Bacterial growth rates, production and estimates of detrital carbon utilization in deep-sea sediments of the Solomon and Coral Seas

Abstract

Surface (0–5 mm) sediments at 10 bathyal and abyssal (695–4350 m) stations were sampled to determine variations in bacterial densities, productivity (tritiated thymidine incorporation at simulated in situ conditions) and specific growth rates (μ) in relation to environmental conditions and phytodetritus in the Solomon and Coral Seas. bacterial direct counts ranged from 2.5 to 60.6 × 10 8 cells g −1 sediment dry wt, but did not decline significantly with ocean depth or correlate with any other variables measured. Bacterial productivity was generally low, ranging from 34 to 7010 μg C m −2 d −1, but specific growth rates varied widely, ranging from 0.001 to 0.12 d −1. Incubation of samples at in situ temperature and atmospheric pressure at two stations resulted in either no detectable growth or significantly decreased thymidine incorporation, indicating a barophilic response. Quantities of macroalgal detritus were found at stations closest to reefs, whereas vascular plant and wood debris were found at most of the other stations, indicating that some detrital material transported from adjacent reefs and by massive riverine export from Papua New Guinea reaches the deep-sea floor in both seas. Significant quantities of sedimentary chlorophyll α and phaeo-pigments also were detected, indicating some deposition of phytoplankton-derived detritus. The fluxes of phytoplankton-derived, detrital carbon to the benthos were estimated (using available plankton data and the flux equation of Suess) as on the order of 2–23 mg C m −2 d −1. Assuming a median carbon assimilation efficiency of 50%, utilization of this material by sedimentary bacteria was estimated to vary widely (2–97%), averaging 40% oof total detrital carbon flux. Rates of bacterial productivity declined significantly with water depth and correlated with no other factors, but specific growth rates correlated positively with the standing amounts of macroalgal debris and were comparatively rapid (= turnover times of 6–63 days) only at the sites where this detritus was found. These results suggest that factors traditionally invoked as regulating bacterial activity in the deep sea may not always be applicable, particularly for habitats receiving significant quantities of fresh algal detritus.