Viral impact on prokaryotic and microalgal activities in the microphytobenthic biofilm of an intertidal mudflat (French Atlantic coast)

International audienceThis is the first report on viriobenthos activity within the microbial biofilm located at the top-surface of the intertidal mudflat during emersion in Marennes-Oléron Bay (France). By combining in situ and ex situ approaches, the viral production (VP) was linked to the dynamics of prokaryotes and microphytobenthos (MPB). VP averaged 2-4 x 108 viruses ml-1 h-1. VP correlated positively with the Virus to Prokaryote Ratio, and both were correlated negatively with the water content. The virus-induced mortality of prokaryotes was lower in winter than in summer (6.8 vs 39.7 % of the production) and the C-shunting may supply 2 to 12% of their Carbon Demand, respectively. VP accounted for 79% of loss in Prokaryotes but the response was delayed compared to the increase in VP suggesting a simultaneous release of viruses of MPB origin. This hypothesis is supported by capsid-sizing of virions by transmission electronic microscopy and bioassays. Harvesting and ex situ maintenance of top-surface sediments was carried out to monitor the dynamics of viruses, prokaryotes and MPB after inoculation with benthic or planktonic viruses. Benthic viruses modified the prokaryotic and MPB dynamics and decreased the photosynthesis efficiency in contrast to planktonic viruses that impacted MPB but not the prokaryotes

In situ structuring of virioplankton through bacterial exoenzymatic activity: interaction with phytoplankton

International audienceThe abundance and composition of microbial communities were investigated during 2007 in Marennes Oléron Bay (France) in order to characterize the biological relationships governing the planktonic food web. Compared to the results of previous years, there was lower abundance and lower magnitude of variations in autotrophic and heterotrophic microbial entities in 2007. Using a Spearman rank correlation analysis, a significant link for biological abundance was only noted between bacteria and ciliates. However, an interesting relationship was highlighted between chlorophyll a and bacterial aminopeptidase and β-glucosidase exoenzymatic activities. Two characteristic periods of virus-bacteria uncoupling were observed in spring and autumn 2007. Using multivariate analysis, the clustering of monthly viral community structures (defined by RAPD-fingerprinting) was related to bacterial proteolysis activity and secondarily to flagellates. Spring and autumn were characterized by phytoplanktonic blooms of large cells (> 10µm) and small cells (< 10µm), respectively, during the virus-bacteria uncoupling phases. High predation by flagellates in the autumn, during a period of microbial food web activity, involved the highest viral fingerprint richness, inferring a top-down control in driving the number of virus-host systems. We discuss the proteolysis level as a descriptor in structuring viral assemblages and argue that changes in the quantity and quality of DOM may bridge phytoplankton to bacteria and reflect changes in phytoplankton composition

Impact of biofilm resuspension on mesozooplankton in a shallow coastal ecosystem characterized by a bare intertidal mudflat

electronic publication at this timeInternational audienceA prey–predator experimental setup was conducted in a shallow coastal ecosystem characterized by a bare intertidal mudflat to test if benthic biofilm resuspension causing microalgae inputs and carbon export toward nanoflagellates would favour the highest planktonic trophic level (i.e. mesozooplankton) when nutrient concentrations are high in the water column. Mesozooplankton predation and somatic production were studied by comparing the evolution of the prey assemblage (diversity and abundances) in the presence and absence of these predators during 24 h experiments. The results were then statistically analysed according to the cross-calculation method. Biofilm resuspension caused (i) a direct input of benthic microorganisms that had changed prey structure in term of diversity and/or size and (ii) a differential growth ability between prey taxa. Both reasons implied a bottom-up control on both micro- and mesozooplankton. The carbon export toward heterotrophic nanoflagellates favoured pelagic ciliate growth while mesozooplankton benefited from largest diatoms with high growth rates, both benthic and R-strategist pelagic species. Even if these microbial and herbivorous pathways are controlled by benthic inputs, they seemed to be totally disconnected since ciliates represented only a small part of mesozooplankton diet. The sensitivity of mesozooplankton production appeared species-dependent with the most tolerant taxa dominating the zooplankton assemblages. This suggests a role of the intensities and the frequencies of biofilm resuspension on the spatio-temporal structuring of mesozooplankton in macrotidal coastal ecosystems

Sequential resuspension of biofilm components (viruses, prokaryotes and protists) as measured by erodimetry experiments in the Brouage mudflat (French Atlantic coast)

International audienceResuspension thresholds in terms of friction velocity were experimentally quantified for the prokaryotes, protists and for the first time, viruses of intertidal mudflat biofilms. Differences in resuspension thresholds could be related to the type, behaviour and size of microorganisms and their association with particles. Free microorganisms (viruses, bacteria and some nanoflagellates) were resuspended by weak flow at friction velocities lower than 2cms-1. Chlorophyll a, some nanoflagellates and attached bacteria were resuspended together with the bed's muddy sediment, which required friction velocities larger than 3cms-1. Diatoms smaller than 60μm were resuspended at velocities between 3 and 5cms-1, while those larger than 60μm were resuspended at higher friction velocities (5.5 to 6.5cms-1).The thresholds of resuspension also depended on the micro-scale position of microorganisms in the sediment (horizontal and vertical distributions). In the field, the vertical distribution of chlorophyll a (a proxy of microphytobenthos) was skewed, with a maximum in the first 2. mm of sediment. Along the neap-spring tidal cycle, chlorophyll a revealed an increase in MPB biomass in the first 2. mm of the sediment, in relation to light increases with exposure durations. The horizontal distribution of chlorophyll a could be inferred from erosion experiments. During the initial phase of biofilm growth, the distribution of chlorophyll a seemed horizontally homogeneous, and was uniformly eroded at the beginning of the increase in chlorophyll a. From these results, we can make a hypothesis: in the subsequent phase of biofilm growth until the maximum of emersion duration, the eroded quantity of chlorophyll a was larger than expected based from chlorophyll a vertical distribution, suggesting that biofilm horizontal distribution became patchy and enriched chlorophyll a was preferentially eroded. When emersion duration and biofilm growth decreased, the trend was reversed, and eroded quantity of chlorophyll a was lower than expected from chlorophyll a vertical distribution, suggesting that areas with low chlorophyll a were preferentially eroded. Such erosion patterns when biofilm growth decreased probably resulted from the bulldozing activity of a surficial sediment bioturbator, the gastropod Peringia ulvae. Our study did not directly prove this horizontal distribution but it should be further discussed. This distribution needs to be studied to acquire real evidence of patchy distribution