Tiny Marine Bacteria Feed their Neighbors

The gospel lesson of the “widow’s mite” tells how the poorest person, a widow (no SSI in those days) offered two mites, the smallest coins in circulation, for the religious community–and was praised for giving proportionally more than the wealthy. So what do we say of Prochlorococcus—among the smallest known bacteria, half a millionth of a meter–donating buckets of its cytoplasm to surrounding marine life? On top of giving us half our atmosphere’s oxygen?

In the micrograph, you can see the tiny cells and the tinier little balloon-like vesicle that emerge from them. Each vesicle contains carbon- and nitrogen-rich food molecules.

To appreciate this more, we have to understand that these one-celled green life forms are extraordinarily competitive. Their entire existence consists of photosynthesis–growing and dividing as fast as they can, making the most of the light they absorb, to outnumber their fellow microbes. Whoever makes the most offspring is the winner.

The problem is that to make more offspring faster, it helps to have a tiny genome–to exclude any kind of genes you don’t need. So Prochlorococcus has jettisoned all kinds of genes, such as those that detoxify the hydrogen peroxide byproducts of oxygen production. (Yes, oxygen and its intermediates are toxic–among the most toxic wastes known.) How does it get away with this? By depending on its neighbors: heterotrophic (organic food eaters) bacteria that “breathe” the cyanos’ oxygen to eat their food. The neighbors produce the enzyme catalase that gets rid of H2O2. Thus the cyanos depend on their non-photosynthetic neighbors, as much as the heterotrophs depend on cyanos’ oxygen production. The basis of this dependence was discovered by Kenyon graduate Erik Zinser and students.

But why would Prochlorococcus give up so much of its hard-earned cytoplasm? As Sallie Chisholm and colleagues point out, one possibility is the need to feed the heterotrophic bacteria that swim over to breathe the oxygen. Marine water is extremely sparse, a nutrient desert. Without organic food, the heterotrophs cannot use the oxygen, and cannot make enzymes to convert hydrogen peroxide to harmless water.

We don’t yet know if that’s the real reason. Another, very different possibility is that the Prochlorococcus bacteria send out vesicle decoys for viruses. Marine viruses are even more numerous than bacteria; and they are actually the main predators on photosynthetic bacteria. But viruses require highly specific host receptors, found on given species. The same receptors would be on the vesicle decoys, which would absorb the viruses harmlessly away from the growing cells.

Whatever the reason, the relationship between these tiny green life forms and their bacterial neighbors is full of intricate details. There is competition, and much more, a social fabric in which many individuals depend upon each other.