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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2007.02.007 Byline: Matthew K. Reuer (a), Bruce A. Barnett (a), Michael L. Bender (a), Paul G. Falkowski (b), Melissa B. Hendricks (a) Keywords: Dissolved gases; Oxygen isotopes; Primary production; Southern Ocean Abstract: We report O.sub.2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O.sub.2 (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1-2[degrees] meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O.sub.2 production, and gross O.sub.2 production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O.sub.2 production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous.sup.18O and.sup.14C incubations along 170[degrees]W. O.sub.2/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O.sub.2/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O.sub.2-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O.sub.2 production scatter about 0.13, corresponding to f ratios of [approximately equal to]0.25. Net community/gross O.sub.2 production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O.sub.2 production rates are in satisfactory agreement with comparable published estimates. Net and gross O.sub.2 production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O.sub.2 production and gross O.sub.2 production with other environmental variables, the strongest correlations are between net O.sub.2 production and sea surface temperature (SST) (direct correlation), climatological [NO.sub.3.sup.-] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO.sub.2 and iron are the leading influence or if lower SSTs promote greater export in this region. Author Affiliation: (a) Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544-1003, USA (b) Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901-8521, USA Article History: Received 6 April 2005; Revised 20 February 2007; Accepted 23 February 2007

The maximum quantum yield of carbon fixation (phi(sub C max)) ranges from 0.005 to 0.063 mol C/mol quanta in the natural phytoplankton colony of the oligotrophic sites in tropical northeast Atlantic Ocean. Photosynthetic parameters are nearly constant with depth at the eutrophic and mesotrophic sites. A deep chlorophyll maximum exists at the oligotrophic site. Nitrate concentration is a likely contributing factor for the two-fold variation in phi(sub C max), while the three-fold variations are explained by the light absorption contribution of non-photosynthetic pigments.

A pump and and probe fluorometer is used to evaluate the minimum quantum efficiency (phi II) of phytoplankton photosystem II photochemistry on an offshore-onshore transect from the oligotrophic blue waters of the western Sargasso Sea to the eutrophic waters of lower Delaware Bay. The low volumes of the ratio of variable to maximum fluoroscence (Fv/Fm) of dark adapted samples reveal a reduced quantum efficiencies in surface waters of the Sargasso Sea. The higher values of Fv/Fm in estuarine samples show a near maximum quantum efficiency in lower Delaware Bay. Average values of Fv/Fm is observed over the continental shelf. Nutrient limitation of phytoplankton photsynthetic energy conversion is the cause of reduced values of quantum efficiency in Sargasso Surface Waters.

Abstract: Using an infrared fast-repetition-rate (IRFRR) fluorometer, we measured bacteriochlorophyll a (BChl a) fluorescence kinetics to examine the spatial pattern of the distribution of aerobic and anaerobic photosynthetic bacteria in the Black Sea in June 2001. Aerobic photosynthetic bacteria containing BChl a were distributed together with chlorophyll-containing phytoplankton in the upper, aerobic portion of the water column, confirming their exclusive requirement for oxygen. The BChl a to chlorophyll a ratio averaged 10−2 within the euphotic zone. Seven strains of aerobic photosynthetic bacteria were isolated and partially characterized. They belong to two subgroups closely related to Erythrobacter and Roseobacter genera. Green sulfur photosynthetic bacteria were detected in the central part of the basin, at the boundary between suboxic and anoxic (hydrogen sulfide) zone. However, the population of green sulfur bacteria was extremely small, and confined to a very narrow density layer deep in the water column. [Copyright &y& Elsevier]

Abstract: In all aquatic environments, nitrogen cycling within the water column is strongly influenced by oxygen. We hypothesize that the nitrogen isotopic composition (δ15N) of organic matter deposited in the sediments is a proxy for the redox state of the water column at the time of deposition. We tested the hypothesis by measuring the bulk sedimentary δ15N values in a drill core from the Black Sea, a basin that alternates between oxic, less saline conditions and anoxic, marine conditions on glacial–interglacial time scales. We reconstructed these changes in Black Sea redox conditions using sedimentary δ15N, total organic carbon (TOC), total nitrogen (TN), redox-sensitive metals, and micropaleontological data from a deep-sea core (DSDP Site 380). The sedimentary data reveal that during the transitions between oxic and anoxic conditions, δ15N values increased relative to the preceding and succeeding quasi-steady-state oxic and anoxic periods. The results indicate that the reciprocal transitional states from anoxic to oxic conditions were accompanied by intense denitrification; during the quasi-stable oxic and anoxic states (characterized by glacial fresh water and interglacial marine conditions) nitrification and complete nitrate utilization, respectively, dominate the nitrogen cycle. While other factors may influence the δ15N record, our results support the hypothesis that the variations in nitrogen isotopic composition of organic matter are strongly influenced by changes in redox state in the Black Sea subphotic zone on glacial–interglacial time scales, and can be explained by a relatively simple model describing the effects of oxygen on the microbial processes that drive the nitrogen cycle in marine ecosystems. Our model suggests that the nitrogen isotopic composition of marine sediments, on geological time scales, can be used to reconstruct the redox state of the overlying water column. [Copyright &y& Elsevier]

Highlights: [•] Algal biomass is a promising renewable energy source. [•] We review the potential of diatoms, an algal taxon, to become this energy source. [•] We suggest biological and technical ways to improve production and yield of diatoms. [•] Diatoms can replace fossil fuel using <5% of the USA land area. [Copyright &y& Elsevier]

Abstract: It is widely established that nutritional nitrogen deprivation increases lipid accumulation but severely decreases growth rate in microalgae. To understand the regulatory branch points that determine the partitioning of carbon among its potential sinks, we analyzed metabolite and transcript levels of central carbon metabolic pathways and determined the average fluxes and quantum requirements for the synthesis of protein, carbohydrates and fatty acid in the diatom Phaeodactylum tricornutum. Under nitrate-starved conditions, the carbon fluxes into all major sinks decrease sharply; the largest decrease was into proteins and smallest was into lipids. This reduction of carbon flux into lipids together with a significantly lower growth rate is responsible for lower overall FA productivities implying that nitrogen starvation is not a bioenergetically feasible strategy for increasing biodiesel production. The reduction in these fluxes was accompanied by an 18-fold increase in α-ketoglutarate (AKG), 3-fold increase in NADPH/NADP+, and sharp decreases in glutamate (GLU) and glutamine (GLN) levels. Additionally, the mRNA level of acetyl-CoA carboxylase and two type II diacylglycerol-acyltransferases were increased. Partial suppression of nitrate reductase by tungstate resulted in similar trends at lower levels as for nitrate starvation. These results reveal that the GS/GOGAT pathway is the main regulation site for nitrate dependent control of carbon partitioning between protein and lipid biosynthesis, while the AKG/GL(N/U) metabolite ratio is a transcriptional signal, possibly related to redox poise of intermediates in the photosynthetic electron transport system. [Copyright &y& Elsevier]

Abstract: High light poses a threat to oxygenic photosynthetic organisms. Similar to eukaryotes, cyanobacteria evolved a photoprotective mechanism, non-photochemical quenching (NPQ), which dissipates excess absorbed energy as heat. An orange carotenoid protein (OCP) has been implicated as a blue-green light sensor that induces NPQ in cyanobacteria. Discovered in vitro, this process involves a light-induced transformation of the OCP from its dark, orange form (OCPo) to a red, active form, however, the mechanisms of NPQ in vivo remain largely unknown. Here we show that the formation of the quenching state in vivo is a multistep process that involves both photoinduced and dark reactions. Our kinetic analysis of the NPQ process reveals that the light induced conversion of OCPo to a quenching state (OCPq) proceeds via an intermediate, non-quenching state (OCPi), and this reaction sequence can be described by a three-state kinetic model. The conversion of OCPo to OCPi is a photoinduced process with the effective absorption cross section of 4.5×10−3 Å2 at 470nm. The transition from OCPi to OCPq is a dark reaction, with the first order rate constant of ~0.1s−1 at 25°C and the activation energy of 21kcal/mol. These characteristics suggest that the reaction rate may be limited by cis-trans proline isomerization of Gln224–Pro225 or Pro225–Pro226, located at a loop near the carotenoid. NPQ decreases the functional absorption cross-section of Photosystem II, suggesting that formation of the quenched centers reduces the flux of absorbed energy from phycobilisomes to the reaction centers by ~50%. [Copyright &y& Elsevier]

Abstract: In this paper we summarise the photo-physiological responses of phytoplankton to upwelling of macronutrients in mesoscale eddies in the subtropical North Atlantic (EDDIES project, Sargasso Sea) and subtropical North Pacific (E-FLUX project, Hawaii). The observations, obtained on two sets of cruises over 2 years, occupied six cyclonic eddies and two mode-water eddies. The photosynthetic physiological parameters were measured using a bench-top fluorescence induction and relaxation (FIRe) system and a submersible in situ fast repetition rate fluorometer (FRRF) deployed on an undulating towed vehicle. Both of these instruments were used to provide highly sensitive and well-resolved data on community responses. The responses are dependent on both the type of eddy and its stage of development. Our results indicate that, while cyclonic eddies in the Atlantic and Pacific can increase primary photosynthetic production early in their development, mode-water eddies in the subtropical North Atlantic can support patchy blooms of large diatoms for long periods of time (more than 3 months). [Copyright &y& Elsevier]

Abstract: We report O2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O2 (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1–2° meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O2 production, and gross O2 production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O2 production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous 18O and 14C incubations along 170°W. O2/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O2/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O2-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O2 production scatter about 0.13, corresponding to f ratios of ∼0.25. Net community/gross O2 production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O2 production rates are in satisfactory agreement with comparable published estimates. Net and gross O2 production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O2 production and gross O2 production with other environmental variables, the strongest correlations are between net O2 production and sea surface temperature (SST) (direct correlation), climatological [NO3 −] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO2 and iron are the leading influence or if lower SSTs promote greater export in this region. [Copyright &y& Elsevier]

Abstract: In this paper, we review the state of the art and major challenges in current efforts to incorporate biogeochemical functional groups into models that can be applied on basin-wide and global scales, with an emphasis on models that might ultimately be used to predict how biogeochemical cycles in the ocean will respond to global warming. We define the term “biogeochemical functional group” to refer to groups of organisms that mediate specific chemical reactions in the ocean. Thus, according to this definition, “functional groups” have no phylogenetic meaning—these are composed of many different species with common biogeochemical functions. Substantial progress has been made in the last decade toward quantifying the rates of these various functions and understanding the factors that control them. For some of these groups, we have developed fairly sophisticated models that incorporate this understanding, e.g. for diazotrophs (e.g. Trichodesmium), silica producers (diatoms) and calcifiers (e.g. coccolithophorids and specifically Emiliania huxleyi). However, current representations of nitrogen fixation and calcification are incomplete, i.e., based primarily upon models of Trichodesmium and E. huxleyi, respectively, and many important functional groups have not yet been considered in open-ocean biogeochemical models. Progress has been made over the last decade in efforts to simulate dimethylsulfide (DMS) production and cycling (i.e., by dinoflagellates and prymnesiophytes) and denitrification, but these efforts are still in their infancy, and many significant problems remain. One obvious gap is that virtually all functional group modeling efforts have focused on autotrophic microbes, while higher trophic levels have been completely ignored. It appears that in some cases (e.g., calcification), incorporating higher trophic levels may be essential not only for representing a particular biogeochemical reaction, but also for modeling export. Another serious problem is our tendency to model the organisms for which we have the most validation data (e.g., E. huxleyi and Trichodesmium) even when they may represent only a fraction of the biogeochemical functional group we are trying to represent. When we step back and look at the paleo-oceanographic record, it suggests that oxygen concentrations have played a central role in the evolution and emergence of many of the key functional groups that influence biogeochemical cycles in the present-day ocean. However, more subtle effects are likely to be important over the next century like changes in silicate supply or turbulence that can influence the relative success of diatoms versus dinoflagellates, coccolithophorids and diazotrophs. In general, inferences drawn from the paleo-oceanographic record and theoretical work suggest that global warming will tend to favor the latter because it will give rise to increased stratification. However, decreases in pH and Fe supply could adversely impact coccolithophorids and diazotrophs in the future. It may be necessary to include explicit dynamic representations of nitrogen fixation, denitrification, silicification and calcification in our models if our goal is predicting the oceanic carbon cycle in the future, because these processes appear to play a very significant role in the carbon cycle of the present-day ocean and they are sensitive to climate change. Observations and models suggest that it may also be necessary to include the DMS cycle to predict future climate, though the effects are still highly uncertain. We have learned a tremendous amount about the distributions and biogeochemical impact of bacteria in the ocean in recent years, yet this improved understanding has not yet been incorporated into many of our models. All of these considerations lead us toward the development of increasingly complex models. However, recent quantitative model intercomparison studies suggest that continuing to add complexity and more functional groups to our ecosystem models may lead to decreases in predictive ability if the models are not properly constrained with available data. We also caution that capturing the present-day variability tells us little about how well a particular model can predict the future. If our goal is to develop models that can be used to predict how the oceans will respond to global warming, then we need to make more rigorous assessments of predictive skill using the available data. [Copyright &y& Elsevier]

Focuses on the Synthesis and Modeling Project (SMP) of the United States Joint Global Ocean Flux Study program on marine biogeochemistry and ocean carbon cycle. Objectives of the SMP; Theoretical background on marine biogeochemistry; Results of some SMP research projects.