sets encode proteins that are involved in the
interface that the cell presents to the external environment (e.g., by modifying the cell
surface). In future studies, it will be of interest to ascertain the rates of diversification of
these proteins and how they have varied over
time. It will also be important to go beyond
genome sequencing and examine the expression of these alleles in the different populations throughout the year.
What must also be addressed is how the
diversity is maintained and on which scale
it is acting (8). With the cell surface impli-
cated as a major driver of microdiversity, the
biotic environment seems likely to be play-
ing a large role (9). This might be a result
of the arms race between predator and prey
and/or between virus and host. Depending
on physical transport of cells (e.g., by ocean
currents and wind-driven mixing), this
might further mean that an individual geno-
type is never eaten by the same grazer twice
(or infected by the same phage twice)—
which would slow the pace of biotic selec-
tion—or that locally there is some sort of
biological network where more rapid coevo-
lution occurs (10, 11).
The work by Kashtan et al. is necessarily
descriptive; the oceanographic context is simplified, the study is based on only three sampling dates, and the mechanisms that drive
genome structure changes over the proposed
time scales are not addressed. The authors
also focused only on high light–adapted (HL)
ecotypes; it is likely that low light–adapted
(LL) ecotypes will display even greater
diversity. Although the results essentially
support an “everything is everywhere but
the environment selects” regime (12), further
work is required to show whether the same
genotypes come back year after year, and if
not why not. Furthermore, it will be interesting to extend the observations beyond the
Sargasso Sea to assess whether ocean circulation can ensure a global distribution of
the same seed populations. Given the huge
effective population size of
Prochlorococcus and the modeling of physical mixing that
the authors present, this appears likely to be
the case, but it will be important to show that
the different genotypes are indeed everywhere, at least at population levels that can
be detected by ultradeep sequencing.
By providing a new window into natural microbial communities, DNA sequencing–based technologies seem likely to continue to provide dramatic increases in our
understanding of the evolution and ecology
of phytoplankton populations. Key questions
in microbial oceanography that can now be
addressed include how adaptation to the environment organizes populations into biomes
(networks of interacting organisms) having
specific biogeochemical functions, the relative role of acclimation and genetic adaptation in shaping these networks, and the
importance of physical transport and intermingling of populations by ocean circulation
with respect to local adaptation. No doubt
Prochlorococcus will continue to lead the
way in revealing the ocean’s secrets.
1. P. Flombaum et al., Proc. Natl. Acad. Sci. U.S. A. 110,
2. N. Kashtan et al., Science 344, 416 (2014).
3. L. R. Moore, G. Rocap, S. W. Chisholm, Nature 393, 464
4. Z. I. Johnson et al., Science 311, 1737 (2006).
5. R. R. Malmstrom et al., ISME J. 4, 1252 (2010).
6. A. D. Barton, S. Dutkiewicz, G. Flierl, J. Bragg, M. J. Follows, Science 327, 1509 (2010).
7. F. Rodriguez-Valera et al., Nat. Rev. Microbiol. 7, 828
8. F. Azam, F. Malfatti, Nat. Rev. Microbiol. 5, 782 (2007).
9. M. L. Coleman, S. W. Chisholm, Trends Microbiol. 15, 398
10. C.-E. T. Chow et al., ISME J. 8, 816 (2014).
11. K. Zwirglmaier et al., Environ. Microbiol. 11, 1767 (2009).
12. L. Tirichine, C. Bowler, Plant J. 66, 45 (2011).
Winter Spring Summer Autumn
Prochlorococcus diversity in the ocean. Seasonal
selection of genotypes (different-colored cells) from
similar-sized populations of the Prochlorococcus collective throughout a schematic annual cycle. (Inset)
Bottom left: An idealized population of plankton in
which different genotypes of Prochlorococcus coexist
with different protist grazers (flagellated cells) and
different phages (not to scale). (Inset) Bottom right
[adapted from (9)]: Diversification based on physical
parameters (temperature, light, and nutrients) may
be operative at higher taxonomic levels, whereas the
fine-scale “leaves” of the tree are likely maintained
by biological phenomena such as nutrient acquisition and phage/grazer resistance. Stars and squiggles
indicate the importance of the cell surface in fine-scale genotype selection (HL, high light–adapted
ecotypes; LL, low light–adapted ecotypes).
10.1126/science.1253817 C R