Genomics of viruses infecting green and purple sulfur bacteria in two euxinic lakes

Abstract

Viral infections of marine bacteria modulate primary production rates and the cycling of organic and inorganic matter in the world's oceans. Here, we investigated the hypothesis that viral infections influence the ecology of purple and green sulfur bacteria (PSB and GSB) in anoxic and sulfidic (euxinic) lakes, modern analogs of early Earth oceans. Over 200 high and medium quality viral contigs were identified in long-read metagenomes from the sediments and water column of Lime Blue and Poison Lake, respectively. We compared these sequences with 94 predicted prophages identified in the complete genomes of PSB (n = 213) and GSB (n = 33). Viral genomes carrying psbA, encoding the small subunit of photosystem II protein, were present in all datasets. The ubiquity of these genes suggests PSB and GSB viruses interfere with the light reactions of sulfur-oxidizing autotrophs. Viruses predicted to infect PSB and GSB encoded auxiliary metabolic genes for reductive sulfur assimilation as cysteine, a pathway not yet described in these sulfur bacteria, as well as genes involved in pigment production (crtF) and carbon fixation (CP12, zwf, PGD). These observations highlight the potential for viral modulation of metabolic markers used as proxies to interpret biogeochemical processes in early Earth oceans.