Environmental conditions influence viral (phage) activity in the oceans’ carbon & nutrient cycling

Environmental conditions influence viral (phage) activity in the oceans’ carbon & nutrient cycling

Viruses that infect bacteria are also called as phages and/or bacteriophages. They are ubiquitously present in the oceans. They play major role in the oceans’ bio-geochemical processes and energy fluxes. They infect bacteria in two characteristic ways. In the lytic cycle, viral replication occurs immediately following their infection and results in newly replicated phages coming out of their bacterial hosts through lysis of the host cell. The other way of infection is lysogenic cycle in which, the genetic material of the infecting phage gets inserted into the host cell’s genome as a prophage. It is transmitted to the next generation of bacteria as bacteria divide. This prophase can be transformed into lytic cycle spontaneously or because of some environmental triggers. Similarly, some environmental agents such as ultra-violet (UV) radiation can induce prophase.  

Few studies have noted the relation between environmental conditions, -associated bacterial abundance and their influence on the occurrences of lytic and lysogenic phage infection of bacteria. Nevertheless, there is knowledge gap in understanding how and what environmental changes induce lytic or lysogenic infection of phage, particularly in the Arctic Ocean. This study investigated seasonal and spatial patterns of lytic and lysogenic viral infection in the Canadian Arctic Shelf and explored if these patterns are related to the environmental conditions or not! Viral reduction approach was used to monitor viral production and viral-induced lysis of bacteria.

This study emphasizes the critical role viral (phage)-induced bacterial lysis plays in the carbon cycling and energy fluxes in the oceans. It transfers significant amounts of carbon from heterotrophic bacteria to the pools of dissolved and particulate organic matter, which further facilitates proliferation of other uninfected microbes present in the ocean. Seasonal changes in the productivity in terms of phytoplankton and bacterial biomass affects viral production. These are dynamic processes, in other words, constantly changing processes as a function of changing environment.

The authors discussed that during the winters of Arctic, primary production and hence bacterial production is low; which results in low viral production. Nevertheless, minimal bacterial activity still occurs in the surface waters. Viral lytic cycles occur on lesser extent and release dissolved and particulate organic matter into the environment. This low-productivity, unfavourable conditions may favour lysogenic infection more over the lytic type of infection.

On the other hand, during the summers of Arctic, primary production and hence bacterial production are highly elevated. This ultimately results in elevated lytic activity and viral production. Rates of lytic activity stay very high during this time. The authors of this study found strong positive correlation between bacterial abundance and lytic activity. Thus, elevated hosts’ productivity and abundance induces lytic type of phage infection. These conditions of high bacterial abundance may suppress the lysogenic type of phage infection.

Hence, environmental conditions that determine physiological state of the bacterial hosts, also decide whether the infecting phages will lyse their hosts or undergo into the lysogenic pathway. Lysogenic type of infection is a kind of survival strategy to remain in the hosts during unfavourable, nutrient poor conditions, particularly during winter season. In contrast, lytic type of infection represents proliferation of viruses which is correlated with the high primary productivity and bacterial biomass, under favourable, nutrient-rich condition of summer.

Findings of this study clearly demonstrates that viral lytic and lysogenic cycles are a function of changing environmental conditions, in particular changes in the host bacterial abundance which depends on primary production.

Payet J. P. & Suttle C. A. (2013) To kill or not to kill: The balance between lytic and lysogenic viral infection is driven by trophic status; Limnol. Oceanogr., 58(2):465-474.