Viral Nature of the Aquatic Ecosystems

Abstract

Viruses infecting microorganisms are ubiquitous and highly abundant in aquatic environments. They considerably affect the dynamics, diversity, and evolution of their host microorganisms. In this review, we discuss the ecological implications of viruses from the perspectives of the biogeochemical cycles, microbial diversity, and virus–host coevolutionary dynamics in aquatic environments. Generally, viruses redirect host metabolism toward reproduction through molecular host–virus interactions characterized by the compositional and stoichiometric changes in intracellular metabolites, which are eventually released into the environment when the infected host cells are lysed, thus also changing the chemical composition of the water. Therefore, the modulation of metabolite biosynthesis and promotion of their recycling are major viral functions. Viruses also maintain microbial community diversity via increased infection and lysis rates of the dominant taxa and genotypes in a frequency-dependent manner, thereby allowing the co-existence of members with various competitive abilities. Finally, viruses can expand their own genotypic diversity and that of the host through complex defense and counter-defense interactions, including loss of host fitness due to the cost of resistance and the possible need for antiviral defense-specific (e.g., intra- vs. extracellular) changes in the hosts genome diversification. Continuous interactions drive the coevolution of hosts and viruses, thereby increasing both the host and viral micro-diversity. Hence, these fundamental functions are viral “raison d’etre” and are essential for the functioning of aquatic ecosystems and its components.