Duration of rapid ligth curves for determining the photosynthetic activity of microphytobenthos biofilm in situ

International audienceThis study is the first field work that examines the use of rapid light curves (RLCs) for in situ assessment of microphytobenthos (MPB) photosynthetic activity. The advantages offered by RLC methodology respond primarily to in situ constraints concerning MPB ecology and behaviour. In particular, the rapidity of RLCs allows for a high number of replicates during emersion, while preventing disturbance of the measurement due to MPB cell migration. It is necessary to maintain this feature in order to thoroughly assess the large spatial and temporal variability of in situ MPB biomass and photosynthetic activity. Consequently, working with intertidal mudflat MPB biofilms dominated by epipelic (motile) species, we investigated the effect of 10-s and 30-s irradiance steps: two durations commonly used for building RLCs. We compared the performance of 10-s and 30-s RLCs in determining the in situ MPB photosynthetic activity without dark acclimation and in high irradiances (summer). Although a similar trend of MPB cell photoacclimation was observed using the two procedures, there were differences, revealed by lower values of α-10 s and rETRmax-30 s compared with α-30 s and rETRmax-10 s on some experimental days (49% and 34%, respectively). Discrepancies could mainly be explained by the unbalanced QA redox state and the occurrence of photoprotective non-photochemical quenching (NPQ) present prior to RLC, which impacted the 10-s and 30-s RLCs differently. The initial slope (α) was strongly impacted by NPQ developed prior to RLCs. A correction performed a posteriori and based on the use of the maximum Fm, obtained during the course of RLCs was not sufficient to remove the bias of pre-existing NPQ on the determination of α completely. Consequently, a good, direct and simultaneous determination of rETRmax and α was virtually impossible in situ even when using RLCs. In contrast to published results from laboratory studies, the shorter irradiance steps (10 s) offers the better (i.e. the most reasonable) compromise for assessing MPB maximum photosynthetic capacity in situ using RLC.