PHYSIOLOGY AND MOLECULAR PHYLOGENY OF COEXISTING PROCHLOROCOCCUS ECOTYPES

The cyanobacterium Prochlorococcus(1,2) is the dominant oxygenic photo troph in the tropical and subtropical regions of the world's (1,3,4). It can grow at a range of depths over which light oceans intensities c an vary by up to 4 orders of magnitude. This broad depth distribution has been hypothesized to stem from the coexistence of genetically diff erent populations adapted for growth at high-and low-light intensities (4-6). Here we report direct evidence supporting this hypothesis, whic h has been generated by isolating and analysing distinct co-occurring populations of Prochlorococcus at two locations in the North Atlantic. Go-isolates from the same water sample have very different light-depe ndent physiologies, one growing maximally at light intensities at whic h the other is completely photoinhibited. Despite this ecotypic differ entiation, the co-isolates have 97% similarity in their 16S ribosomal RNA sequences, demonstrating that molecular microdiversity, commonly o bserved in microbial systems(7-12), can be due to the coexistence of c losely related, physiologically distinct populations. The coexistence and distribution of multiple ecotypes permits the survival of the popu lation as a whole over a broader range of environmental conditions tha n would be possible for a homogeneous population.