What would you guess is the most abundant species on Earth?
You'd be forgiven on a bad day for thinking it would be Homo sapiens, but of course the most abundant species will be very small, and definitely not a vertebrate. (Vertebrates make up only 0.001% of the standing biomass, measured as carbon, on Earth. Now there's a sobering thought.)
The question of whether the most abundant species is terrestrial or marine is an interesting one. While the oceans make up about 70% of the Earth's surface, large parts of that are very infertile and have low biodiversity compared with the land. Then again, the biotically richest part of the land surface is a very thin crust compared with the oceans (although researchers at the Deep Carbon Observatory project have discovered diverse bacterial communities as deep as 5 km below the surface).
Weighing this up, if you guessed the most abundant species on Earth is very small and marine, you'd be right.
Here's another question. How recently do you think the most abundant species on Earth was discovered and named? Would you have guessed that it was only discovered in 2002, and hasn't actually been named yet?
The most likely candidate for the most abundant organism on Earth is an ultra-small marine bacterium called Candidatus Pelagibacter ubique. Candidatus is a term used in bacteriology for 'candidate taxa' - species and other taxa that have been discovered and given an informal name but have not yet passed the requirements for formal naming under the International Code of Nomenclature of Bacteria. Unlike the other Codes, the ICNB requires that a name be formally included on a global taxonomic register in order to be regarded as validly named - before this happens, the taxon is given the prefix Candidatus.
Pelagibacter ubique is well-named, the name being derived from Latin roots meaning 'the ubiquitous pelagic bacterium'. And ubiquitous it is. There are an estimated 2 × 10^28 individuals of P. ubique in the surface waters of temperate oceans, where at times they may comprise around half of all cells. For context, there are only around 10^21 stars in all the galaxies in the observable universe. So there are around 10 million times more of this one species of bacterium in the Earth's oceans than there are stars in the universe. There's another sobering thought.
Not only is Candidatus Pelagibacter ubique ubiquitous, it's also fascinating, and important for understanding the limits of life. Pelagibacter and its relatives in the 'SAR11' clade (another informally named bacterial taxon) live in environments where nutrients are almost vanishingly rare, with concentrations near the lower limits suitable for life. They have adapted to this circumstance by streamlining their genomes and cell structures to only execute the most essential functions of life. That is, they're only just alive.. It's this trick that allows Pelagibacter and its relatives to be so abundant in the ocean's nutrient-poor 'marine deserts'. Detailed studies of such extreme organisms can tell us a great deal about the limits and evolution of life on Earth, and perhaps also elsewhere in the universe.
And, speaking of organisms that are scarcely alive, all known bacteria are plagued by viruses (bacteriophages), which of course are often more abundant than their hosts. This may well be the case for Pelagibacter ubique also - a bacteriophage tagged HTVC010P has recently been discovered infecting Pelagibacter cells. So perhaps it's actually the bacteriophage that's the most abundant organism on Earth. Then again, we're not quite sure whether to regard viruses as living organisms or not - but that's a whole other matter.
Whether it's Pelagibacter or HTVC010P that's the most abundant organism on Earth, once thing is certain - it definitely ain't humans.