Orthophosphate uptake by heterotrophic bacteria, cyanobacteria, and autotrophic nanoflagellates in Villefranche Bay, northwestern Mediterranean: Vertical, seasonal, and short-term variations of the competitive relationship for phosphorus

Abstract

Previous studies have suggested that Mediterranean surface water becomes phosphorus limited for both bacteria and phytoplankton during stratified periods and that orthophosphate uptake in these situations was close to diffusion limitation for both cyanobacteria and autotrophic nanoflagellates. In order to better understand vertical and seasonal variations of this system, we measured orthophosphate uptake by bacteria and phytoplankton in Villefranche Bay (northwestern Mediterranean) monthly for the 0-75-m layer and weekly or biweekly at 10 m from June to December 2002. Turnover time of orthophosphate was relatively short (<2 h) in the surface mixed layer (0-30 m) during the stratified period, and long (>30 h) in the deeper layer during the stratified period and in the whole water column during the mixing period. Short-term fluctuations in turnover time were repeatedly observed from the stratified through the mixing periods. The dominance of PO4 uptake drastically shifted from both bacteria and cyanobacteria to cyanobacteria when there were slight increases in turnover time (1-10 h). Compared to the theoretical maximum calculated for diffusion limitation, mean affinity constants at 10 m were similar for autotrophic nanoflagellates and greater for cyanobacteria in situations with turnover time <2 h, but observed values were much smaller than theoretical for heterotrophic bacteria even in samples with turnover time <1 h.