18 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
Exploring the distance between nitrogen and phosphorus limitation in mesotrophic surface waters using a sensitive bioassay
No full textThe balance in microbial net consumption of nitrogen and phosphorus was investigated in samples collected in two mesotrophic coastal environments: the Baltic Sea (Tvärminne field station) and the North Sea (Espegrend field station). For this, we have refined a bioassay based on the response in alkaline phosphatase activity (APA) over a matrix of combinations in nitrogen and phosphorus additions. This assay not only provides information on which element (N or P) is the primary limiting nutrient, but also gives a quantitative estimate for the excess of the secondary limiting element (P+ or N+, respectively), as well as the ratio of balanced net consumption of added N and P over short timescales (days). As expected for a Baltic Sea late spring–early summer situation, the Tvärminne assays (n = 5) indicated N limitation with an average P+ = 0.30 ± 0.10 µM-P, when incubated for 4 days. For short incubations (1–2 days), the Espegrend assays indicated P limitation, but the shape of the response surface changed with incubation time, resulting in a drift in parameter estimates toward N limitation. Extrapolating back to zero incubation time gave P limitation with N+ ≈ 0.9 µM-N. The N : P ratio (molar) of nutrient net consumption varied considerably between investigated locations: from 2.3 ± 0.4 in the Tvärminne samples to 13 ± 5 and 32 ± 3 in two samples from Espegrend. Our assays included samples from mesocosm acidification experiments, but statistically significant effects of ocean acidification were not found by this method
Mediterranean Pelagic Ecosystem Study: Plankton Dynamics
No full textMediterranean Targeted Project (MTP) - MEDIPELAGOS Project, Contract MAS2-CT93-0063, Synthesis of Final Results.-- 12 pages, 1 figuresPeer Reviewe
Bacterioplankton groups involved in the uptake of phosphate and dissolved organic phosphorus in a mesocosm experiment with P-starved Mediterranean waters
No full text20 pages, 8 figures, 1 tableThe use of inorganic phosphate (Pi) and dissolved organic phosphorus (DOP) by different bacterial groups was studied in experimental mesocosms of P-starved eastern Mediterranean waters in the absence (control mesocosms) and presence of additional Pi (P-amended mesocosms). The low Pi turnover times in the control mesocosms and the increase in heterotrophic prokaryotic abundance and production upon Pi addition confirmed that the bacterial community was originally P-limited. The bacterioplankton groups taking up Pi and DOP were identified by means of microautoradiography combined with catalysed reporter deposition fluorescence in situ hybridization. Incubations with leucine were also performed for comparative purposes. All the probe-identified groups showed a high percentage of cells taking up Pi and DOP in the control, P-limited, mesocosms throughout the experiment. However, in response to Pi addition two contrasting scenarios in Pi use were observed: (i) on day 1 of the experiment Pi addition caused a clear reduction in the percentage of SAR11 cells taking up Pi, whereas Gammaproteobacteria, Roseobacter and Bacteroidetes showed similar percentages to the ones in the control mesocosms and (ii) on day 4 of the experiment, probably when the bacterial community had fully responded to the P input, all the probe-identified groups showed low percentages of cells taking up the substrate as compared with the control mesocosms. These differences are likely related to different P requirements among the bacterial groups and point out to the existence of two contrasting strategies in P useThe mesocosm experiment in the Cretacosmos facility was arranged in cooperation by project Nutritunnel financed by the Research Council of Norway and the MESOAQUA EU network. The processing of the samples was supported by the grants FOSMICRO (CTM2009-07679-E), STORM (CTM2009-09352/MAR), SUMMER (CTM2008-03309/MAR) and MALASPINA (CSD2008 – 00077) funded by the Spanish Ministry of Science and Innovation. We thank Irene Forn and Clara Ruiz for their help with the CARDFISH and MARFISH techniques, Iordanis Magiopoulos for performing the flow cytometer analyses and Tatiana Tsagaraki for performing the inorganic nutrient analyses. Marta Sebastián was supported by the Spanish Ministry of Science and Innovation through a
‘Juan de la Cierva’ awardPeer reviewe
Nutrient dynamics mediated through Turbulence And Plankton interactions - (NTAP)
No full textEuropean Conference on Marine Science & Ocean Technology (EurOCEAN 2004): Celebrating European Marine Science, Building the European Research Area, Communicating Marine Science, 10-13 May 2004, Galway, Ireland.-- 2 pages, 1 figurePeer reviewe
Determining the availability of phosphate and glucose for bacteria in P-limited mesocosms of NW Mediterranean surface waters
No full text11 pages, 6 figuresAvailability of phosphate for phytoplankton and bacteria and of glucose for bacteria was studied in mesocosms using NW Mediterranean coastal water with added phosphate, glucose, or a combination of both. We observed an initial increase of bacterial production in all mesocosms, a continuous increase of particulate P only in the P-enriched mesocosms, and a greater accumulation of dissolved organic C in the glucose-enriched mesocosms compared to those enriched by both glucose and P. This suggests that the water used was initially P-starved with a certain pool of degradable organic C available for bacteria. Specific phosphate affinities indicated a P limitation for the bacterial community in all mesocosms. Specific glucose affinities were different between the mesocosms but much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that the glucose pool was not a strong controlling factor of bacterial growth. In the P-enriched mesocosms, it is indicated that the extent of P limitation shifted from highest to lowest, while the available pool of glucose steadily decreased during the experiment. The explanation suggested for these observations is that phosphate regeneration was enhanced in the plankton food web, by which bacterial carbon demand became higher than the degradable organic C produced in the P-enriched mesocosmsThis work was supported by the EC through project BASICS (Contract: EVK3-CT 2002-00078). T.T. was supported by the EC through project DANLIM (Contract: EVK3-CT-2001-00049). K.S. and J.J. were supported by a project GACR 206/08/0015 and by the CSIC-CAS mobility programPeer reviewe
Does competition for nanomolar phosphate supply explain the predominance of the cyanobacterium Synechococcus ?
No full textInternational audienceExperimental work during a cruise along a W‐E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6–2ߝµm size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (<5 nM) as well as at relatively high (5–25 nM) concentrations could explain both general Synechococcus abundance in P‐depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur
www.biogeosciences.net/5/669/2008/ © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License.
No full textAvailability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO2 levels in a mesocosm stud
Errata and corrigenda: Climate change and biodiversity in the Arctic-Nordic perspectives (vol 26, pg 96, 2007)
No full textIsolation and characterisation of novel viruses infecting marine phytoplankton
No full textViruses are the most abundant biological agents in the global marine environment. Through cellular lysis viruses influence many biogeochemical and ecological processes, including energy and nutrient cycling, host distribution and abundance, algal bloom control and genetic transfer. Nano- and picophytoplankton are ubiquitous in the world’s oceans and are responsible for a high proportion of the annual global carbon fixation. However, relatively few viruses have been isolated and described that infect these key primary producers and little is known of their diversity, dynamics or propagation strategies. The aims of this study were to detect, isolate and characterise novel marine viruses that infect these important members of the phytoplankton assemblage. Screening of seawater samples for viruses that infect a broad representation of nano and picophytoplankton species was undertaken here. To enable this, a large culture collection of 106 phytoplankton species was established and used to screen seawater samples for viruses on a weekly basis over a two year period. A total of 12 novel viruses infecting the prasinophyte species’ Ostreococcus tauri and Micromonas pusilla were isolated from seawater sampled in coastal waters of the Western English Channel. Viruses were purified by plaque purification or liquid serial dilution techniques. Characterisation of novel virus isolates included growth kinetics, visualisation using transmission electron microscopy, host range analysis and estimates of viral genome sizes using pulsed field gel electrophoresis. Phylogenetic analysis of these viruses was conducted based on the sequence of the conserved DNA polymerase gene. Genome sequencing of two of the viruses infecting O. tauri was completed and revealed many exciting features, including a suite of genes hitherto unreported, or with rare occurrence, in viruses. Evidence is presented for horizontal gene transfer between viruses isolated in this study and their hosts, as well as between other eukaryotic and bacterial sources. Functional characterisation of the viral genomes sequenced and described in this study will provide clearer insights into viral dynamics and evolutionary history
