The effect of microorganisms and seasonal factors on the isotopic composition of particulate organic carbon from the black sea

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the¹²C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO₂ fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC δ¹³C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Carbon mass balance methodology to characterize the growth of pigmented marine bacteria under conditions of light cycling

A carbon mass balance methodology employing minimal measurements was applied to heterotrophic and photoheterotrophic marine bacteria grown under constant dilution and exposed to 12-h intervals of light or darkness. Carbon mass balance calculations using measurements taken every 3 h closed to within 93–103% using dissolved organic carbon, biomass carbon and CO₂ production data only, indicating that background interference from dissolved inorganic carbon variations in the amended seawater medium was not significant. Neither strain was observed to sustain a net CO₂ fixation using paramagnetic measurement of oxygen uptake rates (OUR), indicating a need for more sensitive on-line measurement techniques for OUR. Photoheterotrophic growth demonstrated lower carbon-mole biomass yields (0.41±0.026 vs. 0.64±0.013 mol mol^–1) despite higher specific glucose uptake rates (0.025 vs. 0.02 mol mol^–1 h^–1), suggesting that bioreactor-based study of marine bacteria can present growth modes that are different from those encountered in the marine environment.     

Benthic decomposition of Zostera marina roots: a controlled laboratory experiment

The initial benthic decomposition of Zostera marina roots was studied in a controlled flow-through chamber experiment for 23 days. Sediment chambers without added roots served as controls. The inflowing and outflowing artificial seawater (ASW) was analyzed for O₂, ΣCO₂, urea-N, NH₄⁺ and NO₂⁻+NO₃⁻. Sediment profiles of Eh, particulate organic carbon (POC) and nitrogen, dissolved organic nitrogen (DON), dissolved free amino acids (DFAA), urea-N, NH₄⁺, DFAA and urea turnover rates, sulfate reduction and counts of total anaerobic heterotrophic bacteria and different functional groups were determined. Fluxes of O₂, ΣCO₂, urea-N and NH₄⁺ were stimulated during root decomposition compared to the unamended control. There were indications of stimulated bacterial growth based on counts of total anaerobic heterotrophic bacteria, anaerobic phosphatase utilizers, ammonifyers and sulfate reducers. Independent estimates of nitrogen and carbon incorporation into bacterial biomass during root decomposition indicate that a major fraction of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen (PON) pool, whereas the energy source for bacterial growth was mainly obtained from the added eelgrass roots. Most of the nitrogen mineralized during root decomposition was incorporated into the bacterial biomass resulting in a low efflux of urea-N and inorganic nitrogen from the sediment to the water column.     

EVOLUTIONARY RESPONSES OF A COCCOLITHOPHORID GEPHYROCAPSA OCEANICA TO OCEAN ACIDIFICATION

The ongoing ocean acidification associated with a changing carbonate system may impose profound effects on marine planktonic calcifiers. Here, we show that a coccolithophore, Gephyrocapsa oceanica, evolved in response to an elevated CO₂ concentration of 1000 μatm (pH reduced to 7.8) in a long‐term (～670 generations) selection experiment. The high CO₂‐selected cells showed increases in photosynthetic carbon fixation, growth rate, cellular particulate organic carbon (POC) or nitrogen (PON) production, and a decrease in C:N elemental ratio, indicating a greater upregulation of PON than of POC production under the ocean acidification condition. Cells from the low CO₂ selection process shifted to high CO₂ exposure showed an enhanced cellular POC and PON production rates. Our data suggest that the coccolithophorid could adapt to ocean acidification with enhanced assimilations of carbon and nitrogen but decreased C:N ratios.     

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (r_S = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.

     

Seasonal variation of particulate organic carbon, dissolved organic carbon and the contribution of microbial communities to the live particulate organic carbon in a shallow near-bottom ecosystem at the Northwestern Mediterranean Sea

Microbial planktonic communities (i.e. bacteria and protozoa), phytoplankton, dissolved organic carbon (DOC) and particulate organic carbon (POC) were seasonally examined at Medes Islands (Northwestern Mediterranean) to assess their variation in abundance and composition throughout the year in a near-bottom littoral ecosystem. From October 1995 to November 1996, samples were collected between two and six times per month at 0.5 m above the bottom. Mean DOC and POC values throughout the year were 2560 180 (SE) and 387 ± 35 g C l^-1, respectively. All year, detrital organic carbon (detrital=total POC - live carbon) represented the main POC fraction, and mean live carbon was 24 ± 9 g C l^-1. Winter and spring had maximum values of POC, and spring and summer had maximum values of DOC. Heterotrophic bacteria, with a mean abundance of 5.16 ± 0.08 x 10⁵ cells ml^-1, were the main contributor to live carbon (26 ± 7%). During winter, heterotrophic bacterial biomass decreased 40% due to a decrease in mean biovolume per cell. Synechococcus sp. and Prochlorococcus sp. abundance were 2.24 ± 0.09 x 10⁴ and 1.05 ± 0.07 x 10⁴ cells ml^-1, respectively. However, while Synechococcus sp. were present all year, Prochlorococcus sp. were not observed from April to July. Mean phytoplankton (i.e. diatoms and dinoflagellates) abundance was 2.06 ± 0.40 x 10⁴ cells l^-1 with biomass at a maximum during the winter months, the period with the lowest temperature and the highest nutrient concentration. The size composition of live carbon showed two clearly distinct periods: from December to March, live carbon was dominated in biomass by microplankton, while from April to November, pico- and nanoplankton cells were dominant. Overall, the dynamics of the near-bottom planktonic communities was characterized by a low biomass of heterotrophic and autotrophic bacteria, phytoplankton and ciliates in contrast to previous water column studies. This pattern and the high temporal heterogeneity of the different planktonic communities are discussed in relation to the physical and chemical characteristics of the environment, as well as to the potential role that benthic communities may be exerting in the control of the near-bottom planktonic communities.      

Effect of microorganisms and seasonal factors on the isotope composition of organic carbon from Black Sea suspensions

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the 12C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO2 fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC delta 13C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Seasonal and spatial distribution of dissolved and particulate organic carbon and bacteria in the bank of an impounding reservoir on the Enns River, Austria

1. Interstitial bacterial abundance, production and ectoenzyme activity were investigated over an annual cycle in an Austrian river when infiltration of oligotrophic river water into a river-bank was artificially enhanced. These microbial parameters were related to porewater chemistry and the concentration of particulate (POC) and dissolved organic carbon (DOC).
2. Porewater chemistry reflected the hydrodynamic mixing of infiltrating river water with riparian groundwater. Seasonal fluctuations in the microbial parameters resulted mainly from changes in temperature and organic matter supply. Seasonal change in porewater chemistry in the river-bank was detectable laterally only within the first metre of the sediment and decreased rapidly with increasing distance from the sediment–water interface.
3. The DOC concentration decreased only slightly during lateral transport through the aquifer, while total organic carbon (TOC) concentration as well as abundance and activity of interstitial bacteria were reduced by up to one order of magnitude within the top metre of the sediment. Retention of incoming particulate matter structured the lateral distribution pattern of TOC concentration. The POC and not the DOC pool was the main source of carbon for interstitial bacteria and, therefore, the quality of POC determines the distribution of microbial metabolism within the riparian zone.     

Activity and growth efficiency of heterotrophic bacteria in Rybinsk Reservoir

The active fraction, production, and respiration of heterotrophic bacteria are determined to assess their growth efficiency and their role in the carbon cycle in the pelagic zone of Rybinsk Reservoir in summer. The greater part of organic substances assimilated by bacteria is mineralized to CO₂. It has been established that the essential part of the constructive and energy metabolism of bacteria is supported by the input of allochthonous substances. Bacterioplankton, producing the biomass at their expense, performs functions similar to the functions of phytoplankton, and substantially supports the structural and functional organization of the planktonic food web in the reservoir.     

Seasonal dynamics of crustacean zooplankton, heterotrophic nanoflagellates and bacteria in a shallow, eutrophic lake

1. The seasonal development of crustacean zooplankton, heterotrophic nanoflagellates (HNF) and bacteria was examined in Grosser Binnensee, a shallow, eutrophic lake in northern Germany. The grazing impact of Daphnia on bacteria and nanoflagellates was estimated from field data on population abundances and from clearance rates obtained in laboratory experiments. 2. The seasonal succession of zooplankton showed distinct peaks of Daphnia magna, cyclopopid copepods, Bosmina longirostris and Daphnia galeata and D. hynlina. The population dynamics of Dapfinia had the strongest impact on all sestonic components. Daphnia maxima coincided with clearwater phases, and were negatively correlated with particulate organic carbon (POC), HNF and phytoplankton. Bacterial abundance was only slightly affected although daphnids were at times more important as bacterial consumers than HNF, as estimated from measured bacterial clearance rates. Other crustaceans (copepods, Bosmina) were probably of minor importance as grazers of bacteria and nanoplankton. 3. HNF abundance varied from 550 ml^?1 to more than 30000 ml^?1. HNF appeared to be suppressed by daphnids and reached highest densities when copepods dominated the metazooplankton. The variation in HNF abundance was not reflected in the concentration of heterotrophic bacteria, which fluctuated rather irregularly between 5 and 20 ± 10⁶ ml^?1. Long filamentous bacteria which were probably resistant to protozoan grazing, however, appeared parallel to the development of HNF. These bacterial cells, although small in number, could comprise more than 30% of the total bacterial biomass.     

Seasonal Dynamics of Prokaryotic Abundance and Activities in Relation to Environmental Parameters in a Transitional Aquatic Ecosystem (Cape Peloro,Italy)

This study examines the effects of temporal changes on microbial parameters in a brackish aquatic ecosystem. To this aim, the abundances of prokaryotes and vibrios together with the rates of enzymatic hydrolysis of proteins by leucine aminopeptidase (LAP), polysaccharides by β-glucosidase (GLU) and organic phosphates by alkaline phosphatase (AP), heterotrophic prokaryotic production (HPP), respiration (R), were seasonally investigated, during a 2-year period in the coastal area of Cape Peloro (Messina, Italy), constituted by two brackish lakes (Faro and Ganzirri). In addition, physical and chemical parameters (temperature, salinity, nutrients) and particulate organic carbon and nitrogen (POC, PN) were measured. The influence of multiple factors on prokaryotic abundances and activities was analysed. The results showed that Cape Peloro area is characterised by high seasonal variability of the microbial parameters that is higher than the spatial one. Combined changes in particulate matter and temperature (T), could explain the variability in vibrios abundance, GLU and R activities in both lakes, indicating a direct stimulation of the warm season on the heterotrophic prokaryotic metabolism. Positive correlations between T (from 13.3 to 29.6 °C) and HPP, LAP, AP, POC, PN are also observed in Ganzirri Lake. Moreover, the trophic status index and most of the microbial parameters show significant seasonal differences. This study demonstrates that vibrios abundance and microbial activities are responsive to the spatial and seasonal changes of examined area. The combined effects of temperature and trophic conditions on the microbial parameters lead us to suggest their use as potential indicators of the prokaryotic response to climate changes in temperate brackish areas.     

Inorganic carbon promotes photosynthesis,growth, and maximum biomass of phytoplankton in eutrophic water bodies

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Development and ecological importance of phytoplankton in a large lowland river (River Meuse,Belgium)

The ecological importance of the River Meuse phytoplankton with regard to carbon and nutrient transport has been examined in two reaches of the Belgian course of the river.Field measurements of total particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate phosphorus (PP) show that the large autochtonous production of organic matter strongly affects the carbon and nutrient budget of the aquatic system. During the growing season, phytoplankton accounts for nearly 60% of the POC and dominates the PON. Calculations of the carbon and oxygen budget in the upper reach of the Belgian Meuse demonstrates that the ecosystem is autotrophic, i.e. that autochtonous FPOM (fine particulate organic matter) production is the major carbon input. This suggests that in large lowland rivers, primary production (P) may exceed community respiration (R), i.e. P:R>1, whereas they are assumed to be heterotrophic (P:R<1) in the River Continuum concept.The question of maintenance of phytoplankton in turbid mixed water columns is also addressed, and the case of the River Meuse is treated on the basis of studies of photosynthesis and respiration (ETS measurements). The results suggest that the potamoplankton may show some low-light acclimation, through an increase of chlorophyll a relative to biomass, when it comes to deep downstream reaches, and that algal respiration rate may be reduced. A simulation of the longitudinal development of the algal biomass shows the different phases of algal growth and decline along the river and brings support to the importation hypothesis for explaining maintenance of potamoplankton in the downstream reaches.     

Elevated pCO2 alters marine heterotrophic bacterial community composition and metabolic potential in response to a pulse of phytoplankton organic matter

Factors that affect the respiration of organic carbon by marine bacteria can alter the extent to which the oceans act as a sink of atmospheric carbon dioxide. We designed seawater dilution experiments to assess the effect of pCO₂ enrichment on heterotrophic bacterial community composition and metabolic potential in response to a pulse of phytoplankton-derived organic carbon. Experiments included treatments of elevated (1000 p.p.m.) and low (250 p.p.m.) pCO₂ amended with 10 μmol L⁻¹ dissolved organic carbon from Emiliana huxleyi lysates, and were conducted using surface-seawater collected from the South Pacific Subtropical Gyre. To assess differences in community composition and metabolic potential, shotgun metagenomic libraries were sequenced from low and elevated pCO₂ treatments collected at the start of the experiment and following exponential growth. Our results indicate bacterial communities changed markedly in response to the organic matter pulse over time and were significantly affected by pCO₂ enrichment. Elevated pCO₂ also had disproportionate effects on the abundance of sequences related to proton pumps, carbohydrate metabolism, modifications of the phospholipid bilayer, resistance to toxic compounds and conjugative transfer. These results contribute to a growing understanding of the effects of elevated pCO₂ on bacteria-mediated carbon cycling during phytoplankton bloom conditions in the marine environment.     

The production-to-respiration ratio and its implication in Lake Biwa,Japan

Production-to-respiration (P:R) ratio was estimated at an offshore site of Lake Biwa in order to examine whether the plankton and benthic community is subsidized with allochthonous organic carbon, and to clarify the role of this lake as potential source or sink of carbon dioxide. The respiration rate of protozoan and metazoan plankton was calculated from their biomass and empirical equations of oxygen consumption rates, and that of bacterioplankton was derived from their production rate and growth efficiency. In addition, the carbon mineralization rate in the lake sediments was estimated from the accumulation rate of organic carbon, which was determined using a ²¹⁰Pb dating technique. On an annual basis, the sum of respiration rates of heterotrophic plankton was comparable to net primary production rate measured by the ¹³C method. However, when the mineralization rate in the lake sediments was included, the areal P:R ratio was 0.89, suggesting that Lake Biwa is net heterotrophic at the offshore site with the community being subsidized with allochthonous organic carbon. Such a view was supported by the surface water pCO₂ that was on average higher than that of the atmosphere. However, the estimate of net CO₂ release rate was close to that of carbon burial rate in the sediments. The result suggests that the role of Lake Biwa in relation to atmospheric carbon is almost null at the offshore site, although the community is supported partially by organic carbon released from the surrounding areas.     

Predominance of phytoplankton-derived dissolved and particulate organic carbon in a highly eutrophic tropical coastal embayment (Guanabara Bay,Rio de Janeiro,Brazil)

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO₂ (pCO₂), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO₂ undersaturation, and DOC reached 82 mg L^?1, POC 152 mg L^?1, and Chl a 800 μg L^?1. Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m^?2 year^?1, representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.     

Stoichiometry and temperature sensitivity of methanogenesis and CO2 production from saturated polygonal tundra in Barrow,Alaska

Arctic permafrost ecosystems store ~50% of global belowground carbon (C) that is vulnerable to increased microbial degradation with warmer active layer temperatures and thawing of the near surface permafrost. We used anoxic laboratory incubations to estimate anaerobic CO₂ production and methanogenesis in active layer (organic and mineral soil horizons) and permafrost samples from center, ridge and trough positions of water‐saturated low‐centered polygon in Barrow Environmental Observatory, Barrow AK, USA. Methane (CH₄) and CO₂ production rates and concentrations were determined at ?2, +4, or +8 °C for 60 day incubation period. Temporal dynamics of CO₂ production and methanogenesis at ?2 °C showed evidence of fundamentally different mechanisms of substrate limitation and inhibited microbial growth at soil water freezing points compared to warmer temperatures. Nonlinear regression better modeled the initial rates and estimates of Q₁₀ values for CO₂ that showed higher sensitivity in the organic‐rich soils of polygon center and trough than the relatively drier ridge soils. Methanogenesis generally exhibited a lag phase in the mineral soils that was significantly longer at ?2 °C in all horizons. Such discontinuity in CH₄ production between ?2 °C and the elevated temperatures (+4 and +8 °C) indicated the insufficient representation of methanogenesis on the basis of Q₁₀ values estimated from both linear and nonlinear models. Production rates for both CH₄ and CO₂ were substantially higher in organic horizons (20% to 40% wt. C) at all temperatures relative to mineral horizons (<20% wt. C). Permafrost horizon (~12% wt. C) produced ~5‐fold less CO₂ than the active layer and negligible CH₄. High concentrations of initial exchangeable Fe(II) and increasing accumulation rates signified the role of iron as terminal electron acceptors for anaerobic C degradation in the mineral horizons.     

How organic carbon derived from multiple sources contributes to carbon sequestration processes in a shallow coastal system?

Carbon captured by marine organisms helps sequester atmospheric CO₂, especially in shallow coastal ecosystems, where rates of primary production and burial of organic carbon (OC) from multiple sources are high. However, linkages between the dynamics of OC derived from multiple sources and carbon sequestration are poorly understood. We investigated the origin (terrestrial, phytobenthos derived, and phytoplankton derived) of particulate OC (POC) and dissolved OC (DOC) in the water column and sedimentary OC using elemental, isotopic, and optical signatures in Furen Lagoon, Japan. Based on these data analysis, we explored how OC from multiple sources contributes to sequestration via storage in sediments, water column sequestration, and air–sea CO₂ exchanges, and analyzed how the contributions vary with salinity in a shallow seagrass meadow as well. The relative contribution of terrestrial POC in the water column decreased with increasing salinity, whereas autochthonous POC increased in the salinity range 10–30. Phytoplankton‐derived POC dominated the water column POC (65–95%) within this salinity range; however, it was minor in the sediments (3–29%). In contrast, terrestrial and phytobenthos‐derived POC were relatively minor contributors in the water column but were major contributors in the sediments (49–78% and 19–36%, respectively), indicating that terrestrial and phytobenthos‐derived POC were selectively stored in the sediments. Autochthonous DOC, part of which can contribute to long‐term carbon sequestration in the water column, accounted for >25% of the total water column DOC pool in the salinity range 15–30. Autochthonous OC production decreased the concentration of dissolved inorganic carbon in the water column and thereby contributed to atmospheric CO₂ uptake, except in the low‐salinity zone. Our results indicate that shallow coastal ecosystems function not only as transition zones between land and ocean but also as carbon sequestration filters. They function at different timescales, depending on the salinity, and OC sources.     

Competition between Picoplanktonic Cyanobacteria and Heterotrophic Bacteria along Crossed Gradients of Glucose and Phosphate

A laboratory experiment was performed to test whether differences in nutrient and energy demands between picophytoplankton and heterotrophic bacteria can explain the apparent inverse biomass relationship between these organisms in lakes along gradients of organic carbon and nutrients. Growth rates and final yield of cells were analyzed in crossed gradients of glucose and phosphate. Concentrations of phosphate (10, 25, and 60 microg P L(-1)) and glucose (0, 0.3, and 3 mg C L(-1)) were used in all possible combinations giving 9 different treatments. Heterotrophic bacteria had higher maximum growth rates in all treatments and became larger than picophytoplankton in many treatments. The variance in abundance of heterotrophic bacteria between treatments could almost completely be explained by the combined effects of glucose and P. In treatments where carbon limitation slowed down the growth of heterotrophic bacteria, picophytoplankton became abundant and these organisms showed a positive response to P in combination with a negative response to glucose. The negative effect of glucose on picophytoplankton is suggested to be indirect and caused by competition with bacteria that are favored by organic C. The results suggest that competition for phosphate between phytoplankton and bacteria is not size-dependent, that heterotrophic bacteria are superior competitors for P, and that organic C produced by picophytoplankton was of minor importance for heterotrophic bacteria.