The Viral Elevator: Modeling Virus and Bacteria Populations in Europa’s Icy Ocean

Abstract:

While several studies have shown the abundance and importance of viruses in icy ocean environments on Earth (e.g. Collins & Deming 2011, Price & Sowers 2004), the role that viruses may play in the hypothetical biospheres of icy ocean worlds, such as Europa, remains unexplored. On Earth, viruses play a key role in our marine food chain, cycling organic carbon back into the system by creating dissolved organic matter (DOM) through a process known as the ‘viral shunt’ (Breitbart et al. 2018). We hypothesize that on an icy ocean world like Europa, where life is likely to fix carbon near hydrothermal systems at the ocean bottom, a similar process that we call the ‘viral elevator’ could potentially shuttle DOM to the sub-ice ocean layer. We model the virus–bacteria population dynamics of a hypothetical biosphere in the sub-ice ocean layer of Europa, incorporating the concept of the viral elevator and using parameters from various Arctic-based studies as proxies for Europa’s environment. We explore what system properties (benthic biomass, viral lysis rates, burst sizes, vertical mixing parameters, etc.) are required to produce steady-state virus–bacteria populations in the sub-ice ocean layer. Our results show extremely high virus-to-bacteria ratios (VBR ~ 100–1000) as well as long periods of dormancy between bacterial and viral population peaks. This work has implications for near-future Europa astrobiology missions, which will attempt to search for life on the surface, within the ice, and in the ocean directly under the ice, but not the ocean bottom. Our findings suggest that future searches for life on Europa should account for the longevity of viral and bacterial populations in these systems, and motivates the inclusion of viruses in the search for life rather than limiting life detection to bacteria or bacteria-analog organisms.