The many we don’t see

I find it strange and fascinating that the most abundant organisms on Earth are the ones we cannot even see and if it was not for the tools like microscopy, PCR and such, we would happily live absolutely oblivious to them.

So what are the most common organisms on Earth?

Well, I guess it depends on what you would call an organism. As far as the living organisms go that would probably be some kind of bacterium. Why? Well, bacteria are incredibly well adapted to all sorts of environments so can occupy variety of niches. Of course, the size helps too (I mean there are only so many elephants that you can fit in a room, right?). One gram of soil can contain as much as 10^9 bacteria that belong to possibly thousands or even hundreds of thousands different species. But, really, the surface of Earth is not just soil, 70% of it is covered with water so it would make sense, if bacteria are so adaptable, that most abundant type of bacteria would live in water- and they do!!!

Behold the most abundant organisms on Earth (which most of us have never seen…)

Electron micrograph of SAR11 Credit: Michael Rappe, SOEST/UHM
Electron micrograph of SAR11
Credit: Michael Rappe, SOEST/UHM

What you see above is a bacteria that belongs to a group of marine bacteria called SAR11. The estimated number of these bacteria in the ocean is 2.8*10^28 or 28000000000000000000000000000 or- a lot ,which constitutes to around 35% of all prokaryotes in the ocean! Because of its abundance this bacterium is very important for nutrient and especially carbon cycling in the ocean. SAR11 is very good at scavenging organic carbon in the ocean, which possibly contributes to its numbers as it is able to outcompete other organisms. Moreover, SAR11 is quite good at taking up extracellular DNA, which primarily is a way of obtaining more nutrients but is also an important mechanism for acquiring new traits as that DNA can contain genetic elements that can improve bacterium’s fitness.
Interestingly, SAR11 might be more closely related to you than you might think. Based on phylogenetic analyses one study concluded that SAR11 and our mitochondria have a common origin. It is likely that mitochondria has evolved from a marine bacterium that was infecting other organism and one such infection, which ended up as a beneficial relationship between the host and the invader, evolved to be the mitochondria in our cells.

For a long time it has been thought that SAR11 is so abundant because it does not have any predators, namely no bacteriophages that infects it. Bacteriophages are basically viruses that can infect bacteria (from greek ‘phagein’, meaning ‘to eat’, so basically bacteria eaters). However, recently, the so called pelagiphages have been discovered that infect SAR11. Among these phages HTVC010P phage is the most abundant one and might consequently, be the most abundant organism on the Earth.
Now, as I said, it depends on how you define an organism. I mean, viruses are not alive (see my previous post) so whether it is correct to call HTVC010P the most abundant organism on Earth depends on whether you think that organism by definition have to be living entities.

Transmission electron microscopy images of isolated pelagiphages. Credit: Y Zhao et al. Nature  (2013)
Transmission electron microscopy images of isolated pelagiphages.
Credit: Y Zhao et al. Nature (2013)

Talking about viruses in oceans you wouldn’t believe how many there are!!! Viruses account for 94% of all nucleic acid containing entities in the oceans. Obviously, they are incredibly tiny so their actual contribution to the total biomass is only 5%.

Relative biomass and abundances of prokaryotes, protists and viruses. Credit: Curtis A. Suttle, Nature Reviews Microbiology 5, 801-812 (October 2007)
Relative biomass and abundances of prokaryotes, protists and viruses.
Credit: Curtis A. Suttle, Nature Reviews Microbiology 5, 801-812 (October 2007)

Viruses in water are not limited to bacteria though; many of them infect protists such as amoebas. In fact, these protist-infecting viruses are among the largest viruses known. In 2013 Jean-Michel Claverie group has published a paper on the largest (as yet) discovered virus, which they named Pandoravirus salinus. This virus infects Acanthamoeba castellani, was purified from a river sediment samples in Chile and has a genome of 2.77 megabases- that is larger than some of bacterial genomes!!! Like all other viruses Pandora and other giant viruses are completely dependent on their host to survive and replicate, but how did these viruses ever come to be. I mean, if you look at their genomes they have genes for all sorts of things that we think of as requirements for the living cells: amino acyl-tRNA synthetases, polymerases, some metabolic genes, etc. Have these viruses always been present, i.e. they are relics of “The RNA World”, have they once been living and lost so many genes that became completely dependent on the host, or are they just genetic elements that somehow pick genes from the organisms they infect?
As always, many questions but few answers, which what makes science so great!!! ☺

Pandoravirus Credit: Chantal Abergel and Jean-Michel Claverie
Pandoravirus
Credit: Chantal Abergel and Jean-Michel Claverie

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