Heterotrophic bacterial activity and growth rates in sediments of the continental margin of eastern Australia

Abstract

The numbers, growth rates and productivity of heterotrophic bacteria were determined in sediment cores collected on the East Australian continental margin (28 and 32°S) at water depths ranging from 150 to 1000 m. The study was centered on a zone of modern phosphate nodule formation (350–460 m water depth), as bacteria had previously been implicated in the genesis of the modern phosphorites in this area. Numbers of bacteria in surface sediment ranged from 2 × 10 8 g −1 at 200 m water depth to 8 × 10 8 g −1 at about 400 m and 2 × 10 9 g −1 at 1000 m. Numbers decreased exponentially with depth in the sediments to 2 × 10 6 g −1 at 1 m depth. Bacterial growth rates, determined from rates of tritiated thymidine incorporation into DNA, were about 0.01–0.002 h −1 in the upper 10 mm of sediment from 150 to 460 m; 0.004 h −1 at 600 m, and 0.0005 h −1 at 1000 m. Bacterial productivity was greatest (80 mg C m −2 day −1 or 243 μmol C cm −2 y −1 integrated over the top 10 cm sediment depth) in the zone of modern phosphorite formation. About 80% of bacterial productivity occurred in the upper 2 cm of sediment, although bacteria found at depths up to 70 cm in the sediment were capable of metabolic activity. Rates of phospholipid and protein synthesis were also determined and, when converted to productivity, the values in the upper 2 cm of sediment were mostly within a factor of 2 or 3 of values determined from DNA synthesis. Similar values were obtained for the flux of the organic carbon into the sediments, required to support metabolic activity, whether calculated from values for bacterial productivity or from a geochemical model of oxygen diffusion and consumption, using profiles of oxygen concentration in the sediments. The organic carbon input into the sediments in the zone of modern phosphorite formation was much higher than anticipated, probably because the zone acts as a focus for the deposition of organic matter produced on the continental shelf. Rapid bacterial degradation of the organic matter in these sediments resulted in a shallow transition from oxic to anoxic conditions and to the release of phosphate to the pore-waters. These factors, in conjunction with sediment mixing and Fe-cycling processes, appear to be critical to phosphorite formation in this area.