Revealing the Hidden Diversity of Terrestrial Giant Viruses

Abstract:

Giant viruses with physical sizes and genomes bigger than some bacteria, were only discovered in 2003.
We fortuitously discovered 16 new species and deeper lineages of giant viruses in our experimental warming plots
at Harvard Forest. These were the first forest soil giant virus genomes. Half to three quarters of the genes in the
genomes do not having any matches in GenBank suggestive of novel biological enzymes and functions. Giant
viruses are now classified within the phylum Nucleocytoviricota and form clades of viruses that infect protists.
They are abundant in fresh water and marine ecosystems, acting as key regulators of algal populations. Two recent
in depth analyses of all prior metagenomes recovered over two thousand Nucleocytoviricota (which include the
giant viruses) genomes at various levels of completeness, however only 10 of these were from soil samples
compare to the 16 giant viruses in our one study. It is remarkable that our Harvard Forest giant viruses still include
the largest Mimiviridae genome with a size of 2.4 Mb and the second largest of all giant viral genomes, but it raises
the questions “Where are the terrestrial giant viruses?” and “What are their roles in biogeochemical cycling?” We
have developed new strategies to reveal the hidden diversity of giant viruses and other members of the
Nucleocytoviricota using a combination of transmission electron microscopy (TEM) and targeted metagenomics.
TEM directly provided the first images of giant viruses from forest soil and many spectacular novel morphotypes.
The metagenomic data are being analyzed for the presence of Nucleocytoviricota with specific emphasis on genes
that enable host identification and suggest roles in the soil biogeochemical cycles.