Growth and distribution of marine bacteria in relation to nanoplankton community structure

Bacterial productivity and biomass were investigated along with nanoplankton community structure and environmental variables at a number of sites in the Gulf of St. Lawrence (mid-April and mid to late-June) and at additional sites off the coast of Nova Scotia (late June), eastern Canada. Total bacterial cell concentrations were determined in conjunction with actively respiring cells (ARCs) visualized using a redox fluorochrome (5-cyano-2,3-ditolyl tetrazolium chloride, CTC). Bacterial growth rates were estimated by 3H-thymidine uptake. There were strong seasonal differences in bacterial activity within the euphotic zone. The CTC assay indicated that the proportion of ARCs to total bacteria (BN) in the euphotic zone was lower in spring (1–4%) than summer (3–12%). In the aphotic zone bacterial growth (TdR-H3 uptake) was much lower than above and the proportion of ARCs was frequently <1% of total bacteria. Bacterial productivity and water temperature in the euphotic zone were positively correlated, while both ARCs and BN tended to be negatively correlated with inorganic nutrients. The proportion of ARCs was negatively correlated with the proportion of heterotrophic nanoflagellates and positively correlated with that of mixotrophic species. The proportion of ARCs and the apparent potential growth rates of ARCs varied with changes in nanoplankton community structure. Mixotroph-dominated communities were associated with bacterial communities that had a relatively high proportion of ARCs but with low apparent potential growth rates. Conversely, communities dominated by a mixture of phototrophs and heterotrophs had a low proportion of ARCs with high apparent potential growth rates. These observations suggest that nanoplankton community structure plays a major role in controlling bacterial abundance and activity in the sea.