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

Abstract

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 23×10 8 virus-like particles (VLP) cm −3 and 3 to 13×10 7 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 = 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 0.7–3×10 8 VLP cm −3 and 0.7–3×10 7 cells cm −3 at 10 cm. A compilation of all abundance data ( n = 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±2.3×10 13 VLP m −2 seafloor. Investigations at a “cold seep” location showed significantly lower viral abundances (<1×10 8 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 3.6×10 6±4.2×10 5 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.