Single-cell physiological structure and growth rates of heterotrophic bacteria in a temperate estuary (Waquoit Bay, Massachusetts)

Abstract

Flow cytometric determinations of membrane integrity, nucleic acid content, and respiratory activity were combined with dilution cultures in Waquoit Bay Estuary (Massachusetts) to estimate specific growth rates of total, live, high (HNA), and low (LNA) nucleic acid content and actively respiring (CTC+) cells. Bacterial abundance ranged from 106 to 107 cells mL-1, with live cells generally contributing > 85% to total numbers, 42-82% HNA cells, and 3-36% CTC+ cells. Specific growth rates (µ) from all physiological groups were positively correlated, but they showed different temperature dependences, with activation energies ranging from 0.28 (live) to 0.97 eV (LNA). The µ values of live cells (0.14-2.40 d-1) were similar to those of total bacteria (0.06-1.53 d-1). LNA bacteria were not dormant but showed positive growth in most experiments, although HNA cells greatly outgrew LNA cells (µ ranges of 0.28-2.26 d-1 vs. 0-0.69 d-1), and CTC+ cells showed the highest values (0.12-2.65 d-1). Positive correlations of HNA bacteria µ with total and phytoplankton-derived dissolved organic carbon support the previously hypothesized strong bottom-up control of HNA cells. Bacterial production estimated from leucine incorporation and empirical conversion factors agreed well with estimates based on growth rates. HNA cells were always responsible for the largest share of bacterial production in the estuary. The contribution of CTC+ cells significantly increased with temperature in the 7-27°C range, reaching values of 40% at temperatures higher than 20°C.