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Prochlorococcus marinus MIT 9313
   
   
 


Photo: C.Ting, J.King, S.W.Chisholm, 1999

The Prochlorococcus group is speculated to be the most abundant photosynthetic organism on the planet (Partensky et al 1999), and is responsible for a significant fraction of photosynthesis in the world's oceans. In the subtropical Pacific, for example, it often represents 50% of the total chlorophyll. These tiny cyanobacteria are also the smallest known photosynthetic organisms with a genome size of approximately 2 Mb, and possess an unorthodox pigment composition of divinyl derivatives of chlorophyll a and b, a-carotene, zeaxanthin (Goericke and Repeta, 1992, Chisholm et al 1988) and a type of phycoerythrin (Hess et al 1996). The latter has not yet been shown to function in light harvesting. By contrast, the highly related Synechococcus contains chlorophyll a and phycobilins that are more typical of cyanobacteria. To date, Prochlorococcus is the only photosynthetic organism known to contain this particular combination of pigments. As such, it has recently been suggested to be an extant model for the ancestral photosynthetic bacterium that gave rise to cyanobacteria as well as chloroplasts (Tomitani et al 1999). Sequence analysis of the Prochlorococcus genome may shed more light on this hypothesis, and on cyanobacterial radiation in general.

Prochlorococcus: The Smallest and Most Abundant Photosynthetic Microbe in the Oceans

Prochlorococcus is a unicellular cyanobacterium that dominates the temperate and tropical oceans. It lacks phycobilisomes that are characteristic of cyanobacteria, and contains chlorophyll b as its major accessory pigment. This enables it to absorb blue light efficiently at the low-light intensities and blue wavelengths characteristic of the deep euphotic zone. It contributes 30-80% of the total photosynthesis in the oligotrophic oceans, and thus plays a significant role in the global carbon cycle and the Earth's climate. Description of the complete genome of this microbe will greatly advance our understanding of the regulation of these globally important processes. Prochlorococcus is an ideal candidate for complete genome sequencing because (1) it is the smallest known phototroph with a relatively small genome (1.8 Mb), (2) it is widespread and abundant and is easily identified and enumerated in situ using flow cytometry, (3) its unique photosynthetic pigment (divinyl chlorophyll) makes its contribution to total photosynthetic biomass in the oceans easily assessed, and (4) we have an extensive culture collection of isolates from different oceans and environments. At least two ecotypes of Prochlorococcus coexist in the oceans that are distinguished by their photophysiology and molecular phylogeny. One is capable of growth at irradiances where the other is not. Ultimately, a comparison of the complete genomes of these two ecotypes would provide valuable insights into the regulation of this type of microdiversity in marine microbial systems. In addition, the use of microarray technology for the analysis of gene expression patterns will give us unprecedented insights into how these microbes cope with the dilute environment of the oligotrophic oceans.

Light-harvesting apparatus of Prochlorococcus

Prochlorococcus marinus is the type species of an only in 1992 described group of marine cyanobacteria that use (divinyl-)chlorophyll a and b. Prochlorococcus possesses a remarkable adaptability to different environmental conditions and might be the most abundant oxyphototrophic organism on earth. In this project, selected genetic and physiological characteristics of the photosynthetic apparatus of Prochlorococcus are studied. In particular, the function of a Prochlorococcus-specific phycobiliprotein-based putative light harvesting system is investigated that exist in some genotypes but not in other, whereas all Prochlorococcus strains use (divinyl-)chlorophyll a/b-protein complexes as the major antenna system.