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Aquatic Microbial Ecology

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AME 10:29-36 (1996)  -  doi:10.3354/ame010029

Loss rate of an oligotrophic bacterial assemblage as measured by H-thymidine and PO4: good agreement and near-balance with production

Thingstad TF, Dolan JR, Fuhrman JA

Growth and loss of planktonic bacteria are thought to be roughly in balance, but are rarely measured together. The loss rate for a bacterial assemblage in surface waters of Villefranche Bay (NW Mediterranean Sea) was estimated using 2 independent techniques. The disappearance rate of 3H from cold-TCA-insoluble material following a labeling of the natural assemblage with 3H-thymidine gave a turnover of 2.2% h-1, while the disappearance of 32P from the bacterial size fraction (0.2 to 1 um) following an initial uptake period and a subsequent cold chase with orthophosphate gave a bacterial turnover rate of 2.5% h-1. The similarity of the 2 estimates suggests that the same loss processes were measured and that processes independent of bacterial population turnover, such as rapid uptake and release of labels, were of minor importance. The mortality estimates were close to thymidine-based production estimates of 2 to 2.3% h-1. Viral abundance (ca 2 x 106 ml-1) was about 3 to 4 times that of bacteria, and relatively constant. Attempts to measure bacterial mortality due to viral infection were complicated by filtration artifacts. Passage of the thymidine-labelled assemblage through a 0.6 um filter in order to separate bacteria and viruses from larger bacterivorous organisms removed 60% of the bacterial label. Label loss rates were undetectable in the filtered assemblage over 96 h incubations, suggesting that viruses were minor loss agents of this (minority) size fraction of bacteria. In the experiments with 32P, most of the label was transferred directly from the bacterial size fraction to dissolved compounds, with relatively minor amounts (10 to 20%) transferred to larger size fractions.

Bacterial decay . Phosphate metabolism . Viruses

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