AME 43:23-32 (2006)  -  doi:10.3354/ame043023

ABSTRACT: Amino acid uptakes by bacterioplankton of low DNA (LNA) and high DNA (HNA) content, populating the oxygen minimum zone (OMZ: <5 µM O₂) and adjacent oxygen-depleted waters (5 to 50 µM O₂), were determined using a ³⁵S-methionine precursor and flow cytometric sorting. The HNA cells were further differentiated into low light scatter (HNA-ls) and high cell light scatter (HNA-hs) groups. Total bacterioplankton methionine uptake strongly correlated (r > 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 variation in methionine uptake depended on the prokaryote group rather than on ambient oxygen concentration, 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-independent index of growth of the cells in the G₂ stage of the cell cycle. The vertical profiles of the percentage of LNA cells in G₂ showed pronounced peaks at 300 to 600 m in the OMZ that did not correlate 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.

KEY WORDS: Bacterioplankton · Flow cytometric sorting · Oxygen minimum zone · Isotopic tracer · Metabolic activity