The cyanobacterium Prochlorococcus1,2 is the dominant oxygenic phototroph in the tropical and subtropical regions of the world's oceans1,3,4. It can grow at a range of depths over which light intensities can vary by up to 4 orders of magnitude. This broad depth distribution has been hypothesized to stem from the coexistence of genetically different populations adapted for growth at high- and low-light intensities4,5,6. Here we report direct evidence supporting this hypothesis, which has been generated by isolating and analysing distinct co-occurring populations of Prochlorococcus at two locations in the North Atlantic. Co-isolates from the same water sample have very different light-dependent physiologies, one growing maximally at light intensities at which the other is completely photoinhibited. Despite this ecotypic differentiation, the co-isolates have 97% similarity in their 16S ribosomal RNA sequences, demonstrating that molecular microdiversity, commonly observed in microbial systems7,8,9,10,11,12 can be due to the coexistence of closely related, physiologically distinct populations. The coexistence and distribution of multiple ecotypes permits the survival of the population as a whole over a broader range of environmental conditions than would be possible for a homogeneous population.
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We thank M. Sogin for access to an automated DNA sequencer and helpful discussion; A. Shimada for the use of unpublished sequences of Prochlorococcus GP2 and SB; R.Olson and E. Zettler for cell sorting at sea; H. Hsu for technical assistance; and C. Cavanaugh, H. Sosik and S. Sathyendranath for helpful consultation. This work was supported by the US National Science Foundation and the US National Aeronautics and Space Administration.
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Moore, L., Rocap, G. & Chisholm, S. Physiology and molecular phylogeny of coexisting Prochlorococcus ecotypes. Nature 393, 464–467 (1998). https://doi.org/10.1038/30965
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