High fraction of dead and inactive bacteria in coastal marine sediments: comparison of protocols and ecological significance

It is now universally recognized that only a portion of aquatic bacteria is actively growing, but quantitative information on the fraction of living vs dormant or dead bacteria in marine sediments is completely lacking. We compared different protocols for the determination of the dead, dormant and active bacterial fractions in two different marine sediments and at different depths into the sediment core. Bacterial counts ranged from 1.5±0.2 to 53.1±16.0 ´108 cells g-1 in sandy and muddy sediments, respectively. Bacteria displaying intact membrane (Live Bacterial Cells) accounted for 26-30% of total bacterial counts, while dead cells represented the most abundant fraction (70-74%). Among Living Bacterial Cells, Nucleoid Containing Cells represented only 4% of total bacterial counts, indicating that only a very limited fraction of bacterial assemblage was actively growing. Nucleoid-containing cells increased with increasing sediment organic content. The number of bacteria responsive to antibiotic treatment (Direct Viable Count, range 0.3-4.8% of the total bacterial number) was significantly lower than Nucleoid Containing Cells counts. An experiment of nutrient enrichment to stimulate a response of the dormant bacterial fraction determined a significant increase of Nucleoid Containing Cells. After nutrient enrichment, a large fraction of dormant bacteria (6-11% of total bacterial number) was “reactivated”. Bacterial turnover rates estimated ranged from 0.01 to 0.1 d-1, but were 50-80 times higher when only the fraction of active bacteria was considered (on average 3.2 d-1). Our results suggest that the fraction of active bacteria in marine sediments is controlled by nutrient supply and availability and that their turnover rates is at least one order of magnitude higher than previously reported.