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

Abstract

Spatial and temporal variations of nucleic acid (DNA and RNA) concentrations and bacterioplankton density were investigated in a highly oligotrophic region of the Eastern Mediterranean in order to evaluate the relative contribution of the pico-, nano- and microparticulate fractions and to estimate bacterioplankton contribution to the nucleic acid pools. The oligotrophic conditions of the Cretan Sea were reflected by the extremely low particulate nucleic acid content. In particular, RNA concentrations (range: 0. 1 to 3.8 μg l -1 ) were the lowest reported so far in the marine environment. Particulate DNA concentrations (range: 0.8 to 5.9 μg l -1 ) also displayed a reduced temporal variability. Picoparticulate RNA and bacterial secondary production displayed similar spatial patterns, suggesting that RNA concentrations increased as a result of enhanced metabolic activity. High DNA concentrations were generally associated with the high salinity (>38.95) Transition Mediterranean Waters (TMW) in summer (August 1994 and September 1995), whereas a sharp decrease in DNA concentrations was observed in non-stratified conditions (e.g. February 1995). Most of the DNA concentration was associated with picoparticles (more than 60% of total DNA pool), with a ratio of 1:3:6 for micro-, nano- and picoparticulate fractions, respectively. By contrast, particulate proteins and RNA displayed an average annual ratio of about 1:1:2 for micro-, nano-, and picoparticulate fractions, respectively. Bacterial densities in the Cretan Sea ranged from 1.1 to 8.8 × 10 8 cells 1 -1 . The bacterial DNA contribution to the total DNA pool in the Cretan Sea was on average 40%, but in February 1995 the living DNA fraction accounted 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 role of primary importance as a living DNA component in the Cretan Sea.