Key Points
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Prochlorococcus is the numerically dominant phototroph in the oceans and is responsible for a notable fraction of global photosynthesis.
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Prochlorococcus populations contain distinct subgroups with remarkable genetic and physiological diversity, which contributes to their stability, abundance and wide distribution in the oceans.
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Cells have distinct adaptations to environmental factors such as light intensity, temperature and nutrient levels.
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Although each individual cell has a small, 'streamlined' genome, collectively, the global Prochlorococcus population (that is, the pan-genome) contains a vast number of different genes.
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Interactions with phages and heterotrophs have a crucial role in shaping Prochlorococcus physiology and diversity.
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Prochlorococcus represents a useful model system for understanding microbial ecology.
Abstract
The marine cyanobacterium Prochlorococcus is the smallest and most abundant photosynthetic organism on Earth. In this Review, we summarize our understanding of the diversity of this remarkable phototroph and describe its role in ocean ecosystems. We discuss the importance of interactions of Prochlorococcus with the physical environment, with phages and with heterotrophs in shaping the ecology and evolution of this group. In light of recent studies, we have come to view Prochlorococcus as a 'federation' of diverse cells that sustains its broad distribution, stability and abundance in the oceans via extensive genomic and phenotypic diversity. Thus, it is proving to be a useful model system for elucidating the forces that shape microbial populations and ecosystems.
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Acknowledgements
The authors thank members of the Chisholm laboratory, L. Kelly, O. Cordero and M. Polz for providing helpful comments on the manuscript. The authors also thank J. Waldbauer for carrying out the initial calculations that inspired Figure 1b. S.B., P.B. and S.W.C. were supported by grants from the Gordon and Betty Moore Foundation (grant GBMF495 to S.W.C.) and the National Science Foundation (OCE-1153588, OCE-1356460 and DBI-0424599, the NSF Center for Microbial Oceanography Research and Education). D.L. was supported by the Israel Science Foundation (Morasha grant 1504/06) and the European Research Council (starting grant 203406).
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Glossary
- Phytoplankton
-
Free-floating aquatic photosynthetic microorganisms that require sunlight and inorganic nutrients for growth
- Euphotic zone
-
The sunlit upper region of the ocean water column that receives sufficient light energy to sustain photosynthesis. The depth can vary depending on local conditions, but it is generally the upper ∼200 m in oligotrophic waters.
- Oligotrophic
-
A term used to describe an environment with low concentrations of available nutrients.
- Ecotypes
-
Genetically and physiologically differentiated subgroups of a species that occupy a distinct ecological niche.
- ITS sequence
-
(Internal transcribed spacer sequence). A non-functional rRNA sequence located between the 16S and 23S ribosomal RNA genes in bacteria, which is a useful phylogenetic marker.
- Clades
-
Coherent phylogenetic groups of organisms, each of which comprises all the descendents of a single ancestor.
- Siderophore
-
A molecule that can bind iron; it is often used by microorganisms to facilitate the acquisition of iron from the environment.
- Oxygen minimum zones
-
Subsurface ocean regions that are deficient in oxygen owing to poor ventilation and high rates of respiration.
- Gyres
-
Ocean systems bounded by circular rotating winds and currents. The five major ocean gyres are found in the North Atlantic, North Pacific, South Atlantic, South Pacific and Indian Oceans.
- Pan-genome
-
The complete set of genes that is encoded by all the genomes of a defined group of organisms.
- Synteny
-
The conserved ordering of genes along a chromosome.
- Paralogous genes
-
A pair of similar genes that were created by a duplication event.
- Effective population size
-
In population genetics, the size of an idealized population that would be expected to behave in the same manner as the actual population in terms of the effects of selection and genetic drift.
- Genetic drift
-
The change in the frequency of an allele in a population due to chance or random events.
- Cyanophages
-
Phages that infect cyanobacteria.
- Lysogenic phages
-
Bacteriophages that are capable of integrating their genome into the host genome and are replicated along with the cell, without killing it.
- Calvin cycle
-
The biochemical process that converts CO2 into organic carbon.
- Reducing power
-
In redox chemistry, the availability of compounds that can supply electrons.
- Axenic
-
A term used to describe a pure culture of a single organism that is free of any other contaminating organism.
- Reactive oxygen species
-
(ROS). Oxygen-containing compounds, such as H2O2, that readily react with and damage cellular components.
- Extracellular membrane vesicles
-
Small (∼20–200 nm diameter) spherical structures enclosed by a lipid bilayer. In Gram-negative cells, they are thought to be derived from the outer membrane.
- Mixotrophic
-
A term used to describe an organism that can use multiple metabolic modes for acquiring energy or carbon for growth. In the context of this Review, this refers to organisms that can use both CO2 (autotrophy) and organic carbon (heterotrophy).
- Autotrophic
-
A term used to describe an organism that can build complex, energy-containing organic molecules from CO2 using either light or inorganic chemical reactions as an energy source. Prochlorococcus is capable of photoautotrophic growth and uses light energy to turn CO2 into organic carbon via photosynthesis.
- Microbial loop
-
The network of interactions among microorganisms at the base of the marine food web through which carbon and other nutrients move before they are supplied to larger organisms.
- Salps
-
Marine tunicates that consume plankton by filter feeding.
- Ocean stratification
-
The division of the water column into low-density and high-density zones, with a boundary layer (the pycnocline) defined by a gradient of densities across which water will not passively mix. Changes in density that lead to stratification are typically due to differences in temperature and salinity.
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Biller, S., Berube, P., Lindell, D. et al. Prochlorococcus: the structure and function of collective diversity. Nat Rev Microbiol 13, 13–27 (2015). https://doi.org/10.1038/nrmicro3378
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DOI: https://doi.org/10.1038/nrmicro3378
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