Particulate nucleic acid dynamics in a highly oligotrophic system: the Cretan Sea (Eastern Mediterranean)

Spatial and temporal variations of nucleic acid (DNA and RNA) concentration s and bacterioplankton density were investigated in a highly oligotrophic r egion of the Eastern Mediterranean in order to evaluate the relative contri bution of the pico-, nano- and microparticulate fractions and to estimate b acterioplankton contribution to the nucleic acid pools. The oligotrophic co nditions of the Cretan Sea were reflected by the extremely low particulate nucleic acid content. In particular, RNA concentrations (range: 0.1 to 3.8 mu g l(-1)) were the lowest reported so far in the marine environment. Part iculate DNA concentrations (range: 0.8 to 5.9 mu g l(-1)) also displayed a reduced temporal variability. Picoparticulate RNA and bacterial secondary p roduction displayed similar spatial patterns, suggesting that RNA concentra tions increased as a result of enhanced metabolic activity. High DNA concen trations were generally associated with the high salinity (>38.95) Transiti on Mediterranean Waters (TMW) in summer (August 1994 and September 1995), w hereas a sharp decrease in DNA concentrations was observed in non-stratifie d conditions (e.g. February 1995). Most of the DNA concentration was associ ated with picoparticles (more than 60% of total DNA pool), with a ratio of 1:3:6 for micro-, nano- and picoparticulate fractions, respectively. By con trast, particulate proteins and RNA displayed an average annual ratio of ab out 1:1:2 for micro-, nano-, and picoparticulate fractions, respectively. B acterial densities in the Cretan Sea ranged from 1.1 to 8.8 x 10(8) cells l (-1). The bacterial DNA contribution to the total DNA pool in the Cretan Se a was on average 40%, but in February 1995 the living DNA fraction accounte d for 85% of the total DNA pool. As phytoplankton DNA annually accounted on average for only 17% of the total DNA, we conclude that bacteria play a ro le of primary importance as a living DNA component in the Cretan Sea.