Forget gargantuan whales and hefty cephalopods—the real marine mammoths may be the mighty microbes. They constitute at least half, and perhaps up to 90 percent, of the oceans’ total biomass, according to data gathered by the decade-long Census of Marine Life project.

The estimate comes courtesy of high-throughput DNA sequencing, which suggests that there might be as many as 100 times more microbe genera than previously assumed. The increase in genus and species also raises the estimate of individual microbes. A single liter of seawater, once thought to contain about 100,000 microbes, can actually hold more than one billion, the census scientists reported in April.

The tiny creatures can join together to create some of the largest masses of life on the planet. Census scientists found one such seafloor mat off the Pacific coast of South America that is roughly the size of Greece.

Despite their small individual size, microbes play a big role in the planet’s climate. They help to turn atmospheric carbon dioxide into usable carbon, as well as oxygenating sediment and cycling nutrients in the ocean. But little is known about these creatures’ susceptibility to shifts in temperatures, dissolved gases and acidity, which are predicted to occur with climate change. Researchers will present the full census in October in London.

This article was originally published with the title Microscopic Giants.

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