Significant contribution of lytic mortality to bacterial production and DOC cycles in Funka Bay, Japan

Abstract

A modified dilution experiment was conducted to evaluate the relative contribution of viral lysis and protozoan grazing to the mortalities of heterotrophic bacteria in Funka Bay, a subarctic coastal bay. The experiment included the stepwise dilution of the original seawater with virus-free seawater (10 kDa ultrafiltered) to change the encounter rate of both virus and protozoa to heterotrophic bacteria, incubation for 48 h and monitoring the change in the abundance of heterotrophic bacteria. In a parallel experiment, the original seawater was replaced by 1.0 µm fractionated seawater to eliminate protozoa, and the same dilution was conducted with the virus-free seawater to estimate only lytic mortality. The viral lysis and protozoan grazing rates in the surface water ranged from 0.40 to 1.19 and 0.08 to 0.27 days−1, respectively. Viral lysis was the main cause for the bacterial mortality (79.8 ± 3.2 %). The net (in situ) growth rate of heterotrophic bacteria was about 0.15 days−1. In the bottom water (90 m), both mortalities were lower than those at the surface and the net growth rate was mostly a negative value. The contribution of released dissolved organic matter (DOM) through lysis to the bacterial carbon demand (BCD) was evaluated. The lysed bacterial cells might release DOM to the ambient environment, in which bacterial organic matter is recycled in the subsequent bacterial production. The potential contribution was estimated to range from 25 to 27 % in the surface water and to be 31 % in the bottom water, suggesting that the lytic mortality significantly fueled DOM to the subsequent bacterial production.