Nucleic acid concentrations (DNA, RNA) in the continental and deep-sea sediments of the eastern Mediterranean: relationships with seasonally varying organic inputs and bacterial dynamics

In order to study temporal variations of the genetic material in the contin ental shelf and deep-sea sediments of the extremely oligotrophic Cretan Sea , samples were collected on seasonal basis from August 1994 to September 19 95, with a multiple corer, at seven stations (from 40 to 1540 m depth). Sur face sediments (0-1 cm) were sub-sampled and analyzed for nucleic acid cont ent (DNA, RNA) and bacterial density. DNA concentrations in the sediments w ere high (on annual average, 25.0 mu g g(-1)) and declined with increasing water depth, ranging from 3.5 to 55.2 mu g g(-1). DNA concentrations displa yed wide temporal changes also at bathyal depths confirming the recent view of the large variability of the deep-sea environments. Also RNA concentrat ions decreased with increasing water depth (range, 0.4-29.9 mu g g(-1)). Th e ratio of RNA to DNA did not show a clear spatial pattern but was characte rized by significant changes between sampling periods. DNA concentrations w ere significantly correlated with protein and phytopigment concentrations i n the sediment, indicating a possible relationship with the inputs of prima ry organic matter from the photic layer. Bacterial densities were generally high (range: 0.9-4.6 x 10(8) cells g(-1)) compared to other deep-sea envir onments and decreased with increasing water depth. Estimates of the bacteri al contribution to the sedimentary genetic material indicated that bacteria l-DNA accounted, on annual average, for a small fraction of the total DNA p ool (4.3%) but that bacterial-RNA represented a significant fraction of the total sedimentary RNA (26%). Bacterial contribution to nucleic acids incre ased, even though irregularly, with increasing depth. In deep-sea sediments , changes in RNA concentrations appear to be largely dependent upon bacteri al dynamics. Estimates of the overall living contribution to the DNA pools (i.e. microbial plus meiofaunal DNA) indicated that the large majority (abo ut 90%) of the DNA in continental and deep-sea sediments of the eastern Med iterranean was detrital. The non-living DNA pools reach extremely high conc entrations up to 0.41 g DNA m(-2) cm(-1). Thus, especially in deep benthic habitats, characterized by low inputs of labile organic compounds, detrital DNA could represent a suitable and high quality food source or a significa nt reservoir of nucleic acid precursors for benthic metabolism. (C) 1999 El sevier Science Ltd. All rights received.