Vertical partitioning of phosphate uptake among picoplankton groups in the P-depleted Mediterranean Sea

A. Talarmin, F. Van Wambeke, P. Lebaron, T. Moutin
2014 Biogeosciences Discussions  
Radiolabeled orthophosphate (P<sub>i</sub>) incubations coupled with cell sorting were conducted in the Mediterranean Sea to assess the contribution of picoplanktonic groups to total P<sub>i</sub> uptake and to potentially explain their spatial distribution. Under natural P<sub>i</sub> concentrations (P-deficient and stratified conditions during the survey), total P<sub>i</sub> uptake was dominated in the surface by heterotrophic prokaryotes (Hprok; up to 82%) and shifted to a cyanobacterial
more » ... a cyanobacterial dominance around the deep chlorophyll maximum depth. These experiments were completed with concentration bioassays at 4 stations to determine P<sub>i</sub> uptake kinetic constants (maximum cell-normalized uptake rates <i>V</i><sub>max</sub> and the half-saturation constant plus natural concentration <i>K</i> +<i>S<sub>n</sub></i>) in picophytoeukaryotes (Pic), <i>Synechococcus</i> (Syn), <i>Prochlorococcus</i> (Proc) and Hprok. Syn cells were the most efficient in the uptake of P<sub>i</sub> at the cellular level at saturating concentration (100 nmol L<sup>-1</sup>). Indeed, Syn displayed the highest <i>V</i><sub>max</sub> (up to 132 amol P cell<sup>−1</sup> h<sup>-1</sup>) compared to other groups like Hprok (<i>V</i><sub>max</sub> up to 2 amol P cell<sup>−1</sup> h<sup>-1</sup>), and the lowest <i>K</i> + <i>S<sub>n</sub></i>. This suggests that they could be quickly reactive to a pulsed supply of P<sub>i</sub>. Hprok and Proc cells seemed more adapted to take up P<sub>i</sub> at low concentrations, with low <i>K</i> + <i>S<sub>n</sub></i> values (2.9 to 22.7 nmol P L<sup>-1</sup>), conferring them a nutritional advantage under P-deficient conditions. Such characteristics may explain the successful year-long coexistence of osmotrophic autotrophs and heterotrophs in the Mediterranean Sea and their vertical distribution in the photic zone.
doi:10.5194/bgd-11-14639-2014 fatcat:6qtl4zdctna5dglhak5jpvncoy