7.6 ZHANG Rui

Viruses in deep sub-seafloor biosphere

State Key Laboratory of Marine Environmental Sciences; College of Ocean and Earth Sciences; Xiamen University

Viruses are abundant and cause significant mortality in marine bacterial and archaeal communities, thereby affecting biogeochemical cycles. Little is known about the role of viruses in the deep sub-seafloor biosphere which hosts a large fraction of all microbes on Earth. We characterized and quantified viruses in deep sediments from the Baltic Sea retrieved during the IODP 347. The results show that the deep biosphere in the Baltic seabed harbors highly abundant viruses with densities of 0.21-18 × 109 viruses cm-3， which was closely related to prokaryotic cell abundance and to organic carbon content in the sediments. We present experimental measurements of the potential lytic and induced lysogenic viral production in sub-seafloor sediments, suggesting high potential for viral activity in the deep biosphere. Lysogenic viral production accounted for 6-33% (average 19%) of the total viral production. High potential lytic viral production (0.03-0.8 × 109viruses cm-3 h-1) and visibly infected prokaryotic cells demonstrated active viral infection. A mixed virus community detected through T4-type phage g23 gene analysis, together with diverse viral morphotypes observed by transmission electron microscopy, suggested that viruses in the deep biosphere derived from both allochthonous influx from the overlying seawater and autochthonous production via infections of indigenous microbial hosts. The occurrence of cyanophage sequences in the deep subsurface showed that viruses of phototrophic hosts may persist in marine sediments for thousands of years. Our results imply that viruses influence subsurface microbial community dynamics and thereby will affect biogeochemical processes in the deep biosphere.