Subseafloor microbes are some of the most common organisms on earth. There are more of them than there are stars or sand grains. ~ American microbiologist Karen Lloyd
Prokaryotes, especially archaea, are particularly plentiful in marine waters, posing as plankton. There are at least a million bacteria in a milliliter of seawater. They are most abundant in estuaries, such as the mouth of the Mississippi, where they break down organic matter to be recycled. By contrast, in coastal waters, viruses greatly outnumber bacteria (100 million per milliliter), with even higher concentration in the oceans (4×1030 per milliliter).
Moving through water is highly problematic when your size is measured in molecules. But in numbers there is power. So, like shoaling fish, marine microbes flock together to swim.
Flocks can move in a synchronised way over long length scales and several times faster than a single bacterium. ~ Swedish physical chemist Joakim Stenhammar
Marine microbes live all through the water column, flourishing in the deep trenches of the Pacific Ocean, at least as far as 11 km down.
On the seafloor, archaea and bacteria scavenge the marine snow proteins that descend from the deceased above. That is but the top layer of communities stratified through time. 90% of marine microbes are not in the water, but instead in the muck. Even 750 meters beneath the ocean live sediment microbes.
Benthic sediment slowly accumulates. It is seldom stirred, and so remains unaerated and unreplenished, becoming a nutrient and oxygen desert with the passage of millennia. Yet deep under the surface populations of microbes get by.
Microbial communities can subsist at depth in marine sediments without fresh supply of organic matter for millions of years. ~ Danish aquatic microbiologist Hans Røy
The trick is to sip oxygen. The microbes in 1 cubic meter of sea floor sediment take 10 years to consume the amount of oxygen taken by an average person in a single breath. This parsimony allows microbes to live in marine mire deposited over 80 million years ago.
Some microorganisms can exist for millennia. They are metabolically active but in stasis, with less energy than we thought possible of supporting life. ~ Karen Lloyd