Abstract
Summary Characterizing both growth and abundance is important in understanding the role of bacterial communities in biogeochemical cycling of global oceans. However, these two quantities are seldom measured together for specific bacterial clades. Our goal was to examine growth and abundance of three gammaproteobacterial subgroups, including SAR 86, at the single‐cell level by microautoradiography combined with fluorescence in situ hybridization ( FISH ) in coastal waters of the west A ntarctic P eninsula region during two austral summers and one austral fall. We found that the SAR 86 clade was less abundant and grew more slowly than two related gammaproteobacterial clades, A nt4 D 3 and A rctic96 B ‐16. Over 60% of A nt4 D 3 and A rctic96 B ‐16 cells incorporated leucine, while only 25% of SAR 86 cells were active in both summer and fall. We also explored using the size of the FISH image as another measure of single‐cell activity. There was a linear relationship between FISH cell size and incorporation of leucine for all bacteria, A nt4 D 3 and A rctic96 B ‐16, but not for SAR 86. FISH sizes of SAR 86 cells were at least threefold smaller than cells in the other clades. Our results suggest slow growth of SAR 86 in the perennially cold waters of the west Antarctic Peninsula.