Friday, December 29, 2006

One more thing to think about when you swallow seawater at the beach

The ocean is home to a tremendous diversity of organisms. Many animal phyla are found exclusively in marine environments (e.g., Ctenophora, Echinodermata), and even those phyla that contain species that can live elsewhere are often more diverse in the oceans (e.g., Cnidaria, Mollusca, Porifera, Annelida). But, of course, animals aren't alone in the ocean: protists abound (diatoms, dinoflagellates, red algae, green algae, brown algae, etc.), and there are more bacteria than you can shake a stick at: at least one species of bacteria can be found at concentrations of up to 200,000 cells per milliliter of seawater (data for Prochlorococcus; Freeman 2005). Introducing all of this to my classes is always fun, and it usually makes at least a few students squeamish as they think back to all that seawater they've accidentally swallowed.

Now I have something new to add to my list of marine diversity highlights: viruses. Angly et al. (2006) took dozens of samples of seawater from four different ocean regions (see their figure 1) and analyzed them for viral genetic material (double- and single-stranded DNA)1. Their results are astounding:
Taken together, these data indicate that the global marine viral richness could be as high as a few hundred thousand species, with a regional richness sometimes almost as high, likely because of migration processes.
In other words, there are probably more than 100,000 different species (genotypes) of viruses in the ocean right now. That's more than double the number of known species of chordates (the phylum humans are in), and is approaching the number of extant plant species2.

The researchers found 129,000 different genotypes of viruses present in just one of the regions they sampled (coastal British Columbia; see their table 3). While other regions had less diversity (containing hundreds to thousands of genotypes), Angly et al. (2006) found that, overall, there was a lot of overlap in the species present in each area. Even though the dominant species in each region were different, the same viruses were generally present in all four regions (see their figure 4). It sounds like there's some interesting population ecology waiting to be uncovered here.

While Angly et al.'s (2006) diversity data is stunning, their methodology limited them to identifying viruses that use DNA as their genetic material. While many viruses do use DNA as their genetic material, a number of viruses use RNA as their genetic material, and thus Angly et al. are necessarily underestimating the actual number of viral species present in the marine environment.

Angly FE, B Felts, M Breitbart, P Salamon, RA Edwards, C Carlson, AM Chan, M Haynes, S Kelley, H Liu, JM Mahaffy, JE Mueller, J Nulton, R Olson, R Parsons, S Rayhawk, CA Suttle, and F Rohwer. 2006. The Marine Viromes of Four Oceanic Regions. PLoS Biology 4:11 e368. Full-text article; synopsis.

Freeman, S. 2005. Biological Science, 2nd edition. Prentice Hall: NJ. pp. 583.

1 Finding more than 100,000 species of viruses by sequencing DNA isolated from viruses in seawater was no small task; Angly et al. ended up generating 1,768,297 different DNA sequences, which they then had to compare both to known sequences in databases and to each other. Yikes.
2 See the data at the end of this post.

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