New research has shown marine bugs are far more abundant than thought and make up 50 per cent to 90 per cent of all the biological material, or "biomass", in the oceans.
Experts estimate their numbers at about ten to the power of 30 – or 1,000 times a billion times a billion times a billion.
Their combined weight matches that of 240 billion African elephants – or 35 elephants-worth of microbes for every human being on the planet, researchers claim.
The true extent of the marine microbe population is only now being revealed by the Census of Marine Life, a huge project to survey life in the oceans involving more than 2,000 scientists.
In the 1950s, it was estimated that about 100,000 microbial cells inhabited a litre of sea water. Today, the same volume is believed to harbour more than a billion micro-organisms. A gram of ocean floor mud holds about the same number.
"In no other realm of ocean life has the magnitude of census discovery been as extensive as in the world of microbes," said Doctor Mitch Sogin, from the Marine Biological Laboratory at Woods Hole, Massachusetts, US, who heads the International Census of Marine Microbes (ICoMM).
ICoMM scientists have amassed a database that includes 19 million DNA sequences spanning more than 100 phyla – groups of organisms sharing the same body plan.
During one 11-month study in 2007, researchers analysed the DNA of more than 180 specimens dredged up from the western English Channel. One in every 25 readings yielded a new genus, or family, of bacteria – around 7,000 in all.
Another striking find was that of mats of microbial filaments off the coasts of Chile and Peru covering an area the size of Greece. The organisms forming the mats live in an oxygen-starved environment, thriving mostly on hydrogen sulphide.
Dr Victor Gallardo, who led the Chile-based team, said: "Some things are unknown because they are too small to see, and some things are unknown because they are too big to see".
Ocean microbes consist both of bacteria and archaea, a separate class of single-celled organisms that often live in extreme environments.
Marine microbes have a huge impact on the planet's climate and ecosystems by trapping carbon from the atmosphere.
The Census of Marine Life has also revised scientists' estimates of the abundance of zooplankton, the microscopic animals that inhabit the oceans.
When researchers began the zooplankton survey in 2004, scientists had described around 7,000 non-larval organisms which stay tiny all their lives. That figure is expected to have doubled to 14,000 by the time all the census samples have been analysed and described.