137 research outputs found
The role of mixotrophic protists in the biological carbon pump
No full textThe traditional view of the planktonic food web describes consumption of inorganic nutrients by photoautotrophic phytoplankton, which in turn supports zooplankton and ultimately higher trophic levels. Pathways centred on bacteria provide mechanisms for nutrient recycling. This structure lies at the foundation of most models used to explore biogeochemical cycling, functioning of the biological pump, and the impact of climate change on these processes. We suggest an alternative new paradigm, which sees the bulk of the base of this food web supported by protist plankton communities that are mixotrophic – combining phototrophy and phagotrophy within a single cell. The photoautotrophic eukaryotic plankton and their heterotrophic microzooplankton grazers dominate only during the developmental phases of ecosystems (e.g. spring bloom in temperate systems). With their flexible nutrition, mixotrophic protists dominate in more-mature systems (e.g. temperate summer, established eutrophic systems and oligotrophic systems); the more-stable water columns suggested under climate change may also be expected to favour these mixotrophs. We explore how such a predominantly mixotrophic structure affects microbial trophic dynamics and the biological pump. The mixotroph-dominated structure differs fundamentally in its flow of energy and nutrients, with a shortened and potentially more efficient chain from nutrient regeneration to primary production. Furthermore, mixotrophy enables a direct conduit for the support of primary production from bacterial production. We show how the exclusion of an explicit mixotrophic component in studies of the pelagic microbial communities leads to a failure to capture the true dynamics of the carbon flow. In order to prevent a misinterpretation of the full implications of climate change upon biogeochemical cycling and the functioning of the biological pump, we recommend inclusion of multi-nutrient mixotroph models within ecosystem studies
Effect of P and N addition to oligotrophic Eastern Mediterranean waters influenced by near-shore waters: A microcosm experiment
No full textInternational audiencePhosphate (P), nitrate (N) or P+N added in a microcosm experiment to oligotrophic waters of the Eastern Mediterranean influenced by near-shore waters triggered a range of responses in the autotrophic and heterotrophic compartments of the system. Chlorophyll a increased in all treatments, including the no-addition control, implying that nutrients became available also from internal sources (recycling). Larger and faster biomass increase as well as a larger P utilization took place in the P+N treatments. Diatoms bloomed in the P+N treatments whereas coccolithophores bloomed following the addition of P ultimately reaching N-limitation. Bacterial activity responded with a transient peak to both low P-alone and N-alone additions (0.01 and 1 mu M, respectively). For reasons not well understood, no such response was observed at higher P-alone additions (0.05 and 0.5 mu M), whereas at the two highest P+N additions the positive response was delayed. We therefore were unable to conclude conclusively on bacteria] limitation. In most cases, the increase in bacterial activity was not matched by an increase in abundance, suggesting a tight top-down control of the biomass. Instead, heterotrophic nanoflagellate and ciliate abundances increased in all treatments. A slightly elevated orthophosphate turnover-time (T-t) (32h) in the initial waters did not give a clear indication of P-limitation, although the system could absorb the lowest P-addition (0.01 mu M) without increase in T-t N alone lead to a reduction in T-t as would be expected in an N-limited system consuming existing surplus P after N-addition. The response of the near-shore influenced system used in this study was in accord with the `classical' response to nutrient introduction-increase in chlorophyll a and in large size phytoplankton. In contrast, in the ultraoligotrophic Cyprus Eddy [Krom, Thingstad, Carbo, Drakopoulos, Fileman, Flaten, Groom, Herut, Kitides, Kress, Law, Liddicoact, Mantoura, Pasternak, Pitta, Polychronaki, Psarra, Rassoulzadegan, Skjoldal, Spyres, Tanaka, Tselepides, Wassmann, Wexels-Riser, Woodward, Zodiatis, Zohary, 2005. Overview of the CYCLOPS P addition lagrangian experiment in the Eastern Mediterranean. Deep-Sea Research II, this volume.], the short T, (< 4h) indicated P-limitation, the combined addition of P and N (as ammonium) induced a bloom of picocyanobacteria [Zohary, Herut, Krom, Mantoura, Pitta, Psarra, Rasssoulzadegan, Stambler, Tanaka, Thingstad, Woodward, 2005. P-limited bacteria but N&P co-limited phytoplankton in the Eastern Mediterranean-a microcosm experiment. Deep-Sea Research II, this volume.] and the in situ P alone addition led to a decrease in chlorophyll. (c) 2005 Elsevier Ltd. All rights reserved
Analyzing the trophic link between the mesopelagic microbial loop and zooplankton from observed depth profiles of bacteria and protozoa
No full textIt is widely recognized that organic carbon exported to the ocean aphotic layer is significantly consumed by heterotrophic organisms such as bacteria and zooplankton in the mesopelagic layer. However, very little is known for the trophic link between bacteria and zooplankton or the function of the microbial loop in this layer. In the northwestern Mediterranean, recent studies have shown that viruses, bacteria, heterotrophic nanoflagellates, and ciliates distribute down to 2000 m with group-specific depth-dependent decreases, and that bacterial production decreases with depth down to 1000 m. Here we show that such data can be analyzed using a simple steady-state food chain model to quantify the carbon flow from bacteria to zooplankton over the mesopelagic layer. The model indicates that bacterial mortality by viruses is similar to or 1.5 times greater than that by heterotrophic nanoflagellates, and that heterotrophic nanoflagellates transfer little of bacterial production to higher trophic levels
Quantifying the structure of the mesopelagic microbial loop from observed depth profiles of bacteria and protozoa
No full textQuantifying the structure of the mesopelagic microbial loop from observed depth profiles of bacteria and protozoa
No full textInternational audiencet is widely recognized that organic carbon exported to the ocean aphotic layer is significantly consumed by heterotrophic organisms such as bacteria and zooplankton in the mesopelagic layer. However, very little is known for the trophic link between bacteria and zooplankton or the structure of the microbial loop in this layer. In the northwestern Mediterranean, recent studies have shown that viruses, bacteria, heterotrophic nanoflagellates, and ciliates distribute down to 2000 m with group-specific depth-dependent decreases, and that bacterial production decreases with depth down to 1000 m. Here we show that such data can be analyzed using a simple steady-state food chain model to quantify the carbon flow from bacteria to zooplankton over the mesopelagic layer. The model indicates that a similar amount of bacterial production is allocated to viruses and heterotrophic nanoflagellates, and that heterotrophic nanoflagellates are the important remineralizers
Preface "Arctic ocean acidification: pelagic ecosystem and biogeochemical responses during a mesocosm study"
No full textThe growing evidence of potential biological impacts
of ocean acidification affirms that this global change phenomenon
may pose a serious threat to marine organisms
and ecosystems. Whilst ocean acidification will occur everywhere,
it will happen more rapidly in some regions than in
others. Due to the high CO2 solubility in the cold surface
waters of high-latitude seas, these areas are expected to experience
the strongest changes in seawater chemistry due to
ocean acidification. This will be most pronounced in the Arctic
Ocean. If atmospheric pCO2 levels continue to rise at
current rates, about 10% of the Arctic surface waters will
be corrosive for aragonite by 2018 (Steinacher et al., 2009).
By 2050 one-half of the Arctic Ocean will be sub-saturated
with respect to aragonite. By the end of this century corrosive
conditions are projected to have spread over the entire
Arctic Ocean (Steinacher et al., 2009). In view of these
rapid changes in seawater chemistry, marine organisms and
ecosystems in the Arctic are considered particularly vulnerable
to ocean acidification. With this in mind, the European
Project on Ocean Acidification (EPOCA) chose the Arctic
Ocean as one of its focal areas of research
Oil Pollution and Plankton Dynamics. IV. Summary of Enclosure Experiments in Lindåspollene, Norway, with Special Emphasis on the Balance between Autotrophic and Heterotrophic Processes
No full textStepwise building of plankton functional type (PFT) models: A feasible route to complex models?
No full textWe discuss the strategy of building models of the lower part of the planktonic food web in a stepwise manner: starting with few plankton functional types (PFTs) and adding resolution and complexity while carrying along the insight and results gained from simpler models. A central requirement for PFT models is that they allow sustained coexistence of the PFTs. Here we discuss how this identifies a need to consider predation, parasitism and defence mechanisms together with nutrient acquisition and competition. Although the stepwise addition of complexity is assumed to be useful and feasible, a rapid increase in complexity strongly calls for alternative approaches able to model emergent systemlevel features without a need for detailed representation of all the underlying biological detail.Special Issue: Parameterisation of Trophic Interactions in Ecosystem Modellin
Measurements of phosphate affinity constants and phosphorus release rates from the microbial food web in Villefranche Bay, northwestern Mediterranean
No full textInternational audienceUsing P-32, uptake and transfer of phosphorus in the microbial food web were studied in surface water from Villefranche Bay (northwestern Mediterranean) from September to December 2001. During the study, the thermocline gradually declined and vertical mixing started, leading to a transition from a nutrient-depleted period to a nutrient-replete period. Before vertical mixing started, the orthophosphate turnover time ranged from 1 to 5 h. Orthophosphate uptake was dominated by the 0.6-2 mum size fraction (mean, 70%), where the cyanobacteria biomass was dominant. The estimated affinity constants for bacteria, cyanobacteria, and autotrophic nanoflagellates ranged from 0.001 to 0.028. 0.047 to 0.103, and 0.002 to 0.032 L nmol P-1 h(-1) during the period, with relatively short (2 mum fraction suggested P transfer to the larger size fraction by predation. Viruses did not contribute significantly to P-32 release from bacteria during the study period
Orthophosphate uptake by heterotrophic bacteria, cyanobacteria, and autotrophic nanoflagellates in Villefranche Bay, northwestern Mediterranean: Vertical, seasonal, and short-term variations of the competitive relationship for phosphorus
No full textInternational audiencePrevious studies have suggested that Mediterranean surface water becomes phosphorus limited for both bacteria and phytoplankton during stratified periods and that orthophosphate uptake in these situations was close to diffusion limitation for both cyanobacteria and autotrophic nanoflagellates. In order to better understand vertical and seasonal variations of this system, we measured orthophosphate uptake by bacteria and phytoplankton in Villefranche Bay (northwestern Mediterranean) monthly for the 0-75-m layer and weekly or biweekly at 10 m from June to December 2002. Turnover time of orthophosphate was relatively short (30 h) in the deeper layer during the stratified period and in the whole water column during the mixing period. Short-term fluctuations in turnover time were repeatedly observed from the stratified through the mixing periods. The dominance of PO, uptake drastically shifted from both bacteria and cyanobacteria to cyanobacteria when there were slight increases in turnover time (1-10 h). Compared to the theoretical maximum calculated for diffusion limitation, mean affinity constants at 10 m were similar for autotrophic nanoflagellates and greater for cyanobacteria in situations with turnover time <2 h, but observed values were much smaller than theoretical for heterotrophic bacteria even in samples with turnover time <1 h
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