Hélène Montanié

Trophic interactions between viruses, bacteria and nanoflagellates under various nutrient conditions and simulated climate change

Population dynamics in the microbial food web are influenced by resource availability and predator/parasitism activities. Climatic changes, such as an increase in temperature and/or UV radiation, can also modify ecological systems in many ways. A series of enclosure experiments was conducted using natural microbial communities from a Mediterranean lagoon to assess the response of microbial communities to top-down control [grazing by heterotrophic nanoflagellates (HNF), viral lysis] and bottom-up control (nutrients) under various simulated climatic conditions (temperature and UV-B radiations). Different biological assemblages were obtained by separating bacteria and viruses from HNF by size fractionation which were then incubated in whirl-Pak bags exposed to an increase of 3°C and 20% UV-B above the control conditions for 96 h. The assemblages were also provided with an inorganic and organic nutrient supply. The data show (i) a clear nutrient limitation of bacterial growth under all simulated climatic conditions in the absence of HNF, (ii) a great impact of HNF grazing on bacteria irrespective of the nutrient conditions and the simulated climatic conditions, (iii) a significant decrease in burst size (BS) (number of intracellular lytic viruses per bacterium) and a significant increase of VBR (virus to bacterium ratio) in the presence of HNF, and (iv) a much larger temperature effect than UV-B radiation effect on the bacterial dynamics. These results show that top-down factors, essentially HNF grazing, control the dynamics of the lagoon bacterioplankton assemblage and that short-term simulated climate changes are only a secondary effect controlling microbial processes.

Pelagic food web patterns: do they modulate virus and nanoflagellate effects on picoplankton during the phytoplankton spring bloom?

As agents of mortality, viruses and nanoflagellates impact on picoplankton populations. We examined the differences in interactions between these compartments in two French Atlantic bays. Microbes, considered here as central actors of the planktonic food web, were first monitored seasonally in Arcachon (2005) and Marennes-Oléron (2006) bays. Their dynamics were evaluated to categorize trophic periods using the models of Legendre and Rassoulzadegan as a reference framework. Microbial interactions were then compared through 48 h batch culture experiments performed during the phytoplankton spring bloom, identified as herbivorous in Marennes and multivorous in Arcachon. Marennes was spatially homogeneous compared with Arcachon. The former was potentially more productive, featuring a large number of heterotrophic pathways, while autotrophic mechanisms dominated in Arcachon. A link was found between viruses and phytoplankton in Marennes, suggesting a role of virus in the regulation of autotroph biomass. Moreover, the virus-bacteria relation was weaker in Marennes, with a bacterial lysis potential of 2.6% compared with 39% in Arcachon. The batch experiments (based on size-fractionation and viral enrichment) revealed different microbial interactions that corresponded to the spring-bloom trophic interactions in each bay. In Arcachon, where there is a multivorous web, flagellate predation and viral lysis acted in an opposite way on picophytoplankton. When together they both reduced viral production. Conversely, in Marennes (herbivorous web), flagellates and viruses together increased viral production. Differences in the composition of the bacterial community composition explained the combined flagellate-virus effects on viral production in the two bays.

Potential Effect of Freshwater Virus on the Structure and Activity of Bacterial Communities in the Marennes-Oléron Bay (France)

Batch culture experiments using viral enrichment were conducted to test the response of a coastal bacterial community to autochthonous (i.e., co-existing) or allochthonous riverine viruses. The effects of viral infections on bacterial dynamics and activity were assessed by epifluorescence microscopy and thymidine incorporation, respectively, whereas the effect of viral infection on bacterial community composition was examined by polymerase chain reaction-single strand conformation polymorphism 16S ribosomal RNA fingerprinting. The percentages of high nucleic acid-containing cells, evaluated by flow cytometry, were significantly correlated (r2 = 0.91, n = 12, p < 0.0001) to bacterial production, making this value a good predictor of active cell dynamics along the study. While confinement and temperature were the two principal experimental factors affecting bacterial community composition and dynamics, respectively, additions of freshwater viruses had significant effects on coastal bacterial communities. Thus, foreign viruses significantly reduced net bacterial population increase as compared to the enrichment treated with inactivated virus. Moreover, freshwater viruses recurrently and specifically affected bacterial community composition, as compared to addition of autochthonous viruses. In most cases, the combined treatment viruses and freshwater dissolved organic matter helped to maintain or even enhance species richness in coastal bacterial communities in agreement to the ‘killing the winner’ hypothesis. Thus, riverine virus input could potentially influence bacterial community composition of the coastal bay albeit with modest modification of bulk bacterial growth.

Key features of intertidal food webs that support migratory shorebirds.

The migratory shorebirds of the East Atlantic flyway land in huge numbers during a migratory stopover or wintering on the French Atlantic coast. The Brouage bare mudflat (Marennes-Oléron Bay, NE Atlantic) is one of the major stopover sites in France. The particular structure and function of a food web affects the efficiency of carbon transfer. The structure and functioning of the Brouage food web is crucial for the conservation of species landing within this area because it provides sufficient food, which allows shorebirds to reach the north of Europe where they nest. The aim of this study was to describe and understand which food web characteristics support nutritional needs of birds. Two food-web models were constructed, based on in situ measurements that were made in February 2008 (the presence of birds) and July 2008 (absence of birds). To complete the models, allometric relationships and additional data from the literature were used. The missing flow values of the food web models were estimated by Monte Carlo Markov Chain – Linear Inverse Modelling. The flow solutions obtained were used to calculate the ecological network analysis indices, which estimate the emergent properties of the functioning of a food-web. The total activities of the Brouage ecosystem in February and July are significantly different. The specialisation of the trophic links within the ecosystem does not appear to differ between the two models. In spite of a large export of carbon from the primary producer and detritus in winter, the higher recycling leads to a similar retention of carbon for the two seasons. It can be concluded that in February, the higher activity of the ecosystem coupled with a higher cycling and a mean internal organization, ensure the sufficient feeding of the migratory shorebirds.

Microbial interactions in marine water amended by eroded benthic biofilm: A case study from an intertidal mudflat

Abstract In shallow macrotidal ecosystems with large intertidal mudflats, the sediment–water coupling plays a crucial role in structuring the pelagic microbial food web functioning, since inorganic and organic matter and microbial components (viruses and microbes) of the microphytobenthic biofilm can be suspended toward the water column. Two experimental bioassays were conducted in March and July 2008 to investigate the importance of biofilm input for the pelagic microbial and viral loops. Pelagic inocula (

Viral Impact on Prokaryotic and Microalgal Activities in the Microphytobenthic Biofilm of an Intertidal Mudflat (French Atlantic Coast)

This 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 × 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 2–12% 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.

Short-term responses of unicellular planktonic eukaryotes to increases in temperature and UVB radiation

BackgroundSmall size eukaryotes play a fundamental role in the functioning of coastal ecosystems, however, the way in which these micro-organisms respond to combined effects of water temperature, UVB radiations (UVBR) and nutrient availability is still poorly investigated.ResultsWe coupled molecular tools (18S rRNA gene sequencing and fingerprinting) with microscope-based identification and counting to experimentally investigate the short-term responses of small eukaryotes (<6 μm; from a coastal Mediterranean lagoon) to a warming treatment (+3°C) and UVB radiation increases (+20%) at two different nutrient levels. Interestingly, the increase in temperature resulted in higher pigmented eukaryotes abundances and in community structure changes clearly illustrated by molecular analyses. For most of the phylogenetic groups, some rearrangements occurred at the OTUs level even when their relative proportion (microscope counting) did not change significantly. Temperature explained almost 20% of the total variance of the small eukaryote community structure (while UVB explained only 8.4%). However, complex cumulative effects were detected. Some antagonistic or non additive effects were detected between temperature and nutrients, especially for Dinophyceae and Cryptophyceae.ConclusionsThis multifactorial experiment highlights the potential impacts, over short time scales, of changing environmental factors on the structure of various functional groups like small primary producers, parasites and saprotrophs which, in response, can modify energy flow in the planktonic food webs.

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

A 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.