Viral Loop Dynamics in Temperate and Polar Freshwaters
Abstract: All cellular organisms in aquatic environments are susceptible to virus attack. Viruses are the smallest but most abundant biological entities in freshwaters. This thesis describes interactions between viruses and bacteria in temperate and polar freshwaters with particular emphasis on Arctic and Antarctic aquatic systems. Free virus-like particles in freshwaters are vulnerable and exposed and as parasites their survival is dependent on the existence of a suitable host. I found that high concentrations of humic substances in lakes had a negative impact on viral abundance as viruses may be adsorbed to humic matter and replication or viral infectivity may be lost. This indicates that virus-bacteria interactions in humic lakes may differ from those in clear water lakes. Furthermore, there was a high incidence of lysogenic bacteria in humic lakes when compared to other temperate freshwaters which could indicate that lysogeny may act as a viral survival strategy in environments where high rates of destruction and inactivation may occur. On the contrary, I found that lysogenic bacteria were not common in the Arctic and Antarctic freshwaters and during a seasonal study conducted on two Antarctic ultra-oligotrophic lakes lysogenic bacteria were only detected on 3 out of 10 sampling occasions. As viruses are parasites the metabolic state of their host will largely determine how successful they are in replicating. I found that bacterial growth in the Arctic and Antarctic freshwaters was primarily limited by phosphorus however even though an increase in phosphorus had a positive effect on bacteria this was not reflected in the virus abundance. Using electron microscopy I established that a high percentage of the Arctic and Antarctic bacteria population was visibly infected and that the average number of virus-like particles per cell was low relative to previous reported data. This indicates that virus-bacteria interactions in polar freshwaters differ significantly from those in temperate freshwaters. In microbially dominated Antarctic freshwater lakes the major pathway of energy and carbon flow is through the microbial loop. Viral lysis disrupts this flow and shunts organic carbon back into the dissolved organic carbon (DOC) pool making it available for the uninfected bacterial community. I found that viral lysis of bacterial cells could contribute significantly to the DOC pool in Antarctic freshwater lakes. I estimated that over 60% of the carbon supplied to the DOC pool originated from viral lysis during the winter months while during the summer period less than 20% originated from viral lysis. This implies that viral processes in extreme Antarctic lakes may be of quantitative significance with respect to carbon flow.
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