In situ experimentation at the water/sediment interface in the deep sea: 2. Biotransformation of dissolved organic substrates by microbial communities at 2000m depth in the Bay of Biscay

Abstract

Few attempts have been made to quantify the utilization of organic matter by the bacteria of the superficial layers of deep sea sediment. During two BIOCYAN cruises (August 1986 and June 1987) we used the submersible Cyana, to incubate sediment samples in situ in a specially designed box core in presence of 14C-glutamic acid and 3H leucine. These experiments were conducted at 2000m depth in the Bay of Biscay. Bacterial activity was stopped by the injection of formaldehyde. Samples were retrieved with the research submersible Cyana and its accompanying free vehicle shuttle. Sediment organic matter was fractioned into four components: 1) 14CO 2; 2) nucleic components and polysacharids; 3) labile proteins; and 4) condensed hydrolysable polymers. To evaluate the barotolerance of deep-sea bacteria, undisturbed superficial layer samples were also incubated with the same labelled substrates at 4°C at the atmospheric pressure. In both cases, and except for glucose, our results show that distributions of radioactivity in the different components of the organic material were almost similar. However, the rate of incorporation was usually higher for in situ experiments than for decompressed samples. Bacterial utilization of both 14C glutamic acid and 14C glucose were higher in June than in August. Such differences may result from changes in the food supply arriving as sinking particles and deriving from the photosynthetically productive surface waters. Food input was probably more important in June than in August leading to corresponding increases in: 1) the abundance of derived bacteria, and 2) deep-sea bacterial activities.