Breakdown and microbial uptake of marine viruses and other lysis products

To understand the roles of marine viruses in marine microbial food webs, it is important to determine rates and mechanisms of virus degradation and su bsequent uptake of degraded virus material and other cell lysis products by heterotrophic marine bacteria. We radiolabeled and concentrated viruses an d viral lysis products from either pure cultures (H-3) Or natural communiti es (H-3 and P-33) and added them to seawater samples of differing trophic s tatus from coastal (mesotrophic) and offshore (oligotrophic) California wat ers and French Mediterranean waters (oligotrophic). Rates of degradation we re determined by the loss of high molecular weight radiolabel over time and the fate of the degraded material (microbial uptake or accumulation in low molecular weight pools) was followed by size fractionation and/or acid ext raction. Preliminary experiments with H-3-labeled, single-stranded RNA phag e MS2 and marine phage H11/1 demonstrated that MS2 degraded significantly f aster in coastal Santa Monica Bay seawater (2.5 +/- 0.6% h(-1)), than the m arine phage, H11/1 (0.99 +/- 0.1% h(-1)). For labeled virus material from n atural populations, rates of degradation were slower in oligotrophic waters (ranges from 1.0 to 3.3% h(-1)) than in mesotrophic waters (ranges from 4. 9 to 6.0% h(-1)), corresponding to turnover rates of 1 to 4 d for this mate rial. Degradation rates of labeled virus material are likely underestimates , because during preparation, degradation and uptake are continually occurr ing, resulting in accumulation of the less reactive products. The proportio n of radiolabeled material taken up by microbes was greatest in oligotrophi c waters, especially in the phosphate-limited Villefranche Bay, France, whe re most of the (PO4)-P-33-labeled material was taken up in less than 7 h. I n contrast, the majority of degraded 3H-labeled material was not accumulate d into biomass, and in 3 of 4 samples, accumulation was hardly detectable. The results suggest that viruses and lysis products are labile and turn ove r relatively rapidly, but often may not be efficiently incorporated into ba cterial biomass.