The most important microbe you've never heard of

Meet Prochlorococcus -- it might be the most important microbe you've never heard of

Tiny creatures that inhabit the oceans’ well-lit upper waters emit gas or gaseous compounds. One algae Emiliana huxleyi emits dimethyl sulfide, which contributes to what we call the smell of the sea.

An unseen "forest" of these microscopic beings fills the upper 200 meters of ocean, exerting an influence on this planet every bit as profound as the forests on land. The diverse phytoplankton species inhabiting the ocean's surface waters -- which mainly consist of single-celled cyanobacteria, diatoms and other kinds of algae -- form the base of the marine food web. They account for roughly half the photosynthesis on the earth, remove nearly as much carbon dioxide from the atmosphere as all land plants, and supply about half to three-quarters of the oxygen we breathe. Without the activities of these free-floating plantlike organisms, atmospheric carbon dioxide levels would triple.

One of these was just discovered in 1986 – a blue-green algae not known to exist until its discovery. It was given the name Prochlorococcus (pro-cloe-row-cock-us). It’s one of the most abundant living creates on the globe. These minute creatures employ chorlophyll to produce as much as one-fifth of the world’s oxygen.

A young researcher at the Massachussetts Institute of Technology, Penny Chisholm, first found the creature in the Atlantic’s Sargasso Sea. She was on a research vessel traveling from Cape Cod to Bermuda. They were using a device known as a flow cytometer to pick up samples of a particular blue-green algae they knew existed.

What they did not foresee was that the device would show the existence of millions and millions of even tinier creatures – oval shaped entities, around six microns in diameter, one two-hundredth of the width of a human hair. Once examined under an electron microscope, they were found to have incorporated into their minute workings a type of chlorophyll that permitted them to absorb carbon dioxide to extract from the seawater a tiny amount of oxygen which then escaped into the atmosphere.

Taken one by one the amount of oxygen that any one of these algae produce is insignificant; but Penny Chisholm calculated that Prochlorococcus existed in such large numbers – one hundred thousand of them in a single cubic centimeter of water, perhaps a trillion trillion of them in all – that they were quite probably the most common creature in all the world, and would in total produce very large quantities of oxygen.

They live in warmer seas, essentially wafting around in the oceans between 40 degrees north and 40 degrees south. There they drift, at the bottom of the food chain, waiting to be eaten by tiny shrimp that would then be gobbled by small fish, and on and on, up to the humankind’s seafood restaurants.

Chisholm felt that Prochlorococcus was an example of how once again nature had displayed its infinite capacity to humble the world of science, and could readily do so again. Before 1986 we didn’t know it existed; now it is recognized as perhaps the most common being on earth and it plays a central role in keeping land-based creatures alive.

It can be claimed that one in every five breaths each one of us takes contains oxygen created out at sea, specifically by Prochlorococcus. We now know it exists and that if anything disastrous happens to it, the survival of all beings that require oxygen would be placed at risk. In the two decades since its discovery, a lot of research has been done on what might harm it, and how. Researchers have been trying to find out whether the warming of the seas due to global climate change might limit its ability to absorb carbon dioxide and foreclose on its ability to produce oxygen.

It turns out, luckily for us, that Prochlorococcus seems to be resilient to the warming. It likes warm seas and flourishes in them. Any increase of sea temperature might well cause its range to expand beyond the present 40-degree lines, and that might have its own effect on the absorption of carbon dioxide.

“It is tempting – but entirely fanciful – to imagine that such a development might balance some of the expanded emissions of greenhouse gasses that are so troubling humankind today,” writes oceanographer Simon Winchester. “An expansion of the range and population of Prochlorococcus might well turn out to be a component of Earth’s self-regulating mechanism, so crucial to James Lovelock’s famous Gaia hyphothesis – which holds that the world is to be viewed as a self-contained living being, able to change its own ways and to deal with its changing circumstances. This curious animalcule might be even more precious than at first supposed: not merely supplying the air that we breathe but somehow dealing with our most dangerous pollutant. But this is an idle thought: there is no evidence; a lot of research still needs to be done.”

Yet all this concerns a being we were entirely unaware of 20 years after humans first went to the moon. “Those who have long claimed the sea to be far less known than outer space seem suddenly to have a special brand of wisdom,” writes Winchester.