Spatial distribution and activity of viruses in the deep-sea sediments of Sagami Bay, Japan

The vertical and horizontal heterogeneity in benthic viral and bacterial abundance were investigated at spatial scales from centimetres to hundreds of meters at 1450 m water depth in Sagami Bay, Japan. In the surface sediment layer (0–1 cm), viral and bacterial abundance ranged from 3 to 23×108 virus-like particles (VLP) cm−3 and 3 to 13×107 bacteria cm−3, with an average virus–bacteria ratio (VBR) of 16.8±5. The variability in surface viral abundance at the investigated sites (n=38n=38) almost covered the entire range of published VLP abundances for benthic environments, and a detailed statistical analysis showed no pattern in the spatial variability at scales between 5 mm and 150 m. Viral and bacterial abundance decreased gradually with depth to 0.7–3×108 VLP cm−3 and 0.7–3×107 cells cm−3 at 10 cm. A compilation of all abundance data (n=89n=89) showed significant correlation between viral and bacterial abundance (p<0.01), and an average area density of viruses in the upper 16 cm sediment of 5.1±2.3×1013 VLP m−2 seafloor. Investigations at a “cold seep” location showed significantly lower viral abundances (<1×108 VLP cm−3) and VBR (<3) suggesting the existence of “cold spots” with reduced viral activity in an otherwise virus-rich environment. Viral production and microbial respiration were measured in homogenized, undiluted, anaerobic sediment incubations, which were manipulated with respect to temperature and organic carbon (OC) addition. Elevated temperature (14 °C) and OC addition stimulated the VLP production to a maximum of 3.6×106±4.2×105 VLP cm−3 h−1 (population doubling time: 2.7 days). Viral production in the sediment enclosures was positively correlated to the heterotrophic dissolved inorganic carbon (DIC) production suggesting that the production was regulated by the bacterial metabolism. Virus-induced bacterial mortality constituted 7–48% of bacterial production with a decrease in the relative importance of viral lysis with increasing bacterial production. The estimated input of viral lysates could explain <10% of total bacterial metabolism, thus suggesting that viruses had only minor impact on benthic carbon cycling at the investigated deep-sea sites.