Potential biomarkers for tracing kelp carbon sequestration

One key part of our research programme is to establish the contribution of (cultivated) kelp forests to carbon sequestration. This involves analysing the natural process of kelp’s ability to absorb carbon and releasing the organic material sinking to the deeper ocean. When the organic material is stored in the deeper ocean, it is unlikely to be disturbed for thousands of years. Researching this natural process presents an opportunity to mitigate climate change.

To better understand the specific role of kelp in the blue carbon cycle, we need to be able to trace carbon found in marine sediments and samples back to the plant (the kelp). One way to investigate the origin of carbon are biomarkers. Biomarkers are organic molecules containing information about their biological origin that can be maintained after burial in sedimentary environments. A good biomarker is typically considered to be a compound that is produced by a limited group of organisms, and that is present in relatively consistent abundances in the organism. Lipids are a class of biomolecules such as fatty acids and sterols, synthesised by living organisms. Although kelp lipids have been studied previously, kelp specific lipid biomarkers have not been explored in the context of Blue Carbon before despite their potential as useful carbon tracers.

Emilia Heiskanen, MSc student Marine Sciences at Utrecht University (image above), worked on the preliminary investigation of “Macroalgal contribution to blue carbon using organic lipid biomarkers”. The importance of this research lied in “fingerprinting” the carbon burial capacity of three seaweed species found in Luderitz: Macrocystis pyrifera (giant kelp), Ecklonia maxima, and Laminaria pallida. Emilia analysed samples of these species collected from wild and cultivated populations in South Africa and Namibia. The preliminary results showed that one sterol, namely fucosterol, was highly abundant in all three species representing over 80% the total sterol abundance. Furthermore, this sterol was also found in all underlying sediments, albeit in low abundances. Because fucosterol is a commonly found in kelp and other brown macroalgal species, it could provide a potential tool for tracing organic carbon from all brown macroalgae (class Phaeophyceae) rather from one particular species. However, there are limited number of other species that also synthesise this compound and therefore further research is needed to better understand the source of the sedimentary fucosterol. Furthermore, additional research is needed to understand how much of the kelp carbon is stored in the sediments and on what timescales, and whether fucosterol could be traced also in deeper sediments further offshore.

A summary of Emilia’s findings:

• The majority of compounds found were fatty acids (FAME) and sterols.
• There were found only a few FAMEs and they were common across different taxa so not suitable as biomarkers
• Fucosterol, although found in all three macroalgal species, had a high abundance and traceability which highlights its potential as a biomarker for brown macroalgae (but further research is needed).
• Kelp seems to supports the marine food web (as carbon donors) at the study sites, there is no direct carbon burial seen (43% NPP export)
• Potential future study: Can fucosterol trace brown macroalgae in deep offshore sediments?

Emilia’s research is currently being developed into a publication, together with her supervisors Dr. Ir. Francien Peterse and Prof. Jack Middelburg of the Faculty of Geosciences at Utrecht University, the Netherlands