Exploring the relationship between active bacterioplankton and phytoplankton in the Southern Ocean

Bacterioplankton are a heterogenous community composed of cells with different physiological states. The consideration of the active fraction of bacterioplankton as a potential factor affecting the strength of the relationship between bacteria and phytoplankton in the Southern Ocean was evaluated in waters around the Antarctic Peninsula. We estimated active bacterioplankton from uptake of H-3-Leucine (bacterial production [BP]) and using vital stains to estimate their proportion within the total bacterioplankton community, based on their relative nucleic acid content (high [HNA] vs. low [LNA]), and by nucleic acid double staining (NADS), based on their membrane permeability. Then we performed a comparative analysis between total and active bacterioplankton and chlorophyll a (chl a) in this area. Staining with NADS suggested that 61 % of all bacteria were viable, a higher proportion of the total bacterial community than previously reported for the Southern Ocean. HNA bacteria comprised 45 % of all bacteria, indicating that 16 % of bacteria may be viable but with LNA. BP was more strongly related to abundance of LNA cells than NADS-viable or HNA bacteria. The relationship between chl a and bacterial abundance (BA) did not increase when considering the abundance of HNA or NADS-viable cells alone, showing that viability/activity of stains did not enhance the linkage between BA and phytoplankton biomass in the Southern Ocean. In contrast, the relationship between chl a and BP was stronger than those reported in the literature, suggesting that, in this region, BP is closely dependent on phytoplankton.