Bacterial growth in deep-sea sediment trap and boxcore samples

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Abstract
This study was to determine whether heterotrophic bacteria associated with deep-sea particulates are adapted more to the moderate temperatures and pressures of surface waters or to the extremes of the deep sea, and how such microorganisms respond to substrate enrichment. Samples of sinking particulates, fecal pellets, and deposited sediments were collected in bottom-moored sediment traps and boxcores at station depths of 1850, 4120, and 4715 m in the North Atlantic. Homogenized seawater suspensions of samples were incubated for 2 to 7 d under both shallow and deep-sea temperatures and pressures, with and without substrate enrichment (yeast extract or chitin). Increases in total bacterial number or percent dividing cells were measured by epifluorescence microscopy. Probable origins of bacteria in a given sample were evaluated according to temperature and pressure regimes affording bacterial growth. With a few exceptions, results indicated a predominance of shallow water bacteria in sediment-trap, but not boxcore, samples and an increasingly significant fraction of deep-sea bacteria, adapted to low temperature and elevate pressure, in fecal pellet samples trapped at increasing depth. Under deep-sea conditions, bacterial doubling times in sediment core samples were weeks or months, regardless of substrate enrichment, while in some trap samples were days (> 1.5) without enrichment, and hours (7.4 to 14) with enrichment. Doubling times of barophilic bacteria, isolated in pure culture from the deeper trap and core samples, ranged from 6 to 13 h under in situ temperature and pressure. These findings suggest that particulate organic matter, prior to its burial in abyssal sediments, is altered by indigenous, deep-sea bacteria, some of which are capable of rapid activity at low temperature and elevated pressure.