THANK YOU, TINIEST PHOTOSYNTHESIZERS

(Picoplankton, viewed by epifluorescence. Image by Daniel Vaulot, CNRS, Station Biologique de Roscoff, courtesy Wikimedia Commons)

Amazingly, scientists only discovered the most abundant photosynthetic organisms on Earth—the genus Prochlorococcus—in 1986. Before then, we lacked the tools to see them.

These tiniest of plants are members of the picoplankton, the drifting marine life measuring between 0.2 and 2 microns (one micron = 1 millionth of a meter).

Prochlorococcus make up for their indescribably minute size with their abundance. As many as 100 million individual plants can share a single liter of seawater. As far as we currently understand things, these are the most abundant lifeforms on Earth, with many octillions (10²⁷) of individuals inhabiting the deep blue home and producing as much as 20 percent of the oxygen in Earth's atmosphere. Without them, our terrestrial world would be a dead zone.

(Image by Daniel Vaulot, CNRS, Station Biologique de Roscoff, courtesy Wikimedia Commons)

Prochlorococcus make a living in temperate waters as well as in the nutrient-poor oceans of the tropics. In the image above you can see the vertical distribution of all picoplankton in the Pacific Ocean at 16° South, with Prochlorococcus reaching far higher densities than their cohorts. 

These invisible photosynthesizers are far too small to be studied in situ with ordinary microscopes. Instead, they're counted and collated with the aid of some interesting technologies designed to view fluorescence and phosphorescence rather than reflection and absorption. They're also parsed with some of the incredible tools of molecular biology.

The images (above), taken by flow cytometry, show an analysis of three kinds of picoplankton as characterized by the pigments of their chlorophyll .

An individual Prochlorococcus marinus, as seen through an electron microsope, published by the Marine Picocyanobacteria Genome Project.

(The Station Biologique de Roscoff. Photo by Daniel Vaulot, courtesy Wikimedia Commons.)

Many of the images and much of the information on Prochlorococcus that's posted online (and almost everything appearing in this post) comes from the Station Biologique de Roscoff in Brittany. What a spectacular-looking place to work. 

The images posted here are authored by biological oceanographer Daniel Vaulot—aka Dr. Picocyanobacteria (okay, I made that up)—from the Station Biologique de Roscoff.

Other researchers have authored a new study just published in PNAS revealing two new clades of  Prochlorococcus adapted to iron-depleted environments in the tropical Pacific and Indian Oceans. From the abstract:

The two uncharacterized clades ... are genetically distinct from each other and other high-light Prochlorococcus isolates and likely define a previously unrecognized ecotype. Our detailed genomic analysis indicates that these clades comprise organisms that are adapted to iron-depleted environments by reducing their iron quota through the loss of several iron-containing proteins that likely function as electron sinks in the photosynthetic pathway in other Prochlorococcus clades from high-light environments. The presence and inferred physiology of these clades may explain why Prochlorococcus populations from iron-depleted regions do not respond to iron fertilization experiments and further expand our understanding of how phytoplankton adapt to variations in nutrient availability in the ocean. 

In other words, dumping tons of fertilizer into the nutrient-poor parts of the ocean to stimulate plankton growth and thereby offset our carbon emissions doesn't work. The little workhorses living there are already photosynthesizing overtime and thriving in the absence of nutrients we think life needs to survive.

The PNAS paper:

Characterization of Prochlorococcus clades from iron-depleted oceanic regions. Douglas B. Rusch, et al.  PNAS. Published online before print August 23, 2010, doi: 10.1073/pnas.1009513107.