The Biological Carbon Pump: 2 key silicifiers – BioPSis

The ocean is an important climate regulator, absorbing about one third of the anthropogenic carbon (C) through its physical and biological pumps. Therefore, without the biological carbon pump (BCP), today's atmospheric C concentration would be ~50% higher. BCP strength varies with primary production intensity, phytoplankton community structure, formation or breakage of fast sinking particles and remineralization by grazing or by the microbial loop. With a global share of more than 40% of the primary production, diatoms are key players for the BCP. They predominated in eutrophic ecosystems and HNLC (High Nutrient Low Chlorophyll) and build biogenic silica (bSiO2) shell, called frustule. This biomineral is denser than the seawater. Therefore, when diatoms aggregate due to sticky transparent expopolymeric particles (TEP) accumulation or when they get incorporated into faecal pellets, their frustules ballast the large particles formed. Cyanobacteria are ubiquitous phytoplankton dominating oligotrophic ecosystems. Their role in the BCP may have been overlooked. In fact, and surprisingly, like diatoms they may accumulate silicon, they are able to form colonies and even aggregates. The latter can either sink directly to deep oceanic layers or be grazed by larger zooplankton, which are fast sinking faecal pellet-producers. According to the IPCC 2007, long-term sequestration of carbon in the ocean requires removal from the atmosphere for over 100 years, a criterion which is met after carbon is transported below the depth of 1000 m, i.e. below the mesopelagic layer. It appears that diatoms are efficient at exporting carbon, but the proportion of this exported C reaching the sequestration depth, i.e. the transfer efficiency, varies between the different oceanic areas, and could be low in high latitude. Contrariwise, large oligotrophic zones do not export much C to the top of the mesopelagic layers but are efficient at transferring it to the sequestration depth. The processes explaining these differences are poorly understood which challenge our ability to predict how the BCP efficiency will evolve with climate change. Models predict that rising stratification will accentuate nutrient limitations of the primary production and consequently, the contribution of the cyanobacteria to primary production will increase at the expense of diatoms. Are these predictions reliable since models may fail to represent some consequences of nutrient limitations? BioPSis proposes to study the variability of the main processes driving the C sequestration comparing the diatom dominated ecosystem of the Arctic Ocean, to the cyanobacteria's dominated ecosystem of the Sargasso sea. The project proposes to re-evaluate the role of diatom and cyanobacteria, i.e. two key players in the BCP in the context of the climate change. To do so, BioPSis gathers a team of young researchers who propose a complementary set of process studies, observations and modeling to investigate 1) how nutrient limitations change the structure of the diatom biogenic silica, and the form in which silicon is accumulated in cyanobacteria, 2) will compare the ability of these two phytoplankton groups to aggregate and their sinking velocities under different limitations, and 3) how their palatability for mesozooplankton change when their silicification or silicon accumulation vary. BioPSis will clearly established if nutrient limitations and silicification variability explain why polar ecosystems sequester less carbon than upwelling or coastal areas and if silicon accumulation in cyanobacteria contribute to the high transfer efficiency in oligotrophic waters. Altogether, BioPSis results will highly contribute to improve estimates of the future BCP, by identifying, describing and quantifying key processes involved to implement 3D models.