Bacterioplankton of low and high DNA content in the suboxic waters of the Arabian Sea and the Gulf of Oman: abundance and amino acid uptake

Abstract

Amino acid uptakes by bacterioplankton of low DNA (LNA) and high DNA (HNA) con- tent, populating the oxygen minimum zone (OMZ: 0.998, p < 0.0001) with leucine incorporation into protein and with microbial glucose uptake, suggesting that bacterioplankton growth was controlled by dissolved organic matter, and that methionine uptake could be used as a general estimate for the metabolic activity of bacterioplankton. The varia- tion in methionine uptake depended on the prokaryote group rather than on ambient oxygen concen- tration, e.g. the numerically dominant LNA cells took 3 to 5 times less precursor than the HNA cells. A percentage of the LNA cells with double the amount of DNA was proposed as an incubation-inde- pendent index of growth of the cells in the G2 stage of the cell cycle. The vertical profiles of the per- centage of LNA cells in G2 showed pronounced peaks at 300 to 600 m in the OMZ that did not corre- late with peaks of either total bacterioplankton abundance or productivity. The present paper underlines the importance of bacterioplankton group studies in the OMZ since high microbial cell abundance does not necessarily mean high metabolic activity and other mechanisms, such as resilience to mortality pressure, have to be investigated.