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

A Talarmin,T Moutin,P Lebaron,F Van Wambeke

doi:10.5194/bg-12-1237-2015

Abstract

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.

Highlights

Understanding nutrient uptake strategies in microorganisms is a necessity to predict their biogeochemical response to environmental changes
The present study investigates the contribution of sorted picoplankton groups to total Pi uptake flux in the low Pi stratified upper water column of the Mediterranean Sea
soluble reactive phosphorus (SRP) concentrations were in the range of 6–80 nmol L−1, varying with depth and location (Table 1)

Summary

Introduction

Understanding nutrient uptake strategies in microorganisms is a necessity to predict their biogeochemical response to environmental changes. Studies concur on the high contribution of the small size fractions (< 0.8, < 1, < 2 or < 3 μm) to Pi uptake fluxes (Björkman and Karl, 1994; Currie et al, 1986; Moutin et al, 2002; Tanaka et al, 2003; Thingstad et al, 1993, 1998) This contribution generally increases in aquatic systems with short Pi turnover times and in low Pi systems after P amendments, emphasizing the idea that heterotrophic prokaryotes are high competitors in P-deficient areas (e.g., Björkman et al, 2012; Currie et al, 1986; Drakare, 2002; Labry et al, 2002). The development of combined radiolabeling techniques and cell sorting by flow cytometry has improved the level of resolution for studying Pi uptake strategies in heterotrophic and phototrophic microbes (e.g., Björkman et al, 2012; Casey et al, 2009; Duhamel et al, 2012; Lomas et al, 2014; Talarmin et al, 2011b; Zubkov et al, 2007)

Methods

Results

Discussion

Conclusion