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.