12/19/2013
Marine Ecosystems More Complex Than Previously Thought
The Science
Marine microbial communities are complex and dynamic, and their ecology impacts biogeochemical cycles in pelagic ecosystems. Yet, little is known about the relative activities of different microbial populations within genetically diverse communities. Researchers used rRNA as a proxy for activity to quantify the relative specific activities (rRNA/ribosomal DNA [rDNA or rRNA genes]) of the eubacterial populations and to identify locations or clades for which there are uncouplings between specific activity and abundance. After analyzing 1.6 million sequences from 16S rDNA and rRNA (cDNA) libraries from two euphotic depths from a representative site in the Pacific Ocean, researchers show that although there is an overall positive relationship between the abundances (rDNAs) and activities (rRNAs) among populations of the bacterial community, for some populations these measures are uncoupled. Different ecological strategies are exemplified by the two numerically dominant clades at this site: the cyanobacterium Prochlorococcus is abundant but disproportionately more active, while the heterotrophic SAR11 is abundant but less active. Other rare populations, such as Alteromonas, have high specific activities in spite of their low abundances, suggesting intense population regulation.
Summary
The tiny cyanobacterium Prochlorococcus is among the most abundant and important in the oceans, and distinct variants (“ecotypes”) exist at different water depths. An estimated 100 million cells of this unicellular organism can be found in a single liter of seawater. These cyanobacteria help remove some 10 billion tons of carbon from the atmosphere each year. New research addresses a long-held assumption that the size of a microbial population in the marine community corresponds to its level of activity in terms of carbon uptake and growth rate, thus determining its impact on global biogeochemical cycles. Researchers, including scientists at the U.S. Department of Energy’s Joint Genome Institute, studied the activity levels of several Prochlorococcus ecotypes at several locations in the Pacific and Atlantic oceans. The results suggest that the theory does not fully explain the link between abundance levels and activity. In their article, the authors state: “Our results suggest that low abundance microbes may be disproportionately active in certain environments and that some of the most abundant may have low metabolic activity.” “We observed uncoupling of abundance and specific activity of Prochlorococcus in the Sargasso Sea depth profile, which highlights deficiencies in our understanding of marine microbial ecology and population structure.” They conclude that marine ecosystem functioning is likely to be more complex and dynamic than previously thought. This finding has significant implications for understanding the role of the oceans in the global carbon cycle.
BER Program Manager
Ramana Madupu
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Biological Systems Science Division
[email protected]
References
Hunt, D. E., Y. Lin, M. J. Church, D. M. Karl, S. G. Tringe, L. K. Izzo, and Z. I. Johnson. 2012. “The Relationship Between Abundance and Specific Activity of Two Bacterioplankton in Open Ocean Surface Waters,” Applied Environmental Microbiology. DOI: 10.1128/AEM.02155-12.