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The effect of Arctic warming, which is 3 times faster than the global average, on microbial communities was evaluated experimentally to determine how increasing temperatures affect bacterial and viral abundance and production, protist community composition, and bacterial loss rates (bacterivory and lysis) in 2 contrasting Arctic marine systems. In July 2009, we collected samples from open Arctic waters in the Barents Sea and Atlantic-influenced waters in Isfjorden, Svalbard Islands (Fjord waters). The samples were used in 2 microcosm experiments at 7 temperatures, ranging from 1.0 to 10.0°C. In the open Arctic microbial community, collected at <1.0°C, bacterial and viral abundances, bacterial production and grazing rates due to protists increased significantly above 5.5°C, and remained at high values at even higher experimental temperatures. The abundance of protists, such as some heterotrophic pico/nanoflagellates, as well as some ciliates, also increased with warming. In contrast, the biomass of phototrophs decreased above 5.5°C. The water temperature in Fjord waters was 6.2°C at the time of sampling, and the microbial community showed smaller variations than the Arctic community. These results indicate that increases in temperature stimulate heterotrophic microbial biomass and activity compared to that of phototrophs, which has important implications for carbon and nutrient cycling in the system. In addition, open Arctic communities were more vulnerable to warming than those already adapted to the warmer Fjord waters influenced by Atlantic seawater. © Inter-Research 2013.

Growing evidence of inorganic nutrient limitation on oceanic bacteria suggests a global dependence of bacterial activity and production on rates of nutrient supply. The present study examined whether surface bacterial abundance is significantly related to water column stability, and whether bacterial activity and growth rate are related to the rate of diffusive supply of inorganic nutrients to the mixed layer in the Central Atlantic during 2 meridional cruises. The 2 cruises were run under very different oceanic conditions, with relatively low values of bacterial activity in spring 1995 and relatively higher values in fall 1995. We obtained depth-resolved data in the second cruise and found that the integrated value of bacterial production was also related to the rate of nutrient supply, while integrated particulate primary production and chlorophyll concentration were not. There was also no relationship between particulate primary production and bacterial production. The relationship between nutrient supply and integrated bacterial production was tested with data from a mesocosm experiment showing a good fit to the pattern obtained in the Atlantic. Average bacterial production was ~21% of primary production in the Central Atlantic, with values ranging between 5 and 100%, and higher values in the tropical areas. The demonstration of a direct relationship between nutrient supply and bacterial activity helps to explain a relatively large bacterial biomass as compared to phytoplankton biomass, a low bacterial growth efficiency, and a high bacterial carbon demand relative to contemporaneous primary production often measured in the open ocean, as well as the accumulation of dissolved organic carbon (DOC) observed in nutrient-limited oligotrophic seas

Narrow annual ranges of temperature characterize polar waters. Consequently, small increases in temperature could significantly affect the metabolic processes of marine microorganisms. We investigated the response of bacterial heterotrophic production (BHP) and grazing rates to small temperature changes in 3 zones near the western Antarctic Peninsula —Bransfield and Gerlache Straits, and Bellingshausen Sea— during December 2002. We performed 8 grazing experiments with water samples collected from depths where chlorophyll a (chl a) concentration was maximum, and incubated the samples at ambient temperature and at –1, 1, 2 and 5°C. We expected that grazing would increase in parallel with BHP at increasing temperatures; however, temperature differentially affected these 2 microbial activities. Thus, grazing rates increased maximally at temperatures ≤ 2°C, except in 1 station in the Gerlache Strait, while BHP increased maximally at temperatures ≥ 2°C, except in 1 station in the Bellingshausen Sea. The percentage of grazed bacteria to BHP at the highest experimental temperatures was low (56 ± 19%) in the Gerlache Strait, high (395 ± 137%) in the Bransfield Strait and approximately balanced (97 ± 24%) in the Bellingshausen Sea. This suggests that differential microbial processes in each zone at increasing temperatures will also depend on the autochthonous community. The present study contributes to the understanding of the variability of polar biogeochemical fluxes, and may aid in predicting the response of microorganisms in future scenarios with local and seasonal changes in temperature

Bacterioplankton have the potential to significantly affect the cycling of organic matter in the ocean; however, little is known about the linkage between bacterial assemblage structure and carbon metabolism. In this study, we investigated whether changes in the phylogenetic composition of bacterioplankton were associated with changes in bacterial carbon processing (bacterial production, respiration and biomass) in the subtropical NE Atlantic Ocean. We found consistent differences in the composition of the bacterial assemblage, as revealed by denaturing gradient gel electrophoresis (DGGE) and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), along a gradient from the NW African upwelling to the oligotrophic North Atlantic Subtropical Gyre. The percent contribution of Bacteroidetes, Roseobacter and Gammaproteobacteria significantly increased towards more productive waters, whereas the SAR11 clade of the Alphaproteobacteria remained relatively constant (average 28% of DAPI-stained cells) throughout the area. Changes in the composition of the bacterial assemblage detected by DGGE were weakly but significantly correlated with changes in carbon processing variables. The abundances of Roseobacter and Gammaproteobacteria were highly correlated with the concentration of particulate organic carbon and chlorophyll a, reflecting the affinity of these groups to nutrient-enriched conditions. The abundance of Roseobacter was also positively correlated with heterotrophic bacterial production, suggesting their active participation in carbon processing

Functional diversity and aminopeptidase activity (AMA) in bacterial assemblages were determined in western Antarctic waters during late spring 2002. Functional diversity was assayed by the patterns of sole carbon source utilization in Biolog-ECO Microplates™ and AMA with the fluorogenic substrate leucine 7-amido-4-methylcoumarin. N-acetyl-D-glucosamine and D-cellobiose were the most used carbohydrates. This suggested that used dissolved organic carbon (DOC) was mostly of either zoo-or phytoplankton origin. Principal component analysis of the sole carbon source utilization profiles separated the samples according to salinity and temperature. This separation corresponded roughly with the 3 areas of study: Bransfield Strait (BR), Gerlache Strait (GE) and Bellingshausen Sea (BE). AMA was higher in the upper 40 m, probably associated with the higher organic matter load. Phytoplankton biomass was the factor that accounted for the highest variance in AMA, but did not have a clear influence on functional diversity of bacterioplankton. Our findings indicate that differences in functional diversity of bacterioplankton populations in western Antarctic waters are not directly related to phytoplanktonic abundance. This suggests that bacteria could utilize other carbon sources than DOC freshly released by phytoplankton. © Inter-Research 2005

Functional diversity and aminopeptidase activity (AMA) in bacterial assemblages were determined in western Antarctic waters during late spring 2002. Functional diversity was assayed by the patterns of sole carbon source utilization in Biolog-ECO Microplates™ and AMA with the fluorogenic substrate leucine 7-amido-4-methylcoumarin. N-acetyl-D-glucosamine and D-cellobiose were the most used carbohydrates. This suggested that used dissolved organic carbon (DOC) was mostly of either zoo-or phytoplankton origin. Principal component analysis of the sole carbon source utilization profiles separated the samples according to salinity and temperature. This separation corresponded roughly with the 3 areas of study: Bransfield Strait (BR), Gerlache Strait (GE) and Bellingshausen Sea (BE). AMA was higher in the upper 40 m, probably associated with the higher organic matter load. Phytoplankton biomass was the factor that accounted for the highest variance in AMA, but did not have a clear influence on functional diversity of bacterioplankton. Our findings indicate that differences in functional diversity of bacterioplankton populations in western Antarctic waters are not directly related to phytoplanktonic abundance. This suggests that bacteria could utilize other carbon sources than DOC freshly released by phytoplankton. © Inter-Research 2005

The effect of variations in N:P ratios on Mediterranean picoplankton Synechococcus sp. was tested with a nutrient enrichment experiment in large-scale mesocosms in a coastal Mediterranean bay community during summer 1998. By adding either N or P in excess of 42 mmol m-2 d-1 N or 2.1 mmol m-2 d-1, the mesocosm units (16 m 3) received nutrient ratios varying between 2.5 to 160, i.e. 8-fold lower to 8-fold higher than background levels (N:P 20). The total phytoplankton had increased significantly after 1 wk of the experiment on mesocosms receiving high N:P ratios (excess N), with diatoms being mainly responsible for this increase. In contrast, Synechococcus sp., which along with small flagellates initially dominated the water column, rapidly increased in biomass, abundance and primary production, with higher abundance in mesocosms receiving excess N and P, and lower abundance in mesocosms receiving the background N:P load of 20 that is normal for the Bay of Blanes. Our study indicated that this complex pattern resulted from the response of the grazing community, which was highest at the 'background' ratio. Indeed, the potential grazers of Synechococcus sp. (heterotrophic nanoflagellates and phagotrophic ciliates) were highly abundant at the background ratio. Our results also showed that primary production of Synechococcus sp. was significantly inversely correlated (r = -0.98, p < 0.05) with specific grazing rates, suggesting that picophytoplankton responses to N:P loads arose from top-down effects related to the responses of the grazing community to the various N:P levels. The top-down effects were, however, obscured during the early part of the experiment, when there were transient imbalances between growth and loss rates of picophytoplankton owing to time lags between the growth of picophytoplankton and the growth of their grazers (heterotrophic nanoflagellates and phagotrophic ciliates). This resulted in a general net increase in Synechococcus sp. abundance in all m

Grazing can be a substantial loss factor for harmful algal blooms (HABs) and a possible cause of their termination. There is, however, a considerable gap in our knowledge of the relative grazing impact of the different components of the planktonic community on a red tide. In this study, we examine the importance of grazing control on a naturally occurring bloom of the toxic dinoflagellate Alexandrium minutum. We estimated the grazing impact of microzooplankton (by the dilution method) and copepods (by 24 h incubations with cultured populations of Acartia grani and Oithona davisae) on a bloom of A. minutum. Grazing by microzooplankton (0.84 d-1) was equal to and even exceeded A. minutum growth rates (0.79 d-1). On the other hand, copepod impact on the A. minutum population was not significant, although they were actively feeding on the dinoflagellate, which accounted for 78 and 86% of the daily carbon intake of A. grani and O. davisae, respectively. Even if these results were extended to the highest realistic copepod abundance attainable in the area, the grazing impact on highly concentrated HABs would not be sufficient to significantly reduce the algal concentrations. In summary, grazing by microzooplankton can represent an important factor for the regulation of HABs

The hypothesis that increasing nutrient supply increases the biomass of autotrophs proportionately more than the biomass of heterotrophs was tested by increasing (0, 1-, 2-, 4-, 8-, and 16-fold over the background loading of 5 mmol N m-2 d-1, 1.6 mmol Si m-2 d-1, and 0.25 mmol P m-2 d-1) the addition of nutrients to large (33 000 l) mesocosm units enclosing an oligotrophic coastal Mediterranean planktonic community. Autotrophic plankton biomass increased 50-fold along the range of nutrient inputs, whereas heterotrophic biomass increased only 10-fold. Heterotrophic biomass increased as the 1/5 power of the increase in the biomass of autotrophs, implying that the ratio of heterotroph to autotroph biomass (HB/AB ratio) declined rapidly as the biomass of autotrophs increases with increasing nutrient inputs. The biomass distribution within the community shifted from an 'inverted pyramid' distribution, involving greater biomass of heterotrophs than that of autotrophs, at low nutrient inputs, to the conventional 'upward' pyramid pattern, where the biomass of autotrophs exceeds that of consumers, at the highest nutrient inputs. This shift stabilized after 4 d, and the pyramids remained quite constant for the rest of the experiment. The experimental test presented supports the hypothesis that the relative biomass distribution between heterotrophs and autotrophs is regulated by nutrient supply.

Diel and spatial variability in the concentration of virus-like particles (VLP) was determined in a transect from the coast to offshore in the western Mediterranean (7 stations and 6 depths per station). VLP abundance in coastal waters was on average 1.6 x 107 VLP ml-1, in offshore stations this value was 1.2 x 107 VLP ml-1. At the coastal station there was no variability in VLP abundance, bacterial abundance or chlorophyll a concentration through the water column, while offshore VLP decreased with depth. No clear diel pattern was found in 3 representative stations where diel cycles were studied. Viral impact on the bacterial assemblage was estimated at 2 depths (surface and deep chlorophyll a maximum) of the latter 3 stations using 2 approaches: the percentage of visibly infected bacteria and viral decay rates in cyanide amended cultures. Visibly infected bacteria could not be detected in any sample after counting 300 cells per sample, implying that the percentage of visibly infected bacteria was lower than 0.3%. Using the maximal conversion factor to convert this value to bacterial mortality we found that viruses could be responsible at most for 21.5% of the whole bacterial mortality. We calculated a hypothetical viral decay rate if the maximal possible impact of viruses (21.5%) were true. This value corresponded to 0.02 h-1. However, using the cyanide method to measure viral decay rate, no significant decrease could be detected in a total of 8 viral decay experiments. In conclusion, the percent of total bacterial mortality due to viral lysis in the oligotrophic northwestern Mediterranean was low, certainly much lower than 20%

Primary producers must respond to the diel changes in light availability. Therefore, detection of diel cycles in bacterial activity would imply tight coupling between the production of photosynthetic dissolved organic carbon (DOC) and its consumption by bacteria. Absence of diel cycles, on the contrary, would indicate that bacteria depend largely upon allochthonous organic carbon and that bacteria are not tightly dependent on photosynthetically produced autochthonous carbon. In 1993 and 1994 we sampled 3 sites in the NW Mediterranean Sea several times a day, and measured several microbial parameters as well as the vertical profiles of DOC along the diel cycle. The sites were selected so that one was on the continental shelf and, thus, was more influenced by coastal runoff; a second one was over the shelf slope and a third, oceanic one was located further offshore over a depth of 2000 m. We found clear diel cycles in bacterial total and specific activity always in the oceanic stations and sometimes in the shelf slope stations. Diel changes were detected as changes in both DNA and protein synthesis rates. These diel cycles were accompanied by diel changes in the distribution of total DOC, and by diel changes in the proportion of bacteria containing visible nucleoids. Noon estimates of bacterial activity were more than twice the daily average in the oceanic site, but they were less different in the other 2 sites. DOC changed daily by 15 µM (5 to 15% of the total stock). For bacterial activity to explain the diel changes in DOC concentration, bacteria should have growth efficiencies lower than 10% in general, and lower than 2% in the oceanic station.

Examination of the contribution of experimental approaches to marine microbial ecology shows these approaches to receive 34.8% of the present effort in the field. Most of the experiments focus on bacteria or marine microbial communities and generally examine the importance of trophic interactions and associated flows of carbon and nutrients in these food webs. Microbial ecologists use experimental units ranging 8 orders of magnitude in size (10-3 to 105 l), with a geometric median size of 0.8 l, and an exponential decline in the number of experiments performed at increasingly larger and smaller scales. The duration of experiments is scaled linearly with the characteristic linear dimension of the experimental units, corresponding to 20 d for each meter in characteristic dimension. The majority (70.3%) of the experiments performed in the past 5 yr used natural communities or organisms, particularly in mesocosm (>103 l) experiments. Most (84.5%) of the experiments are conducted under closely controlled conditions in the laboratory and involve the manipulation of particles, resources and the food web structure, usually manipulating a single factor at a time, the fraction of experiments published declining exponentially with the increase in the numbers of factors tested. A major difference between experimental marine microbial ecology and other disciplines of marine ecology is the remarkable paucity of field experiments (only 2.6% of the experiments in marine microbial ecology), where only the treatment factors are controlled, and the total absence of ecosystem experiments, which are field experiments where the treatment is applied to an entire ecosystem. Experimental approaches have played an important role in the development of marine microbial ecology, but no single experiment has had a large impact on the progress of the field, suggesting that the experiments so far conducted in marine microbial ecology have failed to provide the crucial tests of the main hypotheses

Protists and viruses are recognized as the main predators on bacteria in a variety of environments. Several factors may potentially influence the grazing rates of protists on bacteria: temperature, bacterial abundance and production, and protist abundance. We inspected the relative importance of these variables in determining the specific grazing rates of flagellates (GRF), and total community grazing rates (GT) by analyzing a data set extracted from the literature. We included freshwater and marine environments, ranging from oligo- to eutrophic systems. As different methods to measure grazing rates (GRF and GT) were used by different authors, the data were also analyzed for possible differences among methods. As there were significant differences among certain methods, we classified them into 2 groups: (1) M1 methods, those that measured grazing rates (GRF and GT) through the uptake of fluorescent particles; and (2) M2 methods, those determining community grazing rates (GT) by dilution, inhibition, filtration, or the uptake of genetically marked bacteria. GT measured with M2 methods were systematically higher than those measured by M1. For studies with M1 methods, both GRF and GT were positively correlated to temperature, bacterial abundance and production. GT were also correlated to heterotrophic nanoflagellate abundance. GT and GRF showed a different pattern of change with temperature, GRF increased with temperature within the range 0 to 30-degrees-C, while GT increased only up to 18-degrees-C. We did not find any satisfactory multiple regression model that explained the variation in the GT measurements with M2 methods. For M1 methods, flagellate abundance and temperatures below 18-degrees-C were the main factors affecting GT (the model explained 78 % of the variance). While bacterial and flagellate abundances were the main factors at higher temperatures, the model in this case explained only 19% of the variance. Grazing rates could be affected by other sources o

The constraints imposed by the geometry of Codium bursa, a balloon-like Mediterranean macroalga with a thick (= 5.6 mm) thallus enclosing a lumen filled with water, on its functional properties were tested by examining the scaling of its form, composition (pigmentation and nutrient content), light absorption, metabolism (respiration rate, light use efficiency, and maximal photosynthetic rate), and growth to individual size. We found that C. bursa absorbs most (98 %) of the incident light, due to its high area1 pigment density (0.52 pg chl a mm-2), but that light absorption was very inefficient, as seen from the low values of absorption per unit carbon (0.045 mZ g-' C), which suggest a low intrinsic growth rate. The pigment and nutrient content of C. bursa increase more slowly than the macroalgal weight, so that large C. bursa balloons have lower pigment and nutrient contents than smaller individuals. As a consequence, photosynthetic capacity declines with increasing size, and light compensation points increase with increasing individual size, suggesting increasing light requirements and reduced potential growth rate as C. bursa grows. This was confirmed by in situ estimates of growth rate that showed C. bursa to rank amongst the slowest-growing macroalgae (specific growth rate 1.8 * 0.3 X lO-= d-') as well as by the significant decline of specific growth rate with increasing algal size. The results obtained confirm the slow growth rates and the strong size-dependence of biochemical composition and metabolism imposed by the spherical geometry of C. bursa

The Hudson River Estuary IS a heterotrophic ecosystem with high rates of planktonic bacterial production. We measured grazing on bacteria in monthly experiments during 1990 to deter- mine: (l) the primary consumers of bacteria; (2) whether absolute rates of grazing on bacteria were high relative to other systems; and (3) whether bacteria were a sufficient carbon resource for consumers. Fluorescent minicells were used to measure the Ingestion of bacteria by heterotrophic flagellates and ciliates. Using the abundances of bacteria, rotifers and cladocerans in the river, we also estimated zooplankton clearance rates. Total grazing of the plankton community was assessed by measuring the disappearance of fluorescent minicells over 24 and 48 h. Heterotrophic flagellates were the primary consumers of bacteria with ciliates and the cladoceran Bosmina longirostris also important seasonally. Ingestion of bacteria by heterotrophic flagellates, ciliates, and B. longirostris was sufficient at most times to satisfy estimated carbon requirements of these consumers. Large bacterial size and relatively low consumer populations in the Hudson account for the apparent sufficiency of bacteria as a carbon source. We conclude that grazing on bacteria in the Hudson River is similar to the levels of grazing observed in other systems. Nevertheless, bacteria are a potentially significant resource, because the dominant planktonic consumers are capable of ingesting bacteria at rates sufficient to support carbon demands

We provide evidence, based on the assessment of bacterial abundance in a series of samples representing geometrically decreasing volumes (5 1 to 0.25 ml), for the existence and scale dependence of patchiness in mixed-layer marine bacterioplankton. Our results demonstrate the existence of strong bacterioplankton patchiness at centimetre scale, the bacterioplankton community appearing as homogeneous when sampling at larger or smaller scales. We also provide, based on the inferred frequency distribution of bacterial densities, a representation of the in situ spatial distribution of the bacterioplankton population sampled This distribution depicts the bacterioplankton community as composed of high density (> 107 bacteria ml-l) bacterial patches scattered within a 'matrix' of low bacterial density (105 bacteria ml-l). These results imply that the processes regulating bacterial abundance operate at centimetre scale

Study of 2 diatom species, Leptocylindrus danicus and Skeletonema costatum, growing in an enclosed plankton community showed that most of the variability in the extent of epialgal colonization (i.e. bacteria algal cell-') was attributable to differences within species, compared to small among-species and temporal variability. Within-species variability was related to the population dynamics of the diatom species examined, because the extent of colonization increased with the time algal cells had been exposed to colonization. Thus, modelling of algal colonization by bacteria requires both consideration of encounter probabilities and phytoplankton demography

Use of algal surfaces as substrate for bacterial growth requires colonization of the algal surface. Based on a study of the dynamics of epialgal bacteria during a bloom episode of an enclosed phytoplankton community, we suggest that the colonization process is partially limited by the alga- bacterium encounter probability. This assertion is based on the demonstration that 86 % of temporal variability in the average number of epialgal bacteria per algal cell can be explained by changes in the abundance of algae and free bacteria, that together constrain the alga-bacterium encounter probability. Encounter probability should, therefore, be of paramount importance for bacterial attachment to particles in the sparser communities inhabiting the oligotrophic sea. We postulate that one of the advantages of maintaining a dense, albeit not very active, bacterial population in the sea should be to maximize the probability of encountering algae or other patchy sources of organic carbon

Effects of turbulence on the dynamics of copepods as well as on the development of phytoplankton biomass were studied in 30 dm³ laboratory microcosms. Two experiments were carried out, with different successional stages of the plankton populations, using seawater from the Masnou nautical harbour, 20 km north of Barcelona. Two experimental conditions were established (stirred microcosms with copepods, and unstirred microcosms with copepods). Enclosure of plankton populations was followed by a phytoplankton bloom, which differed in intensity in stirred and unstirred microcosms. Time changes in the abundance of the different developmental stages of copepods (from eggs to adults), sex ratios (♂/♀ + ♂), fecundity (no. eggs/no. females), total copepod biomass, and the ratio consumers/producers, also differed in stirred and unstirred microcosms. Turbulence seems to reduce consumer biomass through changes in the demographic composition (lower proportion of males and higher development rates) and probably by increasing the metabolic activity of copepods (feeding pressure and excretion rates). The ultimate effect of turbulence would be a reduction of the trophic efficiency of the system, and accelerated turnover rates