Large Fraction of Dead and Inactive Bacteria in Coastal Marine Sediments: Comparison of Protocols for Determination and Ecological Significance

Abstract

It is now universally recognized that only a portion of aquatic bacteria is actively growing, but quantitative information on the fraction of living versus 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 between (1.5 ± 0.2) × 10 8 cells g −1 and (53.1 ± 16.0) × 10 8 cells g −1 in sandy and muddy sediments, respectively. Bacteria displaying intact membrane (live bacterial cells) accounted for 26 to 30% of total bacterial counts, while dead cells represented the most abundant fraction (70 to 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 to 4.8% of the total bacterial number) was significantly lower than nucleoid-containing cell 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 to 11% of the total bacterial number) was “reactivated.” Bacterial turnover rates estimated ranged from 0.01 to 0.1 day −1 but were 50 to 80 times higher when only the fraction of active bacteria was considered (on average 3.2 day −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 are at least 1 order of magnitude higher than previously reported.