Aude Barani

Do human activities affect the picoplankton structure of the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia)?

The spatial variations of the picoplankton (photoautotrophic and heterotrophic microorganisms) in the Ahe atoll lagoon were studied in May and October 2008 to assess whether they were affected by human activities along the atoll. Spatial patterns were studied using 10 sampling stations chosen according to the location of the anthropogenic activities (pearl farming, harbor). Experiments were also carried out to determine whether bacterial growth, with or without predators, was limited by inorganic (N and P) substrates. The results showed that heterotrophic bacterioplankton abundance was superior to the photoautotrophic organisms, especially in May. Significant increases in bacterial abundance were observed in May after 24 h incubation with +P and +N (but not in October). All samples complied with the quality levels for fecal indicator bacteria (FIB) defined by the European Union and there was no evidence that human sewage had any impact on picoplankton over the whole atoll.

Seasonal distribution of ultraphytoplankton and heterotrophic prokaryotes in relation to abiotic variables on the north coast of Sfax after restoration.

The Taparura project was set up to restore the north Sfax coast (Tunisia) by shutting down the northern phosphate plant responsible for chronic pollution and uncontrolled phosphogypsum dumping. The restoration effect on coastal ultraphytoplankton (<10 μm) and heterotrophic prokaryotes was investigated using conventional flow cytometry over four successive seasons during 2009-2010. Cell concentrations were generally higher than values reported for the open sea, both in the western and eastern Mediterranean basins. One striking point was that chl a concentration on the north Sfax coast was unchanged after restoration but was still one order of magnitude higher than in the Gulf of Gabès. Restoration of pH, following the shutdown of the phosphate processing plants on the north coast, appeared to reach normal levels for seawater during the study, whereas seawater acidification persisted on the south coast where plants are still in operation. The largest ultraphytoplankton biomass was from an unknown cell group, whose identity and role needs to be established.

Spatio-Temporal Variations of Marine Biofilm Communities Colonizing Artificial Substrata Including Antifouling Coatings in Contrasted French Coastal Environments

Surface colonization in seawater first corresponds to the selection of specific microbial biofilm communities. By coupling flow cytometry, microscopy and high throughput sequencing (HTS, 454 pyrosequencing) with artificial surfaces and environmental analyses, we intend to identify the contribution of biofilm community drivers at two contrasted French sites, one temperate and eutrophic (Lorient, Atlantic coast) and the other at a mesotrophic but highly contaminated bay (Toulon, North-Western Mediterranean Sea). Microbial communities were shaped by high temperatures, salinity and lead at Toulon by but nutrients and DOC at Lorient. Coatings including pyrithione exhibited a significant decrease of their microbial densities except for nanoeukaryotes. Clustering of communities was mainly based on the surface type and secondly the site, whereas seasons appeared of less importance. The in-depth HTS revealed that γ- and α-proteobacteria, but also Bacteroidetes, dominated highly diversified bacterial communities with a relative low β-diversity. Sensitivity to biocides released by the tested antifouling coatings could be noticed at different taxonomic levels: the percentage of Bacteroidetes overall decreased with the presence of pyrithione, whereas the α/γ-proteobacteria ratio decreased at Toulon when increased at Lorient. Small diatom cells (Amphora and Navicula spp.) dominated on all surfaces, whereas site-specific sub-dominant taxa appeared clearly more sensitive to biocides. This overall approach exhibited the critical significance of surface characteristics in biofilm community shaping.

Microbial composition and ecological features of phototrophic biofilms proliferating in the Moidons Caves (France): investigation at the single-cell level

The authors investigated the microbial composition of phototrophic biofilms proliferating in a show cave using flow cytometry for the first time in such a context. Results are based on several biofilms sampled in the Moidons Caves (France) and concern both heterotrophic prokaryotes and autotrophic microorganisms. Heterotrophic microorganisms with low nucleic acid content were dominant in biofilms, as can be expected from the oligotrophic conditions prevailing within the cave. Analysis of the biofilm autotrophic components revealed the presence of several taxa, particularly the unicellular green algae Chlorella minutissima, specifically well adapted to this cave. Relationships between flow cytometry results and environmental variables determined in the cave were established and discussed so as to better understand biofilm proliferation processes in caves.

Evolution of the scattering properties of phytoplankton cells from flow cytometry measurements.

After the exponential growth phase, variability in the scattering efficiency of phytoplankton cells over their complete life cycle is not well characterised. Bulk measurements are impacted by senescent cells and detritrus. Thus the analysis of the evolution of the optical properties thanks to their morphological and/or intra-cellular variations remains poorly studied. Using the Cytosense flow cytometer (CytoBuoy b.v., NL), the temporal course of the forward and sideward efficiencies of two phytoplankton species (Thalassiosira pseudonana and Chlamydomonas concordia) were analyzed during a complete life-cycle. These two species differ considerably from a morphological point of view. Over the whole experiment, the forward and sideward efficiencies of Thalassiosira pseudonana were, on average, respectively 2.2 and 1.6 times higher than the efficiencies of Chlamydomonas concordia. Large intra-species variability of the efficiencies were observed over the life cycle of the considered species. It highlights the importance of considering the optical properties of phytoplankton cells as a function of the population growth stage of the considered species. Furthermore, flow cytometry measurements were combined with radiative transfer simulations and biogeochemical and optical measurements. Results showed that the real refractive index of the chloroplast is a key parameter driving the sideward signal and that a simplistic two-layered model (cytoplasm-chloroplast) seems particularly appropriate to represent the phytoplankton cells.