|Transmission electron micrograph of one of the smallest known eukaryotic algae, Micromonas. Credit for TEM: A.Z. Worden, T. Deerinck, M. Terada, J. Obiyashi and M. Ellisman (MBARI and NCMIR). Click to enlarge.|
Scientists from two-dozen research organizations led by the US Department of Energy (DOE) Joint Genome Institute (JGI) and the Monterey Bay Aquarium Research Institute (MBARI) have decoded the genomes of two strains of the photosynthetic algal genus Micromonas, highlighting the genes enabling them to capture carbon and maintain their delicate balance in the oceans.
The analysis of the genomes, sequenced by a team led by Alexandra Z. Worden of MBARI and published in the 10 April edition of the journal Science, offers insights into ecological differentiation and the dynamic nature of early plant evolution with relevance for work on climate change and cellular processes related to algae-derived biofuels being pursued by DOE scientists.
The study sampled two geographically diverse isolates of Micromonas—one from the South Pacific, the other from the English Channel. The analysis identified approximately 10,000 genes in each, compressed into genomes totaling about 22 million nucleotides.
Yet, surprisingly, they are far more diverse than we originally thought. These two picoeukaryotes, often considered to be the same species, only share about 90 percent of their genes.—Alexandra Worden
By contrast, humans and some primates have about 98% genes in common. Worden said that the algae’s divergent gene complement may cause them to access and respond to the environment differently. “This also means that as the environment changes, these different populations will be subject to different effects, and we don’t know whether they will respond in a similar fashion.”
Worden said that their apparently broad physiological range (exemplified by their expansive geographical range) may result in increased resilience as compared to closely related species, enabling them to survive environmental change better than organisms with a narrower geographic range. Testing the hypotheses developed through cataloging their respective inventory of genes, Worden said, will go a long way towards understanding their biology and ecology.
Tiny Micromonas, less than two microns in diameter, or roughly a 50th of the width of a human hair, are one of the few globally distributed marine algal species, thriving throughout the world’s oceans from the tropics to the poles.
Micromonas is a representative of a well-sampled group of green algae with the largest number of sequenced genomes. With these four genomes in hand—two Micromonas and two Ostreococcus—we can observe patterns of genome organization as well as the diversity between different organisms in this group.—JGI’s Igor Grigoriev, one of the senior authors
Embedded in the genetic code are clues about how photosynthesis transformed from a barren orb into the earth we know today.
The Micromonas genomes encapsulate features that now appear to have been common to the ancestral algae that initiated the billion-year trajectory that led to the rise of land plants, said Worden. As highlighted in the Science article, comparing the strains to each other and in turn to the other characterized algal and plant genomes, will help to illustrate the dynamic nature of evolutionary processes and provide a springboard for unraveling the functional aspects of these and other phytoplankton populations.
The Micromonas genomes reveal features of the ancestral algae that initiated the billion-year trajectory of the green lineage and the greening of Earth. Their divergence, combined with acquisition strategies that are consistent with HGT [horizontal gene transfer], highlight the dynamic nature of marine protistan evolution and provide a springboard for unraveling functional aspects of phytoplankton populations. The challenge now is to identify biogeochemically important features within this natural diversity and apply them in assessing ecological transformations caused by environmental change.Worden et al. (2009)
Motility is another distinguishing aspect of the ecology of Micromonas. Unlike other algae genera sequenced to date, these swift swimmers can cut through the water column at a rate of 50 body lengths per second, and are phototactic, meaning that they can swim towards the sunlight from which they derive their energy.
In previous studies, Worden and her colleagues showed that picoeukaryotes such as Micromonas comprised, on average, only a quarter of the picophytoplankton cells in a Pacific Ocean sampling, but were responsible for three-quarters of the net carbon production. They were also shown to be subject to heavy grazing pressure; their lack of a cell wall may make them more digestible as prey. In this case carbon may be efficiently sequestered by the “biological pump,” the suite of processes that enable the algae to capture atmospheric carbon and transport it from the ocean surface zones to the depths below.
This research serves as a complement to field studies seeking to confirm emerging key players in global carbon fixation.
By understanding which genes a specific strain employs under certain conditions, we gain a view into the factors that influence the success of one group over another. We may then be able to develop models that could more effectively predict a range of possible future scenarios, that will result from current climate change.—Alexandra Worden
Micromonas may well serve as a bellwether for current and future ocean conditions, helping to guide appropriate decision making.
The genome sequencing of Micromonas was conducted under the auspices of the DOE JGI Community Sequencing Program (CSP), supported by the DOE Office of Science. The CSP was created to provide the scientific community at large with access to high-throughput sequencing for projects selected on the criteria of scientific merit—judged through independent peer review—and relevance to the DOE missions.
The US Department of Energy Joint Genome Institute, supported by DOE’s Office of Science, is committed to advancing genomics in support of DOE missions related to clean energy generation and environmental characterization and cleanup. DOE JGI, located in Walnut Creek, Calif., provides integrated high-throughput sequencing and computational analysis that enable systems-based scientific approaches to these challenges.
Alexandra Z. Worden et al. (2009) Green Evolution and Dynamic Adaptations Revealed by Genomes of the Marine Picoeukaryotes Micromonas. Science Vol. 324, no. 5924, pp. 268-272 doi: 10.1126/science.1167222
John M. Archibald (2009) Green Evolution, Green Revolution. Science Vol. 324. no. 5924, pp. 191 - 192 doi: 10.1126/science.1172972