Determining the availability of phosphate and glucose for bacteria in P-limited mesocosms of NW Mediterranean surface waters

Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria was studied in mesocosms using NW Mediterranean coastal water with added phosphate, glucose, or a combination of both. We observed an initial increase of bacterial production in all mesocosms, a continuous increase of particulate P only in the P-enriched mesocosms, and a greater accumulation of dissolved organic C in the glucose-enriched mesocosms compared to those enriched by both glucose and P. This suggests that the water used was initially P-starved with a certain pool of degradable organic C available for bacteria. Specific phosphate affinities indicated a P limitation for the bacterial community in all mesocosms. Specific glucose affinities were different between the mesocosms but much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that the glucose pool was not a strong controlling factor of bacterial growth. In the P-enriched mesocosms, it is indicated that the extent of P limitation shifted from highest to lowest, while the available pool of glucose steadily decreased during the experiment. The explanation suggested for these observations is that phosphate regeneration was enhanced in the plankton food web, by which bacterial carbon demand became higher than the degradable organic C produced in the P-enriched mesocosms.