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It is widely accepted that in many aquatic ecosystems bacterioplankton is dependent on and regulated by organic carbon supplied by phytoplankton, leading to coupled algae-bacteria relationship. In this study, an in-depth analysis of this relationship has been carried out by combining two approaches: (i) a correlation analyses between heterotrophic bacterial production (BP) vs. primary production (PP) or algal excretion of organic carbon (EOC), (ii) the balance between bacterial carbon demands (BCD) and the supply of C as EOC, measured as BCD:EOC ratio. During the study period (2013-2016), the algae-bacteria relationship was constantly changing from a coupling in 2013, uncoupling in 2014 and 2015, and an incipient return to coupling (in 2016). Our results show that top-down control (bacterivory) by algal mixotrophy acts as a decoupling force since it provides a fresh C source different to algal EOC to satisfy bacterial carbon demands. Notably, a relationship between the BCD:EOC ratio and the ecosystem metabolic balance (Primary production (PP): respiration (R)) was found, suggesting that PP:R may be a good predictor of the algae-bacteria coupling. This analysis, including the comparison between basal and potential ecosystem metabolic balance, can be a tool to improve knowledge on the interaction between both biotics compartments, which the traditional analyses on coupling may not capture., (© 2022. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment (15 d) to test the effect of this on the planktonic ecosystem expecting reduced primary production and shifts in the phytoplankton community composition. The experiment was set up in 2.2 m 3 mesocosm bags using four treatments, each with three replicates: control (Contr) without any manipulation, organic carbon additive HuminFeed (Hum; 2 mg L -1 ), inorganic nutrients (Nutr; 5.7 μM NH 4 and 0.65 μM PO 4 ), and combined Nutr and Hum (Nutr + Hum) additions. Measured variables included organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry. The bags with added inorganic nutrients developed a phytoplankton bloom that depleted inorganic N at day 6, followed by a rapid decline in Chla. Brownification did not reduce primary production at the tested concentration. Bacterial production was lowest in the Contr, but similar in the three treatments receiving additions likely due to increased carbon available for heterotrophic bacteria. Picoeukaryotes clearly benefited by brownification after inorganic N depletion, which could be due to more effective nutrient recycling, nutrient affinity, light absorption, or alternatively lower grazing pressure. In conclusion, brownification shifted the phytoplankton community composition towards smaller species with potential effects on carbon fluxes, such as sinking rates and export to the sea floor., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

In recent years, the increasing scarcity of water resources and eutrophication of water have become more serious. Reservoirs that are far from big cities have become important sources of drinking water and were targets of research and protection. Therefore, the abundance and production of bacteria and their correlations with environmental factors were investigated in the Dalongdong Reservoir, Shanglin County, Guangxi Province, using Quantitative Real-time PCR technology and the 14 C tracer technique. The Dalongdong Reservoir is a typical subtropical karst reservoir. The bacterial abundance of the surface water decreased from the upstream to the downstream along the water flow direction and then increases. The vertical distribution of the bacterial abundance at each sampling site shows a similar trend; it is the highest in the surface water and the lowest on the bottom. The correlation analysis results show that the temperature, pH, electrical conductivity, dissolved organic carbon (DOC), chlorophyll- a , dissolved oxygen (DO), and other environmental factors significantly correlate with the bacterial abundance, indicating that these parameters are the main factors limiting the bacterial abundance in this region. The bacterial production is positively correlated with the pH, DOC, and permanganate index; negatively correlated with the conductivity and DIC; and significantly positively correlated with DO. The principal component analyses (PCA) shows that the environmental factors affecting the bacterial abundance and bacterial production can be grouped into two PCAs. PCA1 includes the temperature, pH, electrical conductivity, DIC, DO, chlorophyll- a , DOC, and permanganate index and PCA2 includes TN and TP. The bacterial abundance and production in the Dalongdong Reservoir are affected by various environmental factors and photosynthetic bacteria are the important contributors to the production of organic carbon.

In this study, we examined the response of phytoplankton and heterotrophic bacteria to B12 amendments in microcosm experiments in the coastal southeastern Mediterranean Sea (SEMS) during the summer, where ambient levels of dissolved B12 ranged from as low as ∼2 pmol L−1 to 60 pmol L−1 (median 5.3 pmol L−1). Additions of B12 (20 pmol L−1) to surface seawater triggered a 4‐fold increase in NO3+NO2 uptake compared to unamended seawater, resulting in proliferation of mainly pico/nano‐eukaryotic phytoplankton (∼30%) and increase in primary and bacterial productivity (40%–50%). Complimentary experiments that tested the combined effects of nutrient (NO3 and PO4) and B12 additions suggest that phytoplankton were primarily NO3 and B12 co‐limited, whereas heterotrophic bacteria were PO4 and B12 co‐limited. These results provide valuable information about the marine distribution of nutrient limitation in low nutrients low chlorophyll (LNLC) environments such as the SEMS, and how bacterioplankton might respond to environmental perturbations. Plain Language Summary: Most biogeochemical studies have focused primarily on the effects of dissolved nitrate and orthophosphate on marine phytoplankton and bacterial productivity. However, vitamin B12 is also an important co‐factor for phytoplankton and bacteria, which has been far less investigated. These few studies showed that despite the very low concentrations of dissolved B12 in the marine environment, it has a strong influence on phytoplankton and bacterial productivity. Here, we followed phytoplankton and bacterial abundance and activity in response to B12 additions using high‐resolution daily measurements, and microcosm bioassays with additional nutrient simulations (nitrate, orthophosphate). Our study site was the coastal southeastern Mediterranean Sea, and the experiments were done during the summer when the most nutrient‐poor conditions prevail. Our results indicate that additions of B12 triggered a rapid increase in nitrate uptake, resulting in increased abundance of phytoplankton (mostly pico/nano‐eukaryotes) and photosynthesis rates. We also show that phytoplankton "need" both nitrate and vitamin B12 to proliferate, while bacteria require orthophosphate and B12 to support their growth. These results add to the current knowledge on the limitations of phytoplankton and bacteria in the coastal southeastern Mediterranean Sea and have implications for the whole coastal food‐web in this nutrient‐poor ecosystem. Key Points: Addition of B12 increased pico/nano‐eukaryotes abundance during the illuminated hours and of cyanobacteria during night‐timeB12 amendments accelerated NO3 uptake by phytoplankton, resulting in increased primary production ratesPhytoplankton were co‐limited by NO3 and B12 while heterotrophic bacteria were co‐limited by B12 and PO4 [ABSTRACT FROM AUTHOR]

The seasonal dynamics of production processes in the Baltic Sea are poorly studied. The aim of our research was to study the seasonal features of primary productivity (including the balance with bacterial production) and its redistribution in plankton in the southeastern part of the Baltic Sea in different seasons. More than 70% of primary production is formed in the 0–10 m layer (74–97% of the PP in the euphotic layer). In the same layer, PP accounted for almost 100% of the sum of primary and bacterial production in April and October, and almost 60% in June. Photosynthetic efficiency (PP/rETR) increased in June and October, demonstrating an increase in phytoplankton utilization of absorbed light energy. The depth-integrated values of PP, Chl a, bacterial, and phytoplankton biomasses were maximal in October. The maximum values of zooplankton biomass were determined in June, and they were significantly (5–14 times) higher than in other seasons. The maximum values of bacterial production were also in June. [ABSTRACT FROM AUTHOR]

Biogeochemical responses to mixing were examined in two cruise surveys along a transect across the Kuroshio Current (KC) in May and July 2020. Two stations located at the South China Sea (SCS)–KC mixing and the KC waters were chosen for the diel study. In the euphotic zone (~100 m depth), the average values of nitrate (0.97–1.62 μM), chlorophyll-a (Chl-a, 0.36–0.40 mg/m3), and primary production (PP; 3.46 ± 1.37 mgC/m3/day) of the mixing water station (MWS) of the two cruises were several folds higher than those of the KC station (KCS; nitrate, 0.03–0.10 μM; Chl-a, 0.14–0.24 mg/m3; and PP, 0.91 ± 0.47 mgC/m3/day). In the July cruise, the maximal bacterial production (BP) at the MWS (3.31 mgC/m3/day) was 82% higher in comparison with that of the KCS (1.82 mgC/m3/day); and the readings of Chl-a showed no trend with BP in the oligotrophic KCS, but a positive relationship was found among these measurements at the mesotrophic MWS. This implies that the trophic status of the system might affect phytoplankton–bacteria interactions. The backward-trajectory analyses conducted by an observation-validated three-dimensional model identified that the prevailing southwest monsoon drove a northeastward “intrusion” of the SCS waters in July 2020, resulted in mixing between SCS and Kuroshio (KC) waters off the east coast of southern Taiwan. For the first time, this study demonstrates that the high biological biomass and activities that occur in the KC are induced by the northward intrusion of the SCS waters.

Mesopelagic prokaryotes (archaea and bacteria), which are transported together with nutrient-rich intermediate-water to the surface layer by deep convection in the oceans (e.g., winter mixing, upwelling systems), can interact with surface microbial populations. This interaction can potentially affect production rates and biomass of surface microbial populations, and thus play an important role in the marine carbon cycle and oceanic carbon sequestration. The Eastern Mediterranean Sea (EMS) is one of the most oligotrophic and warm systems in the world's oceans, with usually very shallow winter mixing (

Three cruises were conducted to investigate the distributions of nutrients, chlorophyll a (Chl-a), new and regenerated primary production, bacterial abundance and production, and microzooplankton grazing rates in the Yellow Sea (YS) and the South China Sea (SCS) during March and May. As the water column moved from low to high temperature, weak to strong stratification and high to low nutrients from the YS to the SCS, Chl-a, primary production and bacterial biomass decreased. In contrast, bacterial production, microzooplankton grazing and size preference increased from the YS to the SCS. The increasing grazing activity and decreasing f-ratio from the YS to the SCS suggest roles of regenerated nutrients in the supporting the community increased and more bacteria played important roles in the carbon flow in the oligotrophic SCS than in the eutrophic YS. These variabilities force the classical food chain dominated community in the eutrophic waters into the microbial loop, which is dominant in oligotrophic waters. As nutrients decrease, temperature and grazing activity increase from the YS to the SCS. The increasing ratio of integrated bacterial production to integrated primary production indicates that communities change from autotrophy to heterotrophy and waters change from a carbon sink to a carbon source. [ABSTRACT FROM AUTHOR]

We investigated bacterial production (BP) and respiration (BR), as well as the physico-chemical properties of the water column, to elucidate the effect of upwelling on heterotrophic bacterial metabolic activities and growth efficiency (BGE) in July 2012 and May 2013 in the Ulleung Basin (UB), East/Japan Sea. The upwelled conditions were characterized by higher chlorophyll- a (Chl- a ) concentrations resulting from the upward shift of the nitracline compared to that of the non-upwelled condition. Analyses of the size fractions of Chl- a and pigment composition revealed that large size phytoplankton (> 20 µm), mainly consisting of diatoms, appeared to be the major phytoplankton component. BP and BR were significantly correlated with Chl- a ( P < 0.001), but the correlations with temperature were not significant ( P > 0.05). These results suggest that bacterial metabolic activities are stimulated by the availability of organic resources enhanced by upwelling in the UB. Further statistical analysis showed that the difference in BP and BGE with variations in upwelling intensity were significant ( P = 0.018 for BP, P = 0.035 for BGE), but the difference in BR was not significant ( P = 0.321). These results suggest that metabolic energy is partitioned more for BP under a strong upwelling condition, i.e. high nutrient and Chl- a conditions. In contrast, the energy generated via respiration was partitioned more for maintaining metabolism rather than for biomass production under weakly or non-upwelled conditions, i.e. stratified and low Chl- a conditions. Overall, our results suggest that any changes in upwelling intensity would significantly affect the carbon cycle associated with the fate of primary production, and the role of the microbial loop in the UB where changes in the intensity and frequency of upwelling associated with climatic changes are in progress. [ABSTRACT FROM AUTHOR]

In the Kongsfjorden-Krossfjorden system (Spitsbergen), increasing temperatures enhance glacier melting and concomitant intrusion of freshwater. These altered conditions affect the timing, intensity, and composition of the phytoplankton spring bloom in Kongsfjorden; yet, the effects on prokaryotes (bacteria and archaea) are not well understood. The aim of this study was to examine springtime prokaryote communities in both fjords as a function of hydrographic and phytoplankton variability. Prokaryote community composition was studied in two consecutive years by molecular fingerprinting of the 16S rRNA gene. In addition, we measured bacterial abundance, productivity (H-Leucine uptake), and single-cell activity using catalyzed reporter deposition fluorescence in situ hybridization combined with microautoradiography. Differences in bacterial and archaeal communities were found between Kongsfjorden and Krossfjorden. Furthermore, an increase in productivity, abundance, and proportion of active bacterial cells was observed during the course of spring. Bacteroidetes were the most abundant bacterial group among the assessed taxa in both Kongsfjorden and Krossfjorden. Multivariate analysis of the microbial community fingerprints revealed a strong temporal shaping of both the bacterial and archaeal communities in addition to a spatial separation between the two fjords. A significant part of the observed bacterial variation could be explained by cyanobacterial biomass, as deduced from pigment analysis, and by phosphate concentration. Archaea were mainly controlled by abiotic factors. We speculate that the bacterial response to hydrographic changes and glacier meltwater is mediated through shifts in phytoplankton abundance and composition, whereas archaea are directly influenced by abiotic environmental variables. [ABSTRACT FROM AUTHOR]

We simultaneously measured bacterial production (BP), bacterial respiration (BR), alkaline phosphatase activity ( phos) and ectoaminopeptidase activity ( prot) in relation to biogeochemical parameters, nutritive resources and in situ temperature over a 1-year survey at the long-term observatory the SOLEMIO station (Marseille bay, NW Mediterranean Sea). Despite its proximity to the coast, oligotrophic conditions prevailed at this station (yearly mean of Chl a = 0.43 μg dm, NO = 0.55 μmol dm and PO = 0.04 μmol dm). Episodic meteorological events (dominant winds, inputs from the Rhone River) induced rapid oscillations (within 15 days) in temperature and sometimes salinity that resulted in rapid changes in phytoplankton succession and a high variability in C/P ratios within the particulate and dissolved organic matter. Throughout the year, BP ranged from 0.01 to 0.82 μg C dm− h− and bacterial growth efficiency varied from 1 to 39 %, with higher values in summer. Enrichment experiments showed that BP was limited most of the year by phosphorus availability (except in winter). A significant positive correlation was found between in situ temperature, BP, BR and phos. Finally, we found that temperature and phosphate availability were the main factors driving heterotrophic bacterial activity and thus play a fundamental role in carbon fluxes within the marine ecosystem. [ABSTRACT FROM AUTHOR]

Dominant primary producer in macrophyte- or phytoplankton-dominated shallow lakes might imply differences in dissolved organic carbon (DOC) composition. We compared chromophoric dissolved organic matter (CDOM), plankton respiration (R), and bacterial (BP) and primary production (PP), in two contrasting shallow lakes. We hypothesized that DOC from the macrophyte-dominated lake would be qualitatively inferior, so that it can support a lower yield than DOC from the phytoplankton-dominated one. Macrophyte-dominated lake had more humic and aromatic CDOM, though molecular weight was similar in both lakes. A clear synchronism between lakes was observed in mean depth and several CDOM absorption coefficients, suggesting an external driver of the variation in DOC concentration and CDOM quality. The positive BP-PP and BP-Chl- a correlations in the macrophyte-dominated lake point out to a dependence of bacteria on phytoplankton for a supply of labile DOC. In turn, BP in the phytoplankton-dominated lake was balanced with grazing by HF (heterotrophic flagellates). The significantly higher HB:DOC and HF:DOC carbon ratios in the phytoplankton-dominated lake also suggest that better DOC quality would mean relatively more efficient C transfer to higher trophic levels. According to PP:BP and PP:R ratios both lakes should be considered autotrophic, although the macrophyte-dominated lake would be comparatively more heterotrophic. [ABSTRACT FROM AUTHOR]

The response of planktonic heterotrophic prokaryotes to the NW Mediterranean winter–spring offshore phytoplankton bloom was assessed in 3 cruises conducted in March, April–May and September 2009. Bulk measurements of phytoplankton and bacterioplankton biomass and production were complemented with an insight into bacterial physiological structure by single-cell analysis of nucleic acid content [low (LNA) vs. high (HNA)] and membrane integrity (“Live” vs. “Dead” cells). Bacterial production empirical conversion factors (0.82±0.25 SE kg C mol leucine −1 ) were almost always well below the theoretical value. Major differences in most microbial variables were found among the 3 periods, which varied from extremely high phytoplankton biomass and production during the bloom in March (>1 g C m −2 d −1 primary production) to typically oligotrophic conditions during September stratification (<200 mg C m −2 d −1 ). In both these periods bacterial production was ~30 mg C m −2 d −1 while very large bacterial production (mean 228, with some stations exceeding 500 mg C m −2 d −1 ) but low biomass was observed during the April–May post-bloom phase. The contribution of HNA (30–67%) and “Live” cells (47–97%) were temporally opposite in the study periods, with maxima in March and September, respectively. Different relationships were found between physiological structure and bottom-up variables, with HNA bacteria apparently more responsive to phytoplankton only during the bloom, coinciding with larger average cell sizes of LNA bacteria. Moderate phytoplankton–bacterioplankton coupling of biomass and activity was only observed in the bloom and post-bloom phases, while relationships between both compartments were not significant under stratification. With all data pooled, bacteria were only weakly bottom-up controlled. Our analyses show that the biomass and production of planktonic algae and bacteria followed opposite paths in the transition from bloom to oligotrophic conditions. [ABSTRACT FROM AUTHOR]

Rare spring blooms, > 20 μg l- 1 chlorophyll a, were observed in the San Francisco Bay Delta during the drought year of 2014 in both the upper Sacramento River and in Suisun Bay. The upper Sacramento River bloom was dominated by chlorophytes, but biomass and photosynthetic efficiency (based on variable fluorescence, Fv/Fm) precipitously declined downstream when cells were exposed to sewage effluent and NH4+ levels > 70 μM-N. Further downriver, substantial rates of nitrification occurred, based on increasing levels of NO3- and NO2- in proportion to decreasing NH4+ concentrations, reducing NH4+ levels to < 10 μM-N. The other major tributary, the San Joaquin River, had extremely high nutrient levels (NO3- > 400 μM-N, PO43 - > 13 μM-P, but NH4+ ~ 2 μM-N), very low chlorophyll a levels (~ 3 μg L- 1) and low Fv/Fm values, but elevated bacterial production, suggesting presence of an algal inhibitor, possibly an herbicide. Both rivers converge above Suisun Bay, where elevated NO3- (> 50 μM-N), sufficient PO43 - (> 3 μM-P), and reduced NH4+ levels (as low as 6 μM-N), and reduced flow created conditions conducive to a spatially large and physiologically healthy (elevated Fv/Fm) diatom bloom dominated by the species Entomoneis sp. We conceptualize this bloom as a "window of opportunity" response by these diatoms to multiple factors promoted by the drought, including longer residence time for cell growth and biomass accumulation, and longer time for in-river nitrification to occur, reducing sewage-derived NH4+ to a level where diatoms could access NO3- for uptake and growth. We suggest that management practices that favor higher rates of flow may narrow the "window of opportunity" for phytoplankton growth, potentially leading to low productivity and food limitation for fish. Under high flow, a condition of "washout" may develop where both chlorophyll and unassimilated nutrients are transported out of the bay, and the phytoplankton that do develop are less favorable in terms of community composition for supporting the upper food web. [ABSTRACT FROM AUTHOR]

The bacterial turnover of organic matter was investigated in Fram Strait at 79°N. Both Atlantic Water (AW) inflow and exported Polar Water (PW) were sampled along a transect from Spitsbergen to the eastern Greenland shelf during a late successional stage of the main annual phytoplankton bloom in summer. AW showed higher concentrations of amino acids than PW, while organic matter in PW was enriched in combined carbohydrates. Bacterial growth and degradation activity in AW and PW were related to compositional differences of organic matter. Bacterial production and leucine-aminopeptidase along the transect were significantly correlated with concentrations of amino acids. Activity ratios between the extracellular enzymes β-glucosidase and leucine-aminopeptidase indicate the hydrolysis potential for polysaccharides relative to proteins. Along the transect, these ratios showed a higher hydrolysis potential for polysaccharides relative to proteins in PW than in AW, thus reflecting the differences in organic matter composition between the water masses. Q10 values for bacterial production ranged from 2.4 (±0.8) to 6.0 (±6.8), while those for extracellular enzymes showed a broader range of 1.5 (±0.5) to 23.3 (±11.8). Our results show that in addition to low seawater temperature also organic matter availability contributes to the regulation of bacterial growth and enzymatic activity in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

We studied production by three key pelagic energy mobilizer communities, phytoplankton (PP), heterotrophic bacteria (HB), and methanotrophic bacteria (MOB), in five boreal lakes of varying size and concentration of dissolved organic carbon (DOC). Production by PP was responsible for most (>55%) of the total pelagic energy mobilization in all five lakes. Production by HB and PP estimated for the whole water column during the ice-free period were positively correlated, but with the exception of the clearest and most eutrophic lake PP apparently could not support the total carbon demand of bacteria. However, the DOC concentration did not explain the variability of heterotrophic bacterial production (HBP) within or between the lakes. Thus, our results provide circumstantial evidence for the 'priming effect' whereby labile organic matter from autochthonous production enhances decomposition of allochthonous DOC. However, HBP was only 10-23% of the total pelagic energy mobilization in the lakes, suggesting that only a minor fraction of allochthonous DOC became available for higher trophic levels. High MOB activity was detected in the water columns of the stratified lakes when the molar ratio of CH:O varied between 0.5 and 12. In the small stratified lakes (area < 0.01 km), MOB production contributed 13-52% of the total pelagic energy mobilization, being greatest during the autumn mixing period. Our results indicate that in small stratified lakes (area < 0.01 km) bacteria, especially MOB, are potentially quantitatively important supplementary food resources for zooplankton. However, in larger lakes primary producers are the most important (>70%) potential food source for zooplankton. [ABSTRACT FROM AUTHOR]

Both temperature and terrestrial organic matter have strong impacts on aquatic food-web dynamics and production. Temperature affects vital rates of all organisms, and terrestrial organic matter can act both as an energy source for lower trophic levels, while simultaneously reducing light availability for autotrophic production. As climate change predictions for the Baltic Sea and elsewhere suggest increases in both terrestrial matter runoff and increases in temperature, we studied the effects on pelagic food-web dynamics and food-web efficiency in a plausible future scenario with respect to these abiotic variables in a large-scale mesocosm experiment. Total basal (phytoplankton plus bacterial) production was slightly reduced when only increasing temperatures, but was otherwise similar across all other treatments. Separate increases in nutrient loads and temperature decreased the ratio of autotrophic:heterotrophic production, but the combined treatment of elevated temperature and terrestrial nutrient loads increased both fish production and food-web efficiency. CDOM: Chl a ratios strongly indicated that terrestrial and not autotrophic carbon was the main energy source in these food webs and our results also showed that zooplankton biomass was positively correlated with increased bacterial production. Concomitantly, biomass of the dominant calanoid copepod Acartia sp. increased as an effect of increased temperature. As the combined effects of increased temperature and terrestrial organic nutrient loads were required to increase zooplankton abundance and fish production, conclusions about effects of climate change on food-web dynamics and fish production must be based on realistic combinations of several abiotic factors. Moreover, our results question established notions on the net inefficiency of heterotrophic carbon transfer to the top of the food web. [ABSTRACT FROM AUTHOR]

Abstract: Phytoplankton and bacterioplankton dynamics were studied around South Shetland Islands (Antarctica) with special emphasis on the Drake Passage region, during austral summer, in order to expand our knowledge on the coupling between the autotrophic and heterotrophic microbial plankton compartments in polar ecosystems. In addition, we directly estimated bacterial growth efficiency in the Drake Passage with the aim of better constraining total bacterial carbon utilization in this important polar ecosystem. Integrated chlorophyll-a concentration (21–86mgm−2), primary production rates (0.7–19.3mgCm−3 d−1) and mean water-column photochemical efficiency (0.24–0.60) were significantly correlated with Si⁎ tracer (r 2=0.55, 0.46 and 0.64, respectively), which indirectly points to iron as the major limiting factor for phytoplankton growth in the area. Bacterial production was considerably low (0.002–0.3mgCm−3 d−1) and was best explained by chlorophyll-a concentration, protein-like fluorescence of dissolved organic matter and temperature (r 2=0.53, p<0.001). Water temperature appeared to influence bacterial activity when organic substrate availability is high. Bacterial production accounted on average for only 3.9% of co-occurring primary production, which has been frequently interpreted as an indicator of the marked uncoupling between bacteria and phytoplankton in cold waters. However, using the experimentally derived mean bacterial growth efficiency for the photic zone (6.1±1.3%) the bacterial carbon demand represented on average 63±18% of concomitant primary production, similar to what is found in warmer productive waters. Thus, our study suggests that bacterioplankton and phytoplankton appear to be connected in this polar area. [Copyright &y& Elsevier]

Abstract: We present results on phytoplankton and bacterial production and related hydrographic properties collected on nine annual summer cruises along the western Antarctic Peninsula. This region is strongly influenced by interannual variations in the duration and extent of sea ice cover, necessitating a decade-scale study. Our study area transitions from a nearshore region influenced by summer runoff from glaciers to an offshore, slope region dominated by the Antarctic Circumpolar Current. The summer bacterial assemblage is the product of seasonal warming and freshening following spring sea ice retreat and the plankton succession occurring in that evolving water mass. Bacterial production rates averaged 20mgCm−2 d−1 and were a low (5%) fraction of the primary production (PP). There was significant variation in BP between regions and years, reflecting the variability in sea ice, chlorophyll and PP. Leucine incorporation was significantly correlated (r2 ranging 0.2–0.7, p<0.001) with both chlorophyll and PP across depths, regions and years indicating strong phytoplankton–bacteria coupling. Relationships with temperature were variable, including positive, negative and insignificant relationships (r2 <0.2 for regressions with p<0.05). Bacterial production is regulated indirectly by variations in sea ice cover within regions and over years, setting the levels of phytoplankton biomass accumulation and PP rates; these in turn fuel BP, to which PP is coupled via direct release from phytoplankton or other less direct pathways. [Copyright &y& Elsevier]

Abstract: The distributions of heterotrophic bacterial abundance and production were investigated in the East China Sea and the Yellow Sea during the autumn of 2000 and spring of 2001. Bacterial abundance varied in the range 3.2–15.7 (averaging 5.7) × 105 and 2.3–13.6 (averaging 6.2) × 105 cells cm−3 in the spring and autumn, respectively. During autumn, bacterial production (BP) (0.27–7.77mg C m−3 day−1) was on average 3 fold that in spring (0.001–2.04mg C m−3 day−1). Bacterial average turnover rate (ratio of bacterial production:bacterial biomass, μ=0.21 day−1) in autumn was 3 times as high as in spring (0.07 day−1). The ratio of integrated bacterial biomass to integrated phytoplankton biomass in the euphotic zone ranged from 4 to 101% (averaging 35%) in spring and 24 to 556% (averaging 121%) in autumn. The results indicate that the distributions of heterotrophic bacteria were controlled generally by temperature in spring and additionally by substrate supply in autumn. [Copyright &y& Elsevier]

The strength of the bacteria–phytoplankton coupling and the importance of the microbially mediated carbon fluxes have been investigated in a microtidal lagoon (Lagoon of Venice), with emphasis on the trophic variations, in a within-system perspective. The large trophic heterogeneity of the three stations considered corresponded to an elevated variability of phytoplankton biomass and production (from 0.1 up to 300 μg C L−1 h−1), while bacteria standing stock and production (from 2 to 8 μg C L−1 h−1) appeared, in comparison, to be much more constant. The relationships between bacteria and the phytoplankton community could not be related to the trophic state in a straightforward way; rather, some patterns common to the three stations could be evidenced. In particular, the two communities appeared to be clearly uncoupled, bacterial carbon demand (BCD) always exceeding dissolved primary production (DPP) and, mostly, also total primary production, independent of the station and the season considered. The occurrence of situations in which bacterial production was larger than primary production and the continuous prevalence of BCD over DPP implied that, quite independent of the trophic variability, sources of organic carbon other than phytoplankton production were necessary and available to sustain the bacterial metabolism in the Lagoon of Venice. [ABSTRACT FROM AUTHOR]

Abstract: Standing stocks and production rates for phytoplankton and heterotrophic bacteria were examined during four expeditions in the western Arctic Ocean (Chukchi Sea and Canada Basin) in the spring and summer of 2002 and 2004. Rates of primary production (PP) and bacterial production (BP) were higher in the summer than in spring and in shelf waters than in the basin. Most surprisingly, PP was 3-fold higher in 2004 than in 2002; ice-corrected rates were 1581 and 458mgCm−2 d−1, respectively, for the entire region. The difference between years was mainly due to low ice coverage in the summer of 2004. The spatial and temporal variation in PP led to comparable variation in BP. Although temperature explained as much variability in BP as did PP or phytoplankton biomass, there was no relationship between temperature and bacterial growth rates above about 0°C. The average ratio of BP to PP was 0.06 and 0.79 when ice-corrected PP rates were greater than and less than 100mgCm−2 d−1, respectively; the overall average was 0.34. Bacteria accounted for a highly variable fraction of total respiration, from 3% to over 60% with a mean of 25%. Likewise, the fraction of PP consumed by bacterial respiration, when calculated from growth efficiency (average of 6.9%) and BP estimates, varied greatly over time and space (7% to >500%). The apparent uncoupling between respiration and PP has several implications for carbon export and storage in the western Arctic Ocean. [Copyright &y& Elsevier]

1. Abundance and bacterial production (BP) of heterotrophic bacteria (HBact) were measured in the north and south basins of Lake Tanganyika, East Africa, during seasonal sampling series between 2002 and 2007. The major objective of the study was to assess whether BP can supplement phytoplankton particulate primary production (particulate PP) in the pelagic waters, and whether BP and particulate PP are related in this large lake. HBact were enumerated in the 0–100 m surface layer by epifluorescence microscopy and flow cytometry; BP was quantified using 3H-thymidine incorporation, usually in three mixolimnion layers (0–40, 40–60 and 60–100 m). 2. Flow cytometry allowed three subpopulations to be distinguished: low nucleic acid content bacteria (LNA), high nucleic acid content bacteria (HNA) and Synechococcus-like picocyanobacteria (PCya). The proportion of HNA was on average 67% of total bacterial abundance, and tended to increase with depth. HBact abundance was between 1.2 × 105 and 4.8 × 106 cells mL−1, and was maximal in the 0–40 m layer (i.e. roughly, the euphotic layer). Using a single conversion factor of 15 fg C cell−1, estimated from biovolume measurements, average HBact biomass (integrated over a 100-m water column depth) was 1.89 ± 1.05 g C m−2. 3. Significant differences in BP appeared between seasons, especially in the south basin. The range of BP integrated over the 0–100 m layer was 93–735 mg C m−2 day−1, and overlapped with the range of particulate PP (150–1687 mg C m−2 day−1) measured in the same period of time at the same sites. 4. Depth-integrated BP was significantly correlated to particulate PP and chlorophyll- a, and BP in the euphotic layer was on average 25% of PP. 5. These results suggest that HBact contribute substantially to the particulate organic carbon available to consumers in Lake Tanganyika, and that BP may be sustained by phytoplankton-derived organic carbon in the pelagic waters. [ABSTRACT FROM AUTHOR]

Abstract: High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22nM at depths between 70 and 690m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14nM) and DMS (20nM) were observed between 113 and 240m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8nM) at 250m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ∼12% of the bacterial carbon demand between 200 and 400m. DMS concentrations peaked at 286m but the maximum concentration (0.42nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea. [Copyright &y& Elsevier]

The effect was determined of organo-mineral detritus (OMD), one of the components of suspended mineral matter in aquatic ecosystems, on the production characteristics of bacterioplankton (bacterial production Pb and destruction of organic matter Rb, as well as bacterial growth efficiency BGE). The relation was determined between these parameters and the ratio of the content of suspended mineral matter M to the total organic carbon content ( M/TOC). More active utilization of organic matter by bacterioplankton in the presence of OMD resulted in its positive effect on specific production characteristics of the phytoplankton. [ABSTRACT FROM AUTHOR]

Abstract: Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates, and growth efficiencies were studied in the Northern region of the Cananéia–Iguape estuarine system, which has recently experienced an intense eutrophication due to anthropogenic causes. Two surveys were carried out during spring and neap tide periods of the dry season of 2005 and the rainy season of 2006. This region receives large freshwater inputs with organic seston and phosphate concentrations that reach as high as 1.0mgl−1 and 20.0μM, respectively. Strong decreasing gradients of seston and dissolved inorganic nutrients were observed from the river/estuary boundary to the estuary/coastal interface. Gradients were also observed in phytoplankton and bacterial production rates. The production rates of phytoplankton were 5.6-fold higher (mean 8.5μg Cl−1 h−1) during the dry season. Primary production rates (PP) positively correlated with salinity and euphotic depth, indicating that phytoplankton productivity was light-limited. On the other hand, bacterial biomass (BB) and production rates (BP) were 1.9- and 3.7-fold higher, respectively, during the rainy season, with mean values of up to 40.4μg Cl−1 and 7.9μg Cl−1 h−1, respectively. Despite such a high BP, bacterial abundance remained <2×106 cellsml−1, indicating that bacterial production and removal were coupled. Mean specific growth rates ranged between 0.9 and 5.5d−1. BP was inversely correlated with salinity and positively correlated with temperature, organic matter, exopolymer particles, and particulate-attached bacteria; this last accounted for as much as 89.6% of the total abundance. During the rainy season, BP was generally much higher than PP, and values of BP/PP>20 were registered during high freshwater input, suggesting that under these conditions, bacterial activity was predominantly supported by allochthonous inputs of organic carbon. In addition, BB probably represented the main pathway for the synthesis of high-quality (low C:N) biomass that may have been available to the heterotrophic components of the plankton food web, particularly nanoheterotrophs. [Copyright &y& Elsevier]

The impact of the scyphomedusa Aurelia sp. on planktonic assemblages was experimentally studied in enclosures incubated in situ in the sea lake of Mljet Island (Big Lake, Southern Adriatic), where jellyfish are present throughout the year. In situ feeding experiments using plankton at natural densities indicated a reduction in abundance for small calanoid and cyclopoid copepods, copepodites, nauplii and ciliates in the presence of Aurelia sp. In addition to direct predatory pressure, Aurelia sp. exerted an indirect cascading effect on autotrophic and heterotrophic microbial plankton. Phytoplankton biomass increases of up to 0.5 μg C·l−1·h−1 were mainly related to 19′-hexanoyloxyfucoxanthin- and 19′-butanoyloxyfucoxanthin-containing phytoplankton. Bacterial production was about twice as high in the presence of Aurelia sp. and biomass was also consistently higher. It appears that the top-down effect of predation along with material release by Aurelia sp. results in increases in microbial biomass and production. [ABSTRACT FROM AUTHOR]

Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates and growth efficiencies were studied in July 2001 and January 2002 during both spring and neap tides, along a tidal cycle, at three sites in a subtropical estuary. Major freshwater inputs located in the Northern region led to differences in both phytoplankton and bacterioplankton biomass and activity along the estuary. While in the Northern region phytoplankton is light-limited, with mean phytoplankton production (PP) between 1.1 and 1.9 μg C l−1 h−1 and mean specific growth rates (PSG) between 0.14 and 0.16 d−1, the Southern region registered values as high as 24.7 μg C l−1 h−1 for PP and 2.45 d−1 (mean PP between 3.4 and 7.3 μg C l−1 h−1; mean PSG between 0.28 and 0.57 d−1). On the other hand, maximum bacterial production (BP: 63.8 μg C l−1 h−1) and specific growth rate (BSG: 32.26 d−1) were observed in the Northern region (mean BP between 3.4 and 12.8 μg C l−1 h−1; mean BSG between 1.98 and 6.67 day−1). These bacterial activity rates are among the highest recorded rates in estuarine and coastal waters, indicating that this system can be highly heterotrophic, due to high loads of allochthonous carbon (mainly derived from mangrove forest). Our results also showed that, despite that BP rates usually exceeded PP, in the Southern region BP may be partially supported (∼45%) by PP, since a significant regression was observed between BP and PP ( r = 0.455, P < 0.001). [ABSTRACT FROM AUTHOR]

Seasonal and spatial variations in bacterial abundance, biomass and production in a recently flooded reservoir were followed for 2 consecutive years, in conjunction with phytoplankton biomass (chlorophyll a) and activity (primary production). Between the 2 years of the study, the mean value of primary production remained constant, while those of the chlorophyll a concentration, bacterial abundance (BA), bacterial biomass (BB) and bacterial production (BP) decreased. The observed trends of the bacterial variables were linked to changes in the relative importance of allochthonous dissolved organic matter. Moreover, this factor would explain discrepancies observed between the slope of the model II regression equations established from results of the present study and those of the predictive models from the literature, relating to bacterial and phytoplankton variables. An estimate of the carbon budget indicated that 22 and 5% of the ambient primary production in the Sep Reservoir might be channeled through the microbial loop via BP during the 1st and 2nd year of the study, respectively. We conclude that heterotrophic BP in the Sep Reservoir may, on occasion, represent a significant source of carbon for higher order consumers. [ABSTRACT FROM AUTHOR]

Abstract: Heterotrophic bacterial standing stocks (total and viable cells) and production were determined in the coastal surface waters of the eastern English Channel, during different stages of a phytoplankton succession. Two coastal zones of variable freshwater influence were surveyed within the ‘coastal flow system’ (Wimereux and Somme Bay) where massive and recurrent Phaeocystis globosa blooms take place in spring. The proportion of intact (MEM+) cells, assessed by the LIVE/DEAD® BacLight™ (L/D) method, varied from 15 to 94% at the two coastal stations studied (median of 46%). MEM+ and total (DAPI) cell counts were significantly correlated over the study period, whereas the higher proportion of MEM+ cells did not correspond to an elevated bacterial cell production (BP). Low levels of living (potentially active) cells were nevertheless responsible for the high productivity levels within the bacterial community when the P. globosa bloom declined. Our study revealed that the bacterial carbon production/primary production ratios (BCP/PP) showed broad variations (7 to 111%) within each site, going from low values (7–16%) when the bloom was the most productive, to higher values (61–111%) at the end of the bloom. This suggested (i) a temporal uncoupling between bacteria and phytoplankton throughout the bloom duration and (ii) a drastic change of the amount of PP potentially processed by the bacterial community among high and low productive periods. The BCP increase after the decline of the P. globosa bloom implies that, at this time, a large part of the phytoplankton-derived organic matter (OM) was remineralised via the bacterial heterotrophic production. With respect to the L/D results, this bacterial remineralisation was due to a small yet productive total cell fraction. [Copyright &y& Elsevier]

Abstract: In the summers of 2000, 2001 and 2002, large amounts of sticky mucilaginous material aggregated to form masses of impressive dimensions over large areas of the Adriatic Sea, particularly in its northern part. Aggregates differing in size were sampled by SCUBA divers and submitted to chemical (nutrient and organic matter concentrations) and biological analysis (virus, bacteria and phytoplankton abundances and bacterial metabolism). Suspended and sinking mucilaginous aggregates were biota-rich environments where the abundance of planktonic organisms and the concentration of nutrients were orders of magnitude higher than in the surrounding seawater. The embedded phytoplankton was mostly composed of diatoms, but the dinoflagellate Gonyaulax fragilis, previously reported in association with the aggregates, was also present. A variety of processes occurred within the mucilaginous aggregates which resulted in the transformation of the organic matter composition and lability and contributed to a partial degradation of mucilage. For an efficient degradation of mucilage, several conditions are necessary: high bacterial abundance and activity and an efficient recycling of nutrients within the aggregates. Most of these conditions, appear to change depending on the type and age of the aggregate. During the first phase of aggregation (cobwebs and ribbons), bacterial activities addressed the degradation of organic matter, particularly that of the nitrogen fraction. The degradation products were rapidly taken up by bacteria, supporting an increase in their abundance and production. In aged mucilage (clouds), the degradation processes decreased and the bacterial metabolism suggested the presence of new organic labile compounds probably due to phytoplankton production. On the basis of our results, stringers, generally considered the first step of the aggregation process, seemed to be the result of a mechanical disruption of other types of aggregates. [Copyright &y& Elsevier]

Abstract: Three years of regular weekly/biweekly monitoring of seasonal changes in temperature, transparency, chlorophyll a (CHL) and bacteria [erythrosine-stained microscopic counts and cultivable colony forming units (CFUs)] at the vertical profile in the South basin of Lake Baikal (51°54′195″N, 105°04′235″E, depth 800 m) were evaluated. In more detail, the structure and function of phytoplankton and the microbial loop in the euphotic layer at the same site were investigated during the late-winter–early-spring period under the ice. The depth of euphotic zone (up to 1% of surface irradiation) was 35 to 40 m. Primary production was measured three times a week with the 14C method in 2, 10, 20, 30 and 40 m. Maximum production was found in 10 m, with lower values towards the surface (light inhibition) and towards the lower layers. The total production in cells larger than 1 μm in the column (0–40 m) was 204–240 mg C d−1 m−2, 30–40% of it being in cells 1–3 μm (mostly picocyanobacteria), which represented roughly 9% of the total chlorophyll a (estimated from pigment analyses). A major part of phytoplankton biomass was formed by diatoms (Synedra acus Hust., Asterionella formosa Hass. and Stephanodiscus meyerii Genkal & Popovskaya). Total production (including extracellular, dissolved organic matter) was 235–387 mg C day−1 m−2, and the exudates were readily used by bacteria (particles 0.2–1 μm). This part amounted to 1–5% of cellular production in 2 to 20 m and 11–77% of cellular production in 20–40 m, i.e., in light-limited layers. From 0 to 30 m, chlorophyll a concentration was 0.8 to 1.3 μg l−1, wherefrom it decreased rapidly to 0.1 μg l−1 towards the depth of 40 m. Bacteria (DAPI-stained microscopic counts) reached 0.5–1.4×106 ml−1; their cell volumes measured via image analysis were small (average 0.05 μm−3), often not well countable when erythrosine stain was used. Bacterial biomasses were in the range of 6–21 μg C l−1. Numbers of colony forming units (CFUs) on nutrient fish-agar were c. 3–4 orders lower than DAPI counts. The amounts of heterotrophic protists were low, whereby flagellates reached 6 to 87 ml−1 and ciliates, 0.2–1.2 ml−1 (mostly Oligotrichida). Bacterial production was measured in the same depths as primary production using 3H-thymidine (Thy) and 14C-leucine (Leu) uptake. Consistently, bacterial abundances, biomasses, thymidine and leucine production were higher by 30–50% in layers 2, 10 and 20 m compared with that in the deeper 30 and 40 m, where cellular primary production was negligible. Leucine uptake in the deeper layers was even three times lower than in the upper ones. From the comparison of primary and bacterial production, bacteria roughly use 20–40% of primary production during 24 h in the layers 2 to 20 m. [Copyright &y& Elsevier]

Rivers are regarded as important sites for processing of dissolved organic matter (DOM) from terrestrial sources on its way to the ocean. However, little is known about the longitudinal change of DOM molecular composition in large rivers. Here we performed a Lagrangian sampling in the lower part of the Middle Elbe at low discharge conditions to test how DOM composition changes along the river stretch and how this is related to microbial processes. The concentration of dissolved organic carbon and fluorescence indices showed only subtle longitudinal differences. In contrast, ultra-high-resolution mass spectrometry analysis of riverine DOM detected pronounced changes in molecular composition. Also, chlorophyll a concentration, bacterial abundance, and bacterial production all increased downstream. The three microbial parameters were positively related to intensities of CHO and CHNO molecular formulas with high hydrogen/carbon and low oxygen/carbon ratios but negatively to several CHOS surfactants. To disentangle the role of autotrophic and heterotrophic processes, we developed a new approach and compared slopes from linear regression of DOM compound intensities versus chlorophyll a concentration and bacteria abundance. As a result, most of the positive related DOM compounds were produced by bacteria. In conclusion, longitudinal changes of river DOM seemed to be largely driven by microbial processes. [ABSTRACT FROM AUTHOR]