Initial estimates of viral abundance in the ocean (and indeed, in any sample) were based on the number of infective virus particles for specific host organisms, rather than direct counts of total virus particles. The conclusion from estimates based on infectious assays was that viruses were a curious component of the marine microbial community, but not particularly significant. But in 1989, the first direct counts of virus particles in seawater revealed something amazing: viruses were incredibly abundant in oceans, outnumbering bacteria by a factor of 10! A single liter of seawater can contain over 3 billion virus particles. This set off a revolution in marine microbial ecology. With those kinds of numbers, scientists began to wonder what sorts of impacts those viruses might be having in marine ecosystems, and design experiments to find out. Now, almost 20 years later, the results of those experiments have established viruses as important members of marine microbial communities, with significant impacts on host population dynamics, nutrient cycles, and host evolution. Here are some interesting findings to consider:
Viral lysis accounts for almost half of the microbial biomass turnover in the global ocean on a daily basis. In other words, viruses kill almost half the microbial population in Earth's oceans every day!
Viruses can sequester carbon in the microbial fraction and prevent the transfer of carbon to higher trophic levels.
Viruses alter the genetic diversity of bacterial populations through the selection for resistant sub-populations, lysogenic conversion, and horizontal gene transfer. One of the most interesting cases involves the transfer of photosynthesis genes into viruses that infect cyanobacteria - even though viruses do not perform photosynthesis and have no innate metabolism.
These are just a few examples of the incredible reports coming from studies of marine viral ecology. By comparison, we know very little about the roles of viruses in freshwater systems, and even less about viruses in terrestrial ecosystems. This is a huge data gap, given the global importance of freshwater for human activities for drinking, cooking, and washing; and the importance of soils for food and fiber production, water purification, and the discovery of valuable biomolecules such as antibiotics. While marine viral ecology has flourished over the past two decades, the first estimates of viral abundance in soils were only published in 2003. Many questions remain regarding virus ecology, and The Williamson Lab aims to dig up the dirt.
The electron micrographs on this page feature phages induced from soil bacteria.