Breakdown and microbial uptake of marine viruses and other lysis products

To understand the roles of marine viruses in marine microbial food webs. ~t is important to determine rates and mechanisms of virus degradation and subsequent uptake of degraded virus material and other cell lysis products by heterotrophic marine bacteria. We radiolabeled and concentrated viruses and viral lysis products from either pure cultures (3H) or natural communities (3H and 33P) and added them to seawater samples of differing trophic status from coastal (mesotrophic) and offshore (oligotrophc) California waters and French Mediterranean waters (oligotrophic). Rates of degradation were 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 extraction. Preliminary experiments with 3H-labeled, single-stranded RNA phage MS2 and marine phage H11/1 demonstrated that MS2 degraded significantly faster in coastal Santa Monica Bay seawater (2.5 t 0.6% h-'), than the marine phage, H11/1 (0.99 k 0.1% h-'). For labeled virus material from natural populations, rates of degradation were slower in oligotrophic waters (ranges from 1.0 to 3.3% h-') than in mesotrophic waters (ranges from 4.9 to 6.0% h-'), corresponding to turnover rates of 1 to 4 d for this material. Degradation rates of labeled virus material are likely underestimates, because during preparation, degradation and uptake are continually occurnng, resulting in accumulation of the less reactive products. The proportion of radiolabeled material taken up by microbes was greatest in oligotrophic waters, especially in the phosphate-limited Villefranche Bay, France, where most of the "PO,-labeled material was taken up in less than 7 h. In contrast, the majority of degraded 'H-labeled material was not accumulated into biomass, and in 3 of 4 samples, accumulation was hardly detectable The results suggest that viruses and lysis products are labile and turn over relatively rapidly, but often may not be efficiently inco~porated into bacterial biomass.