Microbial growth rates in a cold-core gulf stream eddy of the northwestern Sargasso sea

Abstract

Horizontal spatial variations of microbial and bacterial abundance and biomass, and rates of nucleic acid synthesis in surface waters across cold-core Gulf Stream eddy P were investigated in April 1982. The age of eddy P was estimated at 8 to 10 months. Chlorophyll a reached 0.38 μg l −1 in an area of high chlorophyll a fluorescence located in 18 to 19°C waters. Bacteria were mostly (98%) free-floating, and microbial-ATP biomass correlated with chlorophyll a fluorescence across eddy P. Rates of microbial metabolism and growth, and bacterial production were not proportional across areas of high chlorophyll a fluorescence. Estimates of microbial growth rates (δ), were 0.3 to 0.6 h −1 in the mixed surface waters of eddy P and 0.06 to 0.08 h −1 in adjacent waters of the northwestern Sargasso Sea. Bacterial production ranged from 0.12 to 1.41 μgC l −1 d −1, and production: biomass ratios (turnover times) were 0.024 to 0.27 h −1 in the eddy and ca. 0.002 h −1 in the Sargasso Sea. Because microbial metabolism (RNA synthesis) and growth (DNA synthesis) across eddy P were not closely coupled, although intertwined with diurnal variations, we conclude that the microbial communities, based on the experimental time scale, were in a state of unbalanced growth.