Dynamics of transparent exopolymeric particles and their precursors during a mesocosm experiment: Impact of ocean acidification

- A large contribution of TEP to organic carbon.
- A substantial contribution of ultraphytoplankton to phytoplankton carbon pool.
- No effect of ocean acidification on TEP, TEP precursors and size distribution.
- Shift in ultraphytoplankton community during the experiment.
- Vast production of TEP precursors by Synechococcus and/or TEP degradation.

The dissolution of anthropogenic atmospheric CO2 in seawater is suspected to exert a strong pressure on biological processes as well as on carbon and nutrient cycles. Transparent exopolymeric particles (TEP) are polysaccharide particles, formed by aggregation of polymers exuded by phytoplankton and strongly involved in organic matter sedimentation. A mesocosm experiment was performed from February to March 2013 in the coastal waters of the Northwestern Mediterranean Sea, a region characterised by low-nutrient low-chlorophyll (LNLC) levels. We aimed to determine the effect of ocean acidification on the organic carbon pool of TEP produced by a natural phytoplankton community. The experiment was conducted in nine mesocosms of 50 m3 deployed for 12 days, and subjected to seven partial pressures of CO2 (pCO2) levels: one control level in triplicate and six elevated levels between 450 and 1250 μatm. The use of different analytical methods allowed the assessment of TEP density, volume concentration and size distribution as well as both TEP and TEP precursors carbon content. TEP contributed vastly to the particulate organic carbon pool (∼62%), and were mainly produced by small-sized phytoplankton such as pico- and nanophytoplankton. TEP precursors carbon content represented three times the carbon content of particulate TEP, showing that this pool has to be considered in experiments focused on the environmental control of TEP production. There was no evidence that TEP and TEP precursors were dependent on pCO2. These parameters exhibited clear temporal dynamics, with tight links to community composition, nutrient availability and other environmental parameters.