Viruses Manipulate the Marine Environment

A review of: Rohwer, F., and Thurber, R. V. (2009) Viruses manipulate the marine environment. Nature. 459; 207-2012

This paper is a review of current understanding into the ways in which marine viruses can affect their hosts and their environments. The importance of these studies focuses on the influences of marine viruses on both biogeochemistry and host evolution which as economic and conservation related implications. Marine viral diversity is a major area of current study but is difficult to pursue due to viruses lacking universally conserved genes or being difficult to culture. However, techniques such as pulsed field gel electrophoresis and shotgun sequencing have been developed to overcome these problems and it has been found that viruses are astoundingly diverse with more than 5000 viral particles per 100 litres of seawater.

Viruses can carry and transfer host genes and are capable of modifying host physiology with both positive and negative consequences for the host. Horizontal gene transfer between the virus and the host is a consequence of viruses acting as gene reservoirs for the host. An example of this is in cyanobacteria in which bacteriophages contain genes for photosynthesis which they use to maintain photosynthesis in the cyanobacteria during viral infection. The phages use the energy generated from photosynthesis for viral production in the host.

Viruses can also manipulate their hosts and transform them from benign microorganisms to pathogens through the high number of virulence genes that viruses contain, including antibiotic resistance, toxicity and host invasion, found in metagenomic studies. Bacteria can extend their ecological niches by taking up these genes, such as Vibrio cholera, a usually harmless marine bacteria that becomes a pathogen which it incorporates the viral cholera toxin genes.

Studies have found that the rate of transduction between the viruses and their hosts is high with data suggesting that as many as 10²⁴ genes are moved this way each year in the world’s oceans. However, this is actually thought to be higher due to generalised transduction agents (GTAs), similar to bacteriophages but smaller in size and number of genes. They contain only the host DNA which they then inject into the recipient. This is effective transduction as it not only allows gene swapping between organisms but between ecosystems too. It is also thought to contribute to niche differentiation in closely related species.

Viruses are expert manipulators, of protists, metazoans and even themselves. Examples in this paper include Coccolithophores, a group of eukaryotic phytoplankton, with calcium carbonate scales known as coccoliths. The Coccolithophores have two life stages, one (diploid) involves the Coccolithophore in its coccolith shell and the second (haploid) is its sexual stage where the Coccolithophore is naked, can be motile and is resistant to the viruses that can infect in the diploid stage. Coccolithophores’ importance lies in their blooms which influence global temperature and remove atmospheric carbon dioxide when their coccoliths sink. This boom and bust system is primarily influenced by the infection and lysis of Coccolithophore specific viruses, whose genomes contain genes for an apotosis pathway that was once designed by the Coccolithophores to prevent the spread of viruses and but is now used by the viruses to facilitate it. Studies on these have led to formation of a hypothesis which suggests that the host and the virus will continually evolve to become resistant to the other in an 'arms race' until the Coccolithophore moves into its haploid stage and evades the virus altogether.

The solar powered sea slug, Elysia chlorotica, practice kleptoplasty in which the slug harvests the chloroplasts from the algae that it feeds on through specialised epithelial cells in the gut. It can maintain these chloroplasts can gain energy from photosynthesis. Genes from these ‘stolen’ chloroplasts only account for 20% of the genes for photosynthesis, and so gene transfer between the algae and the slug takes place via eukaryotic viruses to make up the remainder. Interestingly, these sea slugs typically die once they have laid their eggs and studies have found that this correlates with the appearance of viral particles that were not found in juvenile slugs suggesting dramatic impacts of viruses on life cycles.