Biomarkers for marine endosymbiotic heterocystous cyanobacteria

The glycolipids synthesized by marine endosymbiotic heterocystous cyanobacteria differ from those synthesized by marine free-living heterocystous cyanobacteria. It has been speculated that these glycolipids could serve as biomarkers to identify endosymbiotic cyanobacteria. In their paper, Bale et al. (2015) outline how they enhanced a pre-existing method to screen endosymbiotic cultures for these specific glycolipids. 

The majority of N₂ fixation in low-nutrient tropical and subtropical oceans is thought to be carried out by cyanobacteria. As N₂ fixation is sensitive to O_2, the microbes have developed different strategies to work around the problem. Some cyanobacteria can form specialised cells, called heterocysts, which degrade their photosynthetic apparatus in order to fix Nitrogen (Munn, 2011).        Recent studies show marine heterocystous cyanobacteria commonly occur as endosymbionts in diatoms and rarely as free-living organisms. The endosymbionts provide the algae with fixed N₂, which enables the diatoms to subsist in low nutrient environments.

The cell walls of heterocystous cyanobacteria contain glycolipids. In free-living cyanobacteria these heterocyst glycolipids (HGs) consist of a Hexose (C₆) head group and long chain diols, triols or hydroxyketones. Traces of C₆-HGs have been found in ancient sediments and could have the potential to serve as evidence for N₂-fixation. However, Schouten et al. (2013) found an endosymbiont containing Pentose (C₅) instead of C₆-HGs, presumably as an adaption to the high concentrations of oxygen in the diatom host. Yet, C₅-HGs had only been reported in two in vitro cultures when Bale et al. carried out their study in the Amazon plume region.

In order to detect C₅-HGs, the High performance liquid chromatography mass spectrometry multiple reaction monitoring (HPLC-MS MRM) system described by Bauersachs et al. (2010) was modified to also detect the alkyl component in C₅-HGs. The system was then applied to samples of suspended particular matter (SPM) and surface sediments (SS) from the marine Amazon plume as well as the freshwater Amazon.

All SPM samples of Richelia intracellularis and its host Hemiaulus hauckii contained at least one of three known C₅-HGs. The analysis of the SS samples from the Amazon shelf and slope yielded similar results. Presumably the HGs are transported vertically through the water column to the sediment, where they can serve as an integrated memory of N₂-fixation. Although the amount of C₅-HGs was higher at the sample sites further out in the ocean, statistical analysis showed no significant correlation between the distance from the river mouth and the abundance of C₅-HGs.

To eliminate the possibility of freshwater sources of C₅-HGs falsifying the results, Bale et al. additionally collected SPM and SS samples from the Amazon river and several floodplain lakes. Their analysis with the MRM method detected no C₅-HGs. Bale et al. conclude, that C₅-HGs seem to be synthesized in situ in the ocean while C₆-HGs seem to be limited to freshwater environments. However, no theory is presented, whether or not the HGs in the freshwater samples originated from free-living or endosymbiotic cyanobacteria, or how the water environment might influence the synthesis of HGs. 

With the development of the HPLC-MS² MRM, Bale et al. have created the opportunity for marine endosymbiotic heterocystous cyanobacteria to be studied on a larger scale. As cyanobacteria are among the major fixers of N₂ in the oceans, their study could help us to better understand the global Nitrogen cycle. Moreover, the potential use of HGs as an integrated memory of N₂-fixation offers the possibility of studying how the N-cycle has changed over the history of life on earth. However, many large scale studies would have to be carried out in order to collect more data. 

References: 

Reviewed Paper:

Bale, N. J., Hopmans, E. C., Zell, C., Sobrinho, R. L., Kim, J. H., Damsté, J. S. S., ... & Schouten, S. (2015). Long chain glycolipids with pentose head groups as biomarkers for marine endosymbiotic heterocystous cyanobacteria. Organic Geochemistry, 81, 1-7. Link: http://www.sciencedirect.com/science/article/pii/S0146638015000169

Further Reading:

Bauersachs, T., Compaoré, J., Hopmans, E. C., Stal, L. J., Schouten, S., & Damsté, J. S. S. (2009). Distribution of heterocyst glycolipids in cyanobacteria. Phytochemistry, 70(17), 2034-2039. Link: http://www.sciencedirect.com/science/article/pii/S0031942209003586

Bauersachs, T., Speelman, E. N., Hopmans, E. C., Reichart, G. J., Schouten, S., & Damsté, J. S. S. (2010). Fossilized glycolipids reveal past oceanic N2 fixation by heterocystous cyanobacteria. Proceedings of the National Academy of Sciences, 107(45), 19190-19194. Link: http://www.pnas.org/content/107/45/19190.short

Munn, C. B. (2011). Marine microbiology: Ecology and applications (2nd ed.). New York: Garland Science, Taylor & Francis Group.

Schouten, S., Villareal, T. A., Hopmans, E. C., Mets, A., Swanson, K. M., & Damsté, J. S. S. (2013). Endosymbiotic heterocystous cyanobacteria synthesize different heterocyst glycolipids than free-living heterocystous cyanobacteria. Phytochemistry, 85, 115-121. Link: http://www.sciencedirect.com/science/article/pii/S0031942212003949