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Original Research ARTICLE

Front. Microbiol., 15 November 2011 | http://dx.doi.org/10.3389/fmicb.2011.00229

Constitutive extracellular polysaccharide (EPS) production by specific isolates of Crocosphaera watsonii

Jill A. Sohm, Bethanie R. Edwards, Brian G. Wilson and Eric A. Webb*
  • Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

Unicellular dinitrogen (N2) fixing cyanobacteria have only recently been identified in the ocean and recognized as important contributors to global N2 fixation. The only cultivated representatives of the open ocean unicellular diazotrophs are multiple isolates of Crocosphaera watsonii. Although constituents of the genus are nearly genetically identical, isolates have been described in two size classes, large ∼5 μm and small ∼3 μm cell diameters. We show here that the large size class constitutively produces substantial amounts of extracellular polysaccharides (EPS) during exponential growth, up to 10 times more than is seen in the small size class, and does so under both N2 fixing and non-N2 fixing conditions. The EPS production exceeds the amount produced by larger phytoplankton such as diatoms and coccolithophores by one to two orders of magnitude, is ∼22% of the total particulate organic C in the culture, and is depleted in N compared to cellular material. The large difference in observed EPS production may be accounted for by consistently higher photochemical efficiency of photosystem II in the large (0.5) vs. small (∼0.35) strains. While it is known that Crocosphaera plays an important role in driving the biological carbon (C) pump through the input of new nitrogen (N) to the open ocean, we hypothesize that this species may also contribute directly to the C cycle through the constitutive production of EPS. Indeed, at two stations in the North Pacific Subtropical Gyre, ∼70% of large Crocosphaera cells observed were embedded in EPS. The evolutionary advantage of releasing such large amounts of fixed C is still unknown, but in regions where Crocosphaera can be abundant (i.e., the warm oligotrophic ocean) this material will likely have important biogeochemical consequences.

Keywords: Crocosphaera, extracellular polysaccharides, diazotroph

Citation: Sohm JA, Edwards BR, Wilson BG and Webb EA (2011) Constitutive extracellular polysaccharide (EPS) production by specific isolates of Crocosphaera watsonii. Front. Microbio. 2:229. doi: 10.3389/fmicb.2011.00229

Received: 20 September 2011; Accepted: 28 October 2011;
Published online: 15 November 2011.

Edited by:

Rex Malmstrom, DOE Joint Genome Institute, USA

Reviewed by:

Kathleen Scott, University of South Florida, USA
Hans Paerl, University of North Carolina–Chapel Hill, USA

Copyright: © 2011 Sohm, Edwards, Wilson and Webb. This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.

*Correspondence: Eric A. Webb, Department of Biological Sciences, University of Southern California, 3616 Trousdale Parkway, AHF 107, Los Angeles, CA 90089-0371, USA. e-mail: eawebb@usc.edu