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Prochlorococcus marinus is the smallest known photosynthetic organism as well as the most abundant in the ocean.

You know the funny thing about trying to Google a bacteria? You can find the complete genetic sequence online, and you can find out that though discovered in the 1980s and described in 1992, the name was not officially recognized until 2001 (apparently there are strict rules about how you get an organism listed in the Bacteriological Code, and there might be an international brouhaha among scientists who concern themselves with these things if you do it wrong)-- but it can be damned hard to find a simple description of what the thing is or even its taxonomy.

Fun facts about the Prochlorococcus genus.

  1. They are prokaryotes.
  2. It's very small. At roughly 0.5 to 0.7 µm in diameter it is the smallest known photosynthetic organism. So small, it wasn't even discovered until epifluoresence microscopy detected a dim red glow from this organism's unique pigments. Its tiny size is an evolutionary advantage: when you're small, you don't need as many nutrients to survive, and you absorb them easily, so you can live even where nutrients (like nitrogen and phosphorus) are below detectable levels.
  3. It's all over, and there is a lot of it. Prochlorococcus is found in the 40°S to 40°N latitudinal band of the world's oceans. You can find it at the surface down to depths of 200 m, making it presumably the most abundant photosynthetic organism on Earth. (Which means, if you remember your carbon cycle, that it plays a significant role in the Earth's climate).
  4. Prochlorococcus typically divides once a day in the subsurface layer of oligotrophic areas, such as the subtropical Pacific, where it may make up to 80% of the photosynthetic biomass. It also possesses a pigment complement which includes divinyl derivatives of chlorophyll a (Chl a) and Chl b, and, in some strains, small amounts of a new type of phycoerythrin.
  5. Phylogenetically, Prochlorococcus is an oddball. It lacks the phycobilisomes (multi-protein light harvesting complexes) of other cyanobacteria. Recent studies suggest that it evolved from an ancestral cyanobacterium by reducing its cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system which relies on chlorophyll b to absorb blue light efficiently.
  6. Biologists have found three genetically distinct ecotypes, which are physiologically and genetically adapted to grow in different ecological niches. The type at the light-abundant surface (the top 100m), nicknamed Strain MED4 may eventually be characterized as a different species than Strains SS120 and MIT 9313, found at light-poor but nutrient-rich depths of 80-200m. Considering that at 200m you'll find only one tenth of one percent of the light hitting the surface, it's probably good to have alternative ways to get energy and absorb light. The types' respective antennae, pigments, and proteins differ enough from each other that the different types may eventually be characterized as different species.
  7. Why can you find its genetic sequence online so easily? Simply stated, MED4 has a mere 1.67 Mega-base pairs in its genome, and MIT 9313 approximately 2.40 Mega-base pairs. In gene-sequencing terms, they are fairly simple to sequence. Also, when describing a species this tiny, the distinguishing characteristics, such as the proteins that make up its energy gathering parts, you may as well look at the genes, since you're not so far from that scale.

Kingdom: Bacteria
Phylum: Cyanobacteria;
Order: Prochlorophytes;
Family: Prochlorococcaceae;
Genus: Prochlorococcus

F. Partensky, W. R. Hess, and D. Vaulot. "Prochlorococcus, a Marine Photosynthetic Prokaryote of Global Significance." Microbiology and Molecular Biology Reviews, March 1999, p. 106-127, Vol. 63, No. 1 <http://mmbr.asm.org/cgi/content/full/63/1/106> (1 October 2003)
Alexis Dufresne and Frederic Partensky. Prochlorococcus marinus SS120 genome project Home Page. 1 September 2003. <http://www.sb-roscoff.fr/Phyto/ProSS120/> (19 December 2003)
Stephanie Malfatti, "Prochlorococcus marinus MED4." JGI Microbes. <http://genome.jgi-psf.org/finished_microbes/prom4/prom4.home.html> (19 December 2003).
Michael Stulberg and Casey M. Smith. "Prochlorococcus." The Microbial Biorealm. <http://biology.kenyon.edu/Microbial_Biorealm/bacteria/prochlorococcus/prochlorococcus.htm> (22 December 2003)