Sophie Richier

Abundances of Iron-Binding Photosynthetic and Nitrogen-Fixing Proteins of Trichodesmium Both in Culture and In Situ from the North Atlantic

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high iron-flux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean.

Carbon export efficiency and phytoplankton community composition in the Atlantic sector of the Arctic Ocean

Arctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234-thorium based estimates of the export flux of POC to onboard incubation-based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE-ratio) showed large variability (0.09 ± 0.19–1.3 ± 0.3). The highest ThE-ratio (1.3 ± 0.3) was recorded in a mono-specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE-ratios (0.1 ± 0.1–0.5 ± 0.2), while mixed and/or prebloom communities showed lower ThE-ratios (0.10 ± 0.03–0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralized in the upper mesopelagic ( 100 m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings.

Cnidarian–Dinoflagellate Symbiosis-Mediated Adaptation to Environmental Perturbations

The symbiotic relationships between corals and their phototroph dinoflagellate, Symbiodinium sp., are at the basis of the ecology of coral reefs flourishing in tropical waters. While coral reefs present a very high biological and economic value, they are presently under stress due to climate change (seawater temperature increase and acidification). In order to live together, animal hosts and their symbionts have evolved and acquired specific adaptations ranging from trophic exchange to acquisition of new mechanisms of CO2- and N-compound uptake, to hyperoxia tolerance and more. We think that these mutualistic relationships have also produced tolerance towards environmental stress.