Disturbed virus–bacteria dynamics in the Otsuchi Bay (Japan) after the mega-earthquake and tsunami in March 2011

On 11 March 2011, a mega-earthquake followed by a huge tsunami hit a large area of the Pacific coast of Tohoku, Japan, and caused widespread disturbances in marine ecosystems; however, the impacts on virus−bacteria systems have not yet been elucidated. To investigate whether virus−bacteria interactions were affected by the earthquake and tsunami, we analyzed time series data of viral and bacterial abundances in Otsuchi Bay. Data were collected every 2 mo, from July 2011 to September 2015. Bacterial abundance exhibited a recurrent seasonal cycle with high abundance during the warm season. The seasonal trend of viruses generally followed that of the bacteria, yielding an average virus to bacteria ratio (VBR) of 10.8 ± 3.6 (mean ± SD; n = 432). A notable exception was found at the first 2 sampling times (July and September 2011) when the VBR was consistently low, with an average value of 5.9 ± 1.2 (n = 32). The average VBR during these time periods was substantially lower than the VBR observed in the same season of subsequent years. An analysis of the subset of data collected in the warm season of 2011 and 2012 revealed that the viral abundance and VBR were negatively correlated with turbidity. These results support the hypothesis that viruses were scavenged by non-host particles from the resuspended sediments and damaged catchment. The earthquake and tsunami thus exerted a prolonged impact, over several months, on the virus−bacteria dynamics in Otsuchi Bay.