Marine bacterioplankton show bursts of rapid growth induced by substrate shifts

Diel dynamics of marine bacterioplankton was monitored at sampling sites in the Bothnian Sea (NB1) and the Mediterranean Sea (Villefranche Bay). At both locations, considerable diel variation was found in bacterial production, with highest values recorded during midday and lower values during night and early morning. The abundance of bacteria scored by the number of nucleoid-containing cells (NUCC) also changed during the day, with minimum numbers during midday. The bacterial turnover rate based on thymidine incorporation and NUCC varied between 0.22-0.31 and 0.48-2.40 d(-1) in the Bothnian Sea and Mediterranean, respectively. There was a significant positive correlation between the community turnover rate and phosphate concentration both in the Bothnian Sea and in the Mediterranean. In the Mediterranean, nutrient addition experiments showed that phosphate was the least available 'commodity' for the bacterial community. Combined addition of phosphate and glucose stimulated bacterial growth more than a single addition of either substrate alone, reaching a bacterial turnover of 2.4 d(-1). This value was equal to the maximum turnover rate found in situ during the daily cycle. Our results suggest that bacterioplankton rapidly adjust their growth rate in response to did changes in nutrient supply.