Bacterial Carbon Demand Research Articles

Effects of microbial processes and CaCO3 dynamics on inorganic carbon cycling in snow-covered Arctic winter sea ice

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 611:31-44 (2019) - DOI: https://doi.org/10.3354/meps12868 Effects of microbial processes and CaCO3 dynamics on inorganic carbon cycling in snow-covered Arctic winter sea ice Dorte Haubjerg Søgaard1,2,*, Jody W. Deming3, Lorenz Meire1,4, Søren Rysgaard1,2,5 1Greenland Climate Research Centre (C/O Greenland Institute of Natural Resources), Kivioq 2, PO Box 570, 3900 Nuuk, Greenland 2Arctic Research Centre, Department of Bioscience, Aarhus University, Ny Munkegade 116, building 1540, 8000 Aarhus C, Denmark 3School of Oceanography, University of Washington, Seattle, WA 98195, USA 4Royal Netherlands Institute for Sea Research, and Utrecht University, 4401 NT Yerseke, Netherlands 5Centre for Earth Observation Science, CHR Faculty of Environment Earth and Resources, University of Manitoba, 499 Wallace Building, Winnipeg, MB R3T 2N2, Canada *Corresponding author: doso@natur.gl ABSTRACT: Few combined measurements of primary and bacterial productivity exist for Arctic sea ice, particularly during winter, making it difficult to assess the relative importance of these microbial processes for carbon cycling in sea ice. Furthermore, the occurrence of calcium carbonate (CaCO3), though well-documented in sea ice, is poorly described for the overlying snow. To address these gaps, we investigated primary and bacterial productivity and carbon dynamics at 2 contrasting locations: (1) a landfast site, with thick snow-covered first-year sea ice, and (2) a polynya site, with thin snow-covered young (<1 wk) sea ice. Comparisons of bacterial carbon demand and primary production indicated net heterotrophy in the sea ice at both locations, with a net carbon consumption rate of 0.87 to 1.86 mg C m-2 d-1 derived from sea ice bacterial carbon demand of 0.93 to 2.00 mg C m-2 d-1 and gross primary production of 0.06 to 0.14 mg C m-2 d-1. As these microbial rates are very low, physical processes largely account for the observed CO2 depletion in the ice. High CaCO3 concentrations of 250 to 430 µmol kg-1 were measured in the snow covers which, though similar to concentrations in the underlying ice, are orders of magnitude higher than those reported from the few studies available on CaCO3 in snow. Together these results suggest that the role of biology in modulating inorganic carbon cycling in ice, which can be important in spring, is minor as compared to abiotic processes. KEY WORDS: High Arctic · First-year sea ice · Polynya ice · Snow cover · Primary production · Bacterial production · CaCO3 Full text in pdf format Supplementary material PreviousNextCite this article as: Søgaard DH, Deming JW, Meire L, Rysgaard S (2019) Effects of microbial processes and CaCO3 dynamics on inorganic carbon cycling in snow-covered Arctic winter sea ice. Mar Ecol Prog Ser 611:31-44. https://doi.org/10.3354/meps12868 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 611. Online publication date: February 14, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

The composition and distribution of semi-labile dissolved organic matter across the southwest Pacific

Abstract. The distribution and dynamics of dissolved organic carbon (DOC) and dissolved combined neutral sugars (DCNS) were studied across an increasing oligotrophic gradient (18 to 22∘ S latitude) in the tropical South Pacific Ocean, spanning from the Melanesian Archipelago (MA) area to the western part of the South Pacific gyre (WGY), in austral summer as a part of the OUTPACE project. Our results show that DOC and DCNS concentrations exhibited no statistical differences between the MA and WGY areas (0–200 m: 47–81 µM C for DOC and 0.2-4.2 µM C for DCNS). However, due to a deepening of the euphotic zone, a deeper penetration of DOC was noticeable at 150 m of depth at the WGY area. Excess DOC (DOCEX) was determined as the difference between surface and deep-sea DOC values, and euphotic zone integrated stocks of both DOC and DOCEX were higher in the WGY than the MA area. Considering DOCEX as representative of semi-labile DOC (DOCSL), its residence time was calculated as the ratio of DOCSL to bacterial carbon demand (BCD). This residence time was 176±43 days (n=3) in the WGY area, about 3 times longer than in the MA area (Tr=51±13 days, n=8), suggesting an accumulation of semi-labile dissolved organic matter (DOM) in the surface waters of WGY. Average epipelagic (0–200 m) DCNS yields (DCNS × DOC−1) based on volumetric data were roughly similar in both areas, accounting for ∼2.8 % of DOC. DCNS exhibited a longer residence time in WGY (Tr=91±41 days, n=3) than in MA (Tr=31±10 days, n=8), further suggesting that this DCNS pool persists longer in the surface waters of the WGY. The accumulation of DOCEX in the surface waters of WGY is probably due to very slow bacterial degradation due to nutrient and/or energy limitation of heterotrophic prokaryotes, indicating that biologically produced DOC can be stored in the euphotic layer of the South Pacific gyre for a long period.

Assessment of bacterial dependence on marine primary production along a northern latitudinal gradient.

Recent observations in polar marine waters have shown that a large fraction of primary production may be lost to respiration by planktonic bacteria due to very low bacterial growth efficiencies in cold waters. Here we report that sea temperature may be a key factor (but not the only one) influencing the interaction between bacteria and primary production in North Atlantic and Arctic waters, suggesting that low primary production rates could not sustain bacterial carbon demand in the coldest Arctic waters. The use of freshly produced phytoplankton exudate by bacteria in early- and mid-summer was assessed, together with the bacterial uptake of dissolved inorganic nitrogen (DIN=nitrate and ammonium), in surface waters along a latitudinal gradient from the North Sea to the Arctic sea ice. Bacterial production was independent of the low primary production measured in the coldest waters. Under these conditions, heterotrophic bacteria can consume a large fraction of DIN and N-rich organic matter, making them strong contributors to N fluxes in these waters.

Bacterioplankton drawdown of coral mass-spawned organic matter

Coral reef ecosystems are highly sensitive to microbial activities that result from dissolved organic matter (DOM) enrichment of their surrounding seawater. However, the response to particulate organic matter (POM) enrichment is less studied. In a microcosm experiment, we tested the response of bacterioplankton to a pulse of POM from the mass-spawning of Orbicella franksi coral off the Caribbean coast of Panama. Particulate organic carbon (POC), a proxy measurement for POM, increased by 40-fold in seawater samples collected during spawning; 68% degraded within 66 h. The elevation of multiple hydrolases presumably solubilized the spawn-derived POM into DOM. A carbon budget constructed for the 275 µM of degraded POC showed negligible change to the concentration of dissolved organic carbon (DOC), indicating that the DOM was readily utilized. Fourier transform ion cyclotron resonance mass spectrometry shows that the DOM pool became enriched with heteroatom-containing molecules, a trend that suggests microbial alteration of organic matter. Our sensitivity analysis demonstrates that bacterial carbon demand could have accounted for a large proportion of the POC degradation. Further, using bromodeoxyuridine immunocapture in combination with 454 pyrosequencing of the 16S ribosomal RNA gene, we surmise that actively growing bacterial groups were the primary degraders. We conclude that coral gametes are highly labile to bacteria and that such large capacity for bacterial degradation and alteration of organic matter has implications for coral reef health and coastal marine biogeochemistry.

Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean: links with diazotrophic and photosynthetic activity

Abstract. Heterotrophic prokaryotic production (BP) was studied in the western tropical South Pacific (WTSP) using the leucine technique, revealing spatial and temporal variability within the region. Integrated over the euphotic zone, BP ranged from 58 to 120 mg C m−2 d−1 within the Melanesian Archipelago, and from 31 to 50 mg C m−2 d−1 within the western subtropical gyre. The collapse of a bloom was followed during 6 days in the south of Vanuatu using a Lagrangian sampling strategy. During this period, rapid evolution was observed in the three main parameters influencing the metabolic state: BP, primary production (PP) and bacterial growth efficiency. With N2 fixation being one of the most important fluxes fueling new production, we explored relationships between BP, PP and N2 fixation rates over the WTSP. The contribution of N2 fixation rates to bacterial nitrogen demand ranged from 3 to 81 %. BP variability was better explained by the variability of N2 fixation rates than by that of PP in surface waters of the Melanesian Archipelago, which were characterized by N-depleted layers and low DIP turnover times (TDIP &lt; 100 h). This is consistent with the fact that nitrogen was often one of the main factors controlling BP on short timescales, as shown using enrichment experiments, followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. However, BP was more significantly correlated with PP, but not with N2 fixation rates where DIP was more available (TDIP &gt; 100 h), deeper in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. The bacterial carbon demand to gross primary production ratio ranged from 0.75 to 3.1. These values are discussed in the framework of various assumptions and conversion factors used to estimate this ratio, including the methodological errors, the daily variability of BP, the bacterial growth efficiency and one bias so far not considered: the ability for Prochlorococcus to assimilate leucine in the dark.

Effects of increasing atmospheric CO2 on the marine phytoplankton and bacterial metabolism during a bloom: A coastal mesocosm study

Increases of atmospheric CO2 concentrations due to human activity and associated effects on aquatic ecosystems are recognized as an environmental issue at a global scale. Growing attention is being paid to CO2 enrichment effects under multiple stresses or fluctuating environmental conditions in order to extrapolate from laboratory-scale experiments to natural systems. We carried out a mesocosm experiment in coastal water with an assemblage of three model phytoplankton species and their associated bacteria under the influence of elevated CO2 concentrations. Net community production and the metabolic characteristics of the phytoplankton and bacteria were monitored to elucidate how these organisms responded to CO2 enrichment during the course of the algal bloom. We found that CO2 enrichment (1000μatm) significantly enhanced gross primary production and the ratio of photosynthesis to chlorophyll a by approximately 38% and 39%, respectively, during the early stationary phase of the algal bloom. Although there were few effects on bulk bacterial production, a significant decrease of bulk bacterial respiration (up to 31%) at elevated CO2 resulted in an increase of bacterial growth efficiency. The implication is that an elevation of CO2 concentrations leads to a reduction of bacterial carbon demand and enhances carbon transfer efficiency through the microbial loop, with a greater proportion of fixed carbon being allocated to bacterial biomass and less being lost as CO2. The contemporaneous responses of phytoplankton and bacterial metabolism to CO2 enrichment increased net community production by about 45%, an increase that would have profound implications for the carbon cycle in coastal marine ecosystems.

Plankton community respiration and bacterial metabolism in a North Atlantic Shelf Sea during spring bloom development (April 2015)

Spring phytoplankton blooms are important events in Shelf Sea pelagic systems as the increase in carbon production results in increased food availability for higher trophic levels and the export of carbon to deeper waters and the sea-floor. It is usually accepted that the increase in phytoplankton abundance and production is followed by an increase in plankton respiration. However, this expectation is derived from field studies with a low temporal sampling resolution (5–15 days). In this study we have measured the time course of plankton abundance, gross primary production, plankton community respiration, respiration of the plankton size classes (>0.8 µm and 0.2–0.8 µm) and bacterial production at ≤5 day intervals during April 2015 in order to examine the phasing of plankton autotrophic and heterotrophic processes. Euphotic depth-integrated plankton community respiration increased five-fold (from 22 ± 4 mmol O2 m−2 d−1 on 4th April to 119 ± 4 mmol O2 m−2 d−1 on 15th April) at the same time as gross primary production also increased five-fold, (from 114 ± 5 to 613 ± 28 mmol C m−2 d−1). Bacterial production began to increase during the development of the bloom, but did not reach its maximum until 5 days after the peak in primary production and plankton respiration. The increase in plankton community respiration was driven by an increase in the respiration attributable to the >0.8 µm size fraction of the plankton community (which would include phytoplankton, microzooplankton and particle attached bacteria). Euphotic depth-integrated respiration of the 0.2–0.8 µm size fraction (predominantly free living bacteria) decreased and then remained relatively constant (16 ± 3 – 11 ± 1 mmol O2 m−2 d−1) between the first day of sampling (4th April) and the days following the peak in chlorophyll-a (20th and 25th April). Recent locally synthesized organic carbon was more than sufficient to fulfil the bacterial carbon requirement in the euphotic zone during this productive period. Changes in bacterial growth efficiencies (BGE, the ratio of bacterial production to bacterial carbon demand) were driven by changes in bacterial production rates increasing from <30 ± 14% on 4th April to 51 ± 11% on 25th of April. This study therefore shows a concurrent rather than a phased increase in primary production and community respiration attributable to cells >0.8 µm during the development of the spring bloom, followed 5 days later by a peak in bacterial production. In addition, the size fractionated respiration rates and high growth efficiencies suggest that free living bacteria are not the major producers of CO2 before, during and a few days after this shelf sea spring phytoplankton bloom.

High bacterial carbon demand and low growth efficiency at a tropical hypereutrophic estuary: importance of dissolved organic matter remineralization

Abstract Simultaneous measurements of bacterioplankton production (BP) and respiration (BR) are imperative to understand the magnitude of carbon cycle in the lower trophic levels of the aquatic systems, but are still scarce in the tropics. The present study was performed in a highly productive estuary (Recife harbor, 08°03'S; 34°52'W, NE Brazil) where bacterial carbon demand (BCD=BP+BR) and growth efficiency (BGE=BP/BCD) were evaluated in order to estimate the major role of bacterioplankton: source or sink of organic carbon. In spite of the high BP rates (0.03-0.4 µMC h-1), the extremely high BR rates (0.5-4.1 µMC h-1) led to low BGE (0.02-0.29), possibly due to the high temperatures (&gt;25ºC) and strong inorganic nitrogen limitation (N:P ratios) The high BCD and low BGE indicate the major role of bacterioplankton as dissolved organic matter remineralizers, fueling the primary productivity of the system. These findings contradict what could be expected from studies in highly productive temperate estuaries (where BGE is usually &gt; 0.30) and highlight the importance of increasing in situ BP and BR estimates in tropical estuarine systems in order to better understand the role of these systems in global carbon cycling.

The Response of Heterotrophic Prokaryote and Viral Communities to Labile Organic Carbon Inputs Is Controlled by the Predator Food Chain Structure.

Factors controlling the community composition of marine heterotrophic prokaryotes include organic-C, mineral nutrients, predation, and viral lysis. Two mesocosm experiments, performed at an Arctic location and bottom-up manipulated with organic-C, had very different results in community composition for both prokaryotes and viruses. Previously, we showed how a simple mathematical model could reproduce food web level dynamics observed in these mesocosms, demonstrating strong top-down control through the predator chain from copepods via ciliates and heterotrophic nanoflagellates. Here, we use a steady-state analysis to connect ciliate biomass to bacterial carbon demand. This gives a coupling of top-down and bottom-up factors whereby low initial densities of ciliates are associated with mineral nutrient-limited heterotrophic prokaryotes that do not respond to external supply of labile organic-C. In contrast, high initial densities of ciliates give carbon-limited growth and high responsiveness to organic-C. The differences observed in ciliate abundance, and in prokaryote abundance and community composition in the two experiments were in accordance with these predictions. Responsiveness in the viral community followed a pattern similar to that of prokaryotes. Our study provides a unique link between the structure of the predator chain in the microbial food web and viral abundance and diversity.

Carbon Bioavailability in a High Arctic Fjord Influenced by Glacial Meltwater, NE Greenland

The land-to-ocean flux of organic carbon is increasing in glacierized regions in response to increasing temperatures in the Arctic (Hood et al., 2015). In order to understand the response in ecosystem metabolism to the organic carbon input it is essential to determine the bioavailability of the different carbon sources in the system. We quantified the bacterial turnover of organic carbon in a high Arctic fjord system (Young Sound, NE Greenland) during the ice-free period (July-October 2014) and assessed the quality and quantity of the 3 major organic carbon sources; 1) local phytoplankton production 2) runoff from land-terminating glaciers and a lowland river and 3) inflow from the ocean shelf. We found that despite relatively low concentrations of DOC in the rivers, the bioavailability of the river–DOC was significantly higher than in the fjord, and characterized by high cell-specific bacterial production and low C:N ratios. In contrast, the allochthonous DOC source entering via inflow of coastal shelf waters had elevated DOC concentrations with low bioavailability, high C:N and low specific bacterial production. The phytoplankton production in the fjord could not sustain the bacterial carbon demand, but was still the major source of organic carbon for bacterial growth. We assessed the bacterial community composition and found that communities were specific for the different water types i.e. the bacterial community of the coastal coastal inflow water could be traced mainly in the subsurface water, while the glacial river community strongly dominated the surface water in the fjord.

CO2 Saturation and Trophic Shift Induced by Microbial Metabolic Processes in a River-Dominated Ocean Margin (Tropical Shallow Lagoon, Chilika, India)

ABSTRACTAn effort has been made for the first time in Asia's largest brackish water lagoon, Chilika, to investigate the spatio-temporal variability in primary productivity (PP), bacterial productivity (BP), bacterial abundance (BA), bacterial respiration (BR) and bacterial growth efficiency (BGE) in relation to partial pressure of CO2 (pCO2) and CO2 air–water flux and the resultant trophic switchover. Annually, PP ranged between 24 and 376 µg C L−1 d−1 with significantly low values throughout the monsoon (MN), caused by light limitation due to inputs of riverine suspended matter. On the contrary, BP and BR ranged from 11.5 to 186.3 µg C L−1 d−1 and from 14.1 to 389.4 µg C L−1 d−1, respectively, with exceptionally higher values during MN. A wide spatial and temporal variation in the lagoon trophic status was apparent from BP/PP (0.05–6.4) and PP/BR (0.10–18.2) ratios. The seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter inputs via rivers, enhanced the bacterial metabolism during the MN, as evident from the high pCO2 (10,134 µatm) and CO2 air–water flux (714 mm m−2 d−1). Large variability in BGE and BP/PP ratios especially during MN led to high bacteria-mediated carbon fluxes which was evident from significantly high bacterial carbon demand (BCD >100% of PP) during this season. This suggested that the net amount of organic carbon (either dissolved or particulate form) synthesized by primary producers in the lagoon was not sufficient to satisfy the bacterial carbon requirements. Lagoon sustained low to moderate autotrophic–heterotrophic coupling with annual mean BCD of 231% relative to the primary production, which depicted that bacterioplankton are the mainstay of the lagoon biogeochemical cycles and principal players that bring changes in trophic status. Study disclosed that the high CO2 supersaturation and oxygen undersaturation during MN was attributed to the increased heterotrophic respiration (in excess of PP) fuelled by allochthonous organic matter. On a spatial scale, lagoon sectors such as south sector, central sector and outer channel recorded “net autotrophic,” while the northern sector showed “net heterotrophic” throughout the study period.

Pelagic microbial heterotrophy in response to a highly productive bloom of Phaeocystis antarctica in the Amundsen Sea Polynya, Antarctica

Abstract Heterotrophic bacteria play a key role in marine carbon cycling, and understanding their activities in polar systems is important for considering climate change impacts there. One goal of the ASPIRE project was to examine the relationship between the phytoplankton bloom and bacterial heterotrophy in the Amundsen Sea Polynya (ASP). Bacterial abundance, production (BP), respiration, growth efficiency, and extracellular enzyme activity (EEA) were compared to nutrient and organic matter inventories, chlorophyll a (Chl a), viral and microzooplankton abundance, and net primary production (NPP). Bacterial production and respiration clearly responded (0.04–4.0 and 10–53 µg C L−1 d−1, respectively) to the buildup of a massive Phaeocystis antarctica bloom (Chl a: 0.2–22 µg L−1), with highest rates observed in the central polynya where Chl a and particulate organic carbon (POC) were greatest. The highest BP rates exceeded those reported for the Ross Sea or any other Antarctic coastal system, yet the BP:NPP ratio (2.1–9.4%) was relatively low. Bacterial respiration was also high, and growth efficiency (2–27%; median = 10%) was similar to oligotrophic systems. Thus, the integrated bacterial carbon demand (0.8–2.8 g C m−2 d−1) was a high fraction (25–128%; median = 43%) of NPP during bloom development. During peak bloom, activity was particle-associated: BP and EEA correlated well with POC, and size fractionation experiments showed that the larger size fraction (&amp;gt; 3 µm) accounted for a majority (∼ 75%) of the BP. The community was psychrophilic, with a 5x reduction in BP when warmed to 20°C. In deeper waters, respiration remained relatively high, likely fueled by the significant downward particle flux in the region. A highly active, particle-associated, heterotrophic microbial community clearly responded to the extraordinary phytoplankton bloom in the ASP, likely limiting biological pump efficiency during the early season.

Changes in the phytoplankton-bacteria coupling triggered by joint action of UVR, nutrients, and warming in Mediterranean high-mountain lakes

From an extensive study, we determined that heterotrophic bacterial production (HBP) variance in Sierra Nevada (Spain) lakes was explained mainly by excretion of organic carbon by algae (EOC), underlining a bacterial dependence on algal carbon. Subsequently, we studied how the interaction among global change factors such as ultraviolet radiation (UVR), nutrient inputs, and increased temperature affected this phytoplankton-bacteria coupling through in situ factorial experiments in two model high-mountain lakes, La Caldera, and Las Yeguas. Bacterioplankton were more sensitive than phytoplankton because the joint action of increased temperature and nutrient-addition unmasked an inhibitory UVR effect on HBP while reducing the inhibitory UVR effect on primary production (PP) (in La Caldera) or augmenting the net PP values (in Las Yeguas). The interaction among the three factors had a different effect on phytoplankton-bacteria coupling depending on the lake. Thus, in the colder lake (La Caldera), EOC was not adequate to meet the bacterial carbon demand (BCD), leading to a mismatch in phytoplankton-bacteria coupling. Contrarily, in the warmer lake (Las Yeguas), the phytoplankton-bacteria coupling was accentuated by the interaction among the three factors, with EOC exceeding BCD. These contrasting responses of phytoplankton-bacteria coupling may affect the microbial loop development, becoming reinforced in warmer and less UVR-transparent high-mountain lakes, but weakened in colder and more UVR-transparent high-mountain lakes, with implications in the C-flux of these sentinel ecosystems in a scenario of global change.

Hydrological pulse regulating the bacterial heterotrophic metabolism between Amazonian mainstems and floodplain lakes.

We evaluated in situ rates of bacterial carbon processing in Amazonian floodplain lakes and mainstems, during both high water (HW) and low water (LW) phases (p < 0.05). Our results showed that bacterial production (BP) was lower and more variable than bacterial respiration, determined as total respiration. Bacterial carbon demand was mostly accounted by BR and presented the same pattern that BR in both water phases. Bacterial growth efficiency (BGE) showed a wide range (0.2–23%) and low mean value of 3 and 6%, (in HW and LW, respectively) suggesting that dissolved organic carbon was mostly allocated to catabolic metabolism. However, BGE was regulated by BP in LW phase. Consequently, changes in BGE showed the same pattern that BP. In addition, the hydrological pulse effects on mainstems and floodplains lakes connectivity were found for BP and BGE in LW. Multiple correlation analyses revealed that indexes of organic matter (OM) quality (chlorophyll-a, N stable isotopes and C/N ratios) were the strongest seasonal drivers of bacterial carbon metabolism. Our work indicated that: (i) the bacterial metabolism was mostly driven by respiration in Amazonian aquatic ecosystems resulting in low BGE in either high or LW phase; (ii) the hydrological pulse regulated the bacterial heterotrophic metabolism between Amazonian mainstems and floodplain lakes mostly driven by OM quality.

Significant contribution of lytic mortality to bacterial production and DOC cycles in Funka Bay, Japan

A modified dilution experiment was conducted to evaluate the relative contribution of viral lysis and protozoan grazing to the mortalities of heterotrophic bacteria in Funka Bay, a subarctic coastal bay. The experiment included the stepwise dilution of the original seawater with virus-free seawater (10 kDa ultrafiltered) to change the encounter rate of both virus and protozoa to heterotrophic bacteria, incubation for 48 h and monitoring the change in the abundance of heterotrophic bacteria. In a parallel experiment, the original seawater was replaced by 1.0 µm fractionated seawater to eliminate protozoa, and the same dilution was conducted with the virus-free seawater to estimate only lytic mortality. The viral lysis and protozoan grazing rates in the surface water ranged from 0.40 to 1.19 and 0.08 to 0.27 days−1, respectively. Viral lysis was the main cause for the bacterial mortality (79.8 ± 3.2 %). The net (in situ) growth rate of heterotrophic bacteria was about 0.15 days−1. In the bottom water (90 m), both mortalities were lower than those at the surface and the net growth rate was mostly a negative value. The contribution of released dissolved organic matter (DOM) through lysis to the bacterial carbon demand (BCD) was evaluated. The lysed bacterial cells might release DOM to the ambient environment, in which bacterial organic matter is recycled in the subsequent bacterial production. The potential contribution was estimated to range from 25 to 27 % in the surface water and to be 31 % in the bottom water, suggesting that the lytic mortality significantly fueled DOM to the subsequent bacterial production.

Bacterial growth efficiency in a partly eutrophicated bay of South China Sea: Implication for anthropogenic impacts and potential hypoxia events.

Bacterial metabolism plays a dual role [bacterial production (BP) and bacterial respiration (BR)] in the aquatic ecosystem and potentially leads to hypoxia in the coastal eutrophic area. Bacterial growth efficiency (BGE) is an important index showing the contribution of bacterial metabolism to marine biological production and carbon budget in the pelagic ecosystem. In this study, the spatial and seasonal variety as well as diurnal variation dynamics of BGE and associated ecological characteristics were investigated in a partly eutrophicated subtropical bay (the Daya Bay) located in the northern South China Sea. Furthermore, the relationship between bacterial metabolism and potential hypoxia event was analyzed. The average BGE was 0.14 and 0.22 in summer and winter, respectively, which was lower than the mean value ever reported in other coastal and estuarine waters. The diurnal variations of BGE and BP were widely fluctuated in the Daya Bay, with approximately 3-8 fold variation of BP and 2-3 fold variation of BR in different seasons, suggesting the importance of short-term ecological dynamics on evaluating the long-term ecological processes in the coastal waters. BR was the predominant contributor to the bacterial carbon demand; however, the variation of BGE was controlled by BP in both seasons. BGE was always high in the near-shore waters with higher eutrophic level and more active BP and BR. The bacterial metabolism could deplete dissolved oxygen (DO) in the Daya bay within about 9 days when the water body was enclosed and photosynthesis was prohibited. Therefore, low DO concentration and potential hypoxia was more likely to be found in the near-shore waters of the Daya Bay in summer, since the water was stratified and enclosed with poor water exchange capacity in this area. While in winter, hypoxia seldom occurred due to vertical mixing throughout the water column. Further biological-physical coupling research is recommended to find out the detailed formation mechanism of hypoxia in the bay, and to predict the potential hypoxia events and their environmental impacts in the future.

Local differences in phytoplankton-bacterioplankton coupling in the coastal upwelling off Galicia (NW Spain)

We simultaneously studied microbial-mediated carbon fluxes at 2 contrasting sites within the coastal upwelling system off Galicia, near A Coruna and Vigo, Spain, over an annual cycle in order to compare the fraction of primary production released as dissolved organic carbon (DOC) and the degree of coupling between bacteria and phytoplankton. A significant fraction of primary production was released as DOC at both sites, averaging ~30%. DOC release ac counted for, on average, 30% of the total bacterial carbon demand, which is indicative of a moderate trophic dependence of bacteria on phytoplankton in these coastal ecosystems. Nevertheless, dif- ferences in hydrographic conditions associated with stronger upwelling pulses off Vigo, and deeper upper mixed layers during the downwelling period off A Coruna, led to significant differ- ences in phytoplankton dynamics and the subsequent direct coupling with heterotrophic bacteria. Strong direct coupling between phytoplankton extracellular release and bacterial production (BP) was found off Vigo, which could be related to the quality of the DOC produced by actively grow- ing phytoplankton. By contrast, DOC release and BP rates were decoupled off A Coruna, likely due to unaccounted DOC associated with indirect trophic processes or to the low availability of freshly produced exudates associated with overflow or photoinhibition mechanisms.

Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

Abstract. An indirect effect of global warming is a reduction in the depth of the upper mixed layer (UML) causing organisms to be exposed to higher levels of ultraviolet (UVR, 280–400 nm) and photosynthetically active radiation (PAR, 400–700 nm). This can affect primary and bacterial production as well as the commensalistic phytoplankton–bacteria relationship. The combined effects of UVR and reduction in the depth of the UML were assessed on variables related to the metabolism of phytoplankton and bacteria, during in situ experiments performed with natural pico- and nanoplankton communities from two oligotrophic lakes with contrasting UVR transparency (high-UVR versus low-UVR waters) of southern Spain. The negative UVR effects on epilimnetic primary production (PP) and on heterotrophic bacterial production (HBP), intensified under increased stratification, were higher in the low-UVR than in the high-UVR lake, and stronger on the phytoplanktonic than on the heterotrophic bacterial communities. Under UVR and increased stratification, the commensalistic phytoplankton–bacteria relationship was strengthened in the high-UVR lake where excretion of organic carbon (EOC) rates exceeded the bacterial carbon demand (BCD; i.e., BCD : EOC(%) ratio &lt; 100). This did not occur in the low-UVR lake (i.e., BCD : EOC(%) ratio &gt; 100). The greater UVR damage to phytoplankton and bacteria and the weakening of their commensalistic interaction found in the low-UVR lake indicates that these ecosystems would be especially vulnerable to UVR and increased stratification as stressors related to global climate change. Thus, our findings may have important implications for the carbon cycle in oligotrophic lakes of the Mediterranean region.

Primary production under hypertrophic conditions and its relationship with bacterial production

In shallow hypertrophic lakes where light availability restricts the growth of macrophytes and benthic phytoplankton, pelagic phytoplankton modulates importantly ecosystem production and the energy transfer to heterotrophic bacteria. Diel and seasonal variations in primary production (PP) were studied in the hypertrophic Albufera de Valencia (Spain). Additionally, the relationship between PP and heterotrophic bacterial production (BP) was assessed. PP was extremely high, exceeding most values reported for hypertrophic lakes to date. PP displayed marked diurnal variations defined by the solar radiation curve. Likewise, PP changed importantly across seasons. Minimum PP coincided with maximum water transparency and short water residence times in winter, whereas maximum PP was observed in late spring associated with high chlorophyll a. The spring PP maximum contrasted with the summer maximum often observed in hypertrophic lakes. When compared to spring PP values, summer PP values were lower as a result of strong nitrogen limitation. In contrast to PP, BP remained fairly constant across seasons. Nonetheless, there was a joint diminution during increased water transparency followed by an increase in early spring. Phytoplankton was always the most relevant input to particulate carbon production, but the BP/PP ratio showed clear seasonal variations. The BP/PP ratio was minimum in spring, low in summer and highest in winter. The extracellular dissolved organic carbon released by phytoplankton was sufficient to meet bacterial carbon demand in all experimental dates, suggesting that allochthonous carbon sources play a minor role in sustaining BP, though they cannot be excluded. However, we hypothesize that high availability of dissolved organic carbon might explain the lack of coupling observed between BP and PP.

Effects of river inputs on nutrient and organic-carbon conditions and net ecosystem metabolism in the Kaoping (Taiwan) coastal sea

This study aims to understand the variability of net ecosystem metabolism in a tropical sea. The contrasting pattern of metabolic state between wet and dry seasons was caused by the pronounced difference of river exports in the Kaoping coastal sea. The depth-integrated gross primary production (IGPP) through the euphotic zone ranged from 2451 to 16 230 mg C m–2 day–1 in summer, and from 844 to 5549 mg C m–2 day–1 in winter, and was apparently regulated by oceanic temperature, nutrients and organic carbon. The depth-integrated dark community respiration (IDCR), attributed largely to bacterial respiration (BR, ~69%), ranged from 861 to 12 418 mg C m–2 day–1 in summer, and from 997 to 5781 mg C m–2 day–1 in winter. GPP and DCR correlated inversely with salinity but positively with nutrients, Chlorophyll a, dissolved organic carbon (DOC) and particulate organic carbon (POC). The autotrophic state (IGPP : IDCR &gt; 1) prevailed in most stations in summer, whereas the heterotrophic state (IGPP : IDCR &lt; 1) occurred in all but Station B1 in winter. Bacterial production (BP) and bacterial respiration (BR) also correlated inversely with salinity but positively with nutrients and DOC. Bacterial carbon demand (BCD) was 0.15 GPP in summer and 0.64 GPP in winter, supporting the autotrophic and heterotrophic conditions in summer and winter, respectively. The metabolic state is apparently determined by seasonal variation of temperature and river exports.