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A trillion different species on the Earth: Prochlorococcus marinus is the most abundant photosynthetic bacterium

2016-05-25

A new study in Proceedings of the National Academy of Sciences predicts, on the basis of known relationships of abundance and diversity, that there are approximately 10 to the 12th power (a trillion) species existing on Earth, give or take an order of magnitude.  The conclusion is based on combining all the surveys of plant, animal, and microbial species numbers that have been done in the last forty years.  The surveys of microbes done recently determine the number of species in a sample by the number of different DNA strands found.

Prochlorococcus marinus, first identified in 1986, turns out to be the most abundant organism in the ocean.  P. marinus is very small, roughly 0.6 micrometer in size, with a small genome.  There are roughly 2000 genes or less on the circular genome; many less important genes have been stripped away, and many genes that might be needed for flexibility are missing as well.  Recall from my previous post that the minimal organism needs about 400-500 genes just to function.  P. marinus has even combined some enzymes into a single all-purpose protein: a single enzyme does the work of four enzymes used by more bulky organisms.

To compare, eukaryotic algae that are photosynthetic have about 10,000 genes, and humans have roughly 25,000.  The small genome and small size means these organisms are abundant: there can be 100,000 cells of P. marinus in a milliliter of water.  There are two main types of P. marinus, one that lives in the upper, brightly lit ocean above 100 meters depth, and a second that lives in deeper regions of 80 to 200 meters depth.  The lower light form is more efficient at absorbing blue light, which penetrates deeper into the water.

P. marinus is important because it is the most efficient fixer of carbon dioxide in the ocean: it takes carbon from the air, fixes it into proteins, and releases oxygen.  This process is powered by the absorption of light from the sun and is the foundation of the oxygen-rich atmosphere that we enjoy.  This organism is joined by an almost equally abundant and similar organism called Synechococcus, which uses a different form of chlorophyll.

Synechococcus is larger than P. marinus.  There are at least five types of Synechococcus, differing in size, chlorophyll types, sensitivity to salt, motility, and so on.  These larger organisms also abundantly inhabit fresh water, whereas P. marinus is solely found in marine waters.  They also use urea as well as ammonia as a nitrogen source, and prefer nutrient-rich waters, where their larger genome allows more flexibility in growth.

The information about these organisms was extracted from Wikipedia and another “wiki” called microbewiki, where you can see micrographs and genetic information as well as a graph of the enzymatic pathways that P. marinus uses.

There has been speculation that seeding the ocean with iron will stimulate the growth of P. marinus so much that all the excess carbon dioxide in the atmosphere would be removed.  This idea has been studied extensively, but there has not been any conclusion, nor, more importantly, any large-scale attempts to prove that this process might work.  There is (or was) a company called Climos that was founded by Dan Whaley, a “veteran Silicon Valley entrepeneur” who founded GetThere in 1994.  He and others put together a consortium called ISIS (in situ iron studies) that has a website, but like Climos, everything on the site is more than five years old.  I suspect that they have hit a wall.

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