Articles | Volume 12, issue 4
Biogeosciences, 12, 1237–1247, 2015
Biogeosciences, 12, 1237–1247, 2015

Research article 26 Feb 2015

Research article | 26 Feb 2015

Vertical partitioning of phosphate uptake among picoplankton groups in the low Pi Mediterranean Sea

A. Talarmin1,2,*, F. Van Wambeke1,2, P. Lebaron3,4, and T. Moutin1,2 A. Talarmin et al.
  • 1Aix Marseille Université, Mediterranean Institute of Oceanography (MIO), 13288, Marseille, CEDEX 9, France
  • 2Université de Toulon, 83957, CNRS-INSU/IRD, France
  • 3Sorbonne Universités, UPMC Univ. Paris 06, USR 3579, LBBM, Observatoire Océanologique, 66650, Banyuls-sur-Mer, France
  • 4CNRS, USR 3579, LBBM, Observatoire Océanologique, 66650 Banyuls-sur-Mer, France
  • *now at: Red Sea Research Center, 4700 King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia

Abstract. Microbial transformations are key processes in marine phosphorus cycling. In this study, we investigated the contribution of phototrophic and heterotrophic groups to phosphate (Pi) uptake fluxes in the euphotic zone of the low-Pi Mediterranean Sea and estimated Pi uptake kinetic characteristics. Surface soluble reactive phosphorus (SRP) concentrations were in the range of 6–80 nmol L−1 across the transect, and the community Pi turnover times, assessed using radiolabeled orthophosphate incubations, were longer in the western basin, where the highest bulk and cellular rates were measured. Using live cell sorting, four vertical profiles of Pi uptake rates were established for heterotrophic prokaryotes (Hprok), phototrophic picoeukaryotes (Pic) and Prochlorococcus (Proc) and Synechococcus (Syn) cyanobacteria. Hprok cells contributed up to 82% of total Pi uptake fluxes in the superficial euphotic zone, through constantly high abundances (2.7–10.2 × 105 cells mL−1) but variable cellular rates (6.6 ± 9.3 amol P cell−1 h−1). Cyanobacteria achieved most of the Pi uptake (up to 62%) around the deep chlorophyll maximum depth, through high abundances (up to 1.4 × 105 Proc cells mL−1) and high cellular uptake rates (up to 40 and 402 amol P cell−1 h−1, respectively for Proc and Syn cells). At saturating concentrations, maximum cellular rates up to 132 amol P cell−1 h−1 were measured for Syn at station (St.) C, which was 5 and 60 times higher than Proc and Hprok, respectively. Pi uptake capabilities of the different groups likely contribute to their vertical distribution in the low Pi Mediterranean Sea, possibly along with other energy limitations.

Short summary
Along a transect in the warm nutrient-depleted Mediterranean Sea, we found that microorganisms shared phosphate resources by using different uptake strategies. Heterotrophic prokaryotes dominated phosphate uptake in the upper layers while cyanobacteria dominated uptake fluxes around the deep chlorophyll maximum depth. Synechococcus seemed well equipped to respond to pulses of phosphate, whereas Prochlorococcus and heterotrophs were more adapted to very low concentrations.
Final-revised paper