase:C quota of 2.9 (μmol:mol) suggests a maintenance requirement for this enzyme. The maintenance Fe:C requirement of 13.5 μmol:mol (derived from cultures of Trichodesmium IMS 101) and values of the IUE yielded an Fe requirement ranging from 27 to 48 Fe: C (μmol:mol) to achieve a diazotrophic growth rate of 0.1 d - 1 . Based on these predicted requirements, the Fe:C contents of Caribbean Sea and most North Atlantic Ocean populations sampled thus far exceed that required to support the observed rates of N 2 fixation.

Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean. 15N:14N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15N content relative to average deep-water nitrate. Herein we show that the low dΔ15N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N2 fixation. These results provide direct, integrative evidence that N2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.

A synthesis of the current understanding of potential iron limitation of pelagic nitrogen fixation is given, considering biochemical bases of Fe requirements and empirical observations of growth and Fe quotas of cultures and field populations of Trichodesmium. The potential for iron limitation of heterotrophic diazotrophy in the marine environment is also evaluated.