11 JULY 2014 • VOL 345 ISSUE 6193 135 SCIENCE sciencemag.org
ocean environment is far too complex to be
adequately mimicked in the laboratory. Yet
Ottesen et al. show that the transcriptional
patterns of wild Prochlorococcus are remarkably similar to those of a cultured
Prochlorococcus strain kept under controlled
laboratory conditions. How can growth in
an artificial setting be so similar to that
in the natural environment? One possible
explanation is that the very small genome
of Prochlorococcus has eliminated nuance,
resulting in a limited range of regulatory responses to a given environmental condition
(4). Alternatively, the input of light energy
and the resulting synchronization of cell
division of wild Prochlorococcus cells may
dominate their regulatory network. The observed similarities illustrate the importance
of developing model marine microbes in
parallel with field studies.
Ottesen et al. also uncover differences between the field and the laboratory. A small
subset of genes that display diel cycling
in the field is missing from the laboratory
strain, reiterating the potential importance
of the genetic diversity in natural
Prochlorococcus populations (5). A larger subset of
genes shows different periodicities in the
field and the laboratory, with a large fraction of these genes encoding proteins of unknown function. Identifying their roles may
help to explain how these organisms adapt
to their environment.
A second motivation for at-sea experi-
Each day, photosynthetic organisms like
ments is that most marine bacteria can-
not yet be cultured in the lab; knowledge
of them comes from environmental DNA
sequence information or from transcrip-
tome studies of natural microbial commu-
nities. In the sunlit surface waters at Station
Aloha, most bacteria that require organic
carbon also possess proteorhodopsin, a
light-driven proton pump that presumably
supplements different metabolic activities,
especially when organic carbon is scarce
(6). Transcription of the proteorhodopsin
gene is tightly synchronized to the day/
night cycle, which Ottesen et al. attribute
to optimized light energy capture by these
organisms. But is proteorhodopsin enough
to create the observed transcriptional syn-
chrony across multiple genes and organ-
isms? Earlier work by the same group (7)
in Monterey Bay suggests not. Despite the
use of similar approaches, a synchronized
response to the light/dark cycle was missing
at the coastal station. What could so tightly
synchronize the bacteria to the day/night
cycle at Station Aloha?
Prochlorococcus harvest light energy to
drive carbon fixation. Prochlorococcus com-
monly releases up to 25% of its fixed organic
carbon into the seawater (8). Previous stud-
ies (9, 10) suggest that bacterial communi-
ties that require organic carbon for growth
could synchronize their growth with the
light/dark cycle because of the light-driven
release of organic matter by photosynthetic
organisms. This synchrony may be particu-
larly important and is more easily observed
in the open ocean, far from the influence
of land-derived organic matter. But the ex-
quisite coupling of waves of gene
expression across different types
of bacteria seen by Ottesen et al.
still comes as a surprise.
The observed synchrony of gene
expression may rely on at least
two factors: The pulse of organic
carbon from Prochlorococcus may
harmonize the overall transcriptional responses of the bacteria,
The ability to simultaneously monitor
transcriptional profiles of multiple organisms in situ over time provides the opportunity to answer questions about how
microorganisms interact with their environment and with each other, and how
these interactions influence ecosystem stability. The large proportion of genes with
unknown functions will continue to present
challenges. The greatest challenge will be
to understand how far-reaching the genetic
choreography observed at Station Aloha is
over time and space, and how these interactions expand across the entire microbial
1. B. K. Swan et al ., Proc. Natl. Acad. Sci. U.S.A. 110, 11463
2. S. J. Giovannoni, T. B. Britschgi, C. L. Moyer, K. G. Field,
Nature 345, 60 (1990).
3. E. A. Ottesen et al., Science 345, 207 (2014).
4. E. R. Zinser et al ., PLOS ONE 4, e5135 (2009).
5. N. Kashtan et al ., Science 344, 416 (2014).
6. E. F. DeLong, O. Béjà, PLOS Biol. 8, e1000359 (2010).
7. E.A.Ottesen et al., Proc. Natl. Acad. Sci. U.S. A. 110,E488
8. S. Bertilsson, O. Berglund, M. J. Pullin, S. W. Chisholm, Vie
Milieu 55, 225 (2005).
9. J. A. Fuhrman, R. W. Eppley, Å. Hagström, F. Azam, Mar.
Ecol. Prog. Ser. 27, 9 (1985).
10. J. M. Gasol et al ., Mar. Ecol. Prog. Ser. 164, 107 (1998).
11. J. J. Morris, R. E. Lenski, E. R. Zinser, MBio 3, e00036-12
12. K. M. Jones, H. Kobayashi, B. W. Davies, M. E. Taga, G. C.
Walker, Nat. Rev. Microbiol. 5, 619 (2007).
Synchronized transcription profles
Exquisite synchrony. Surface waters of subtropical gyres contain relatively few types of abundant microbes, as indicated by the
different colors. Photosynthesis in these regions is dominated by the cyanobacterium Prochlorococcus. Ottesen et al. have used
autonomous sampling technology to obtain in situ transcription profiles of microbial communities over the day/night cycle. Profiles
for a subset of genes from two idealized microbes are shown—one photosynthetic, such as Prochlorococcus, and the other containing
the light-driven proton pump proteorhodopsin. The bar at the bottom indicates night (dark bar) and day (yellow bar). The observed
periodicity in transcription profiles may translate into a periodicity of metabolic processes in these open-ocean communities.
School of Oceanography, University of Washington, Seattle,
WA 98195, USA. E-mail: firstname.lastname@example.org