New paper: What happens to carbon released by marine macrophytes?

We are very pleased to share a new paper led by Postdoctoral Researcher Dr Leah Brinch-Iversen, investigating what happens to dissolved organic carbon (DOC) released by two widespread coastal primary producers: the brown alga Fucus serratus and the seagrass Zostera marina. The work was carried out together with Ronnie N. Glud, Mathias Middelboe and Karl Attard, bringing together expertise across LUMEN, HADAL and Nordcee at SDU and the University of Copenhagen.

A key part of the study was the development of flow-through incubation chambers that allowed fragments of Fucus and Zostera to be followed for 85 days while maintaining continuous water exchange. The fragments were kept in darkness to simulate material exported below the photic zone, while DOC release and physiological condition were tracked through time. The released DOC was then exposed to natural microbial communities to determine how much could be consumed, with microbial degradation quantified using oxygen consumption, dissolved inorganic carbon production and several independent measurements of bacterial production and growth efficiency.

Both species continued releasing DOC throughout the experiment, although Fucus released around 30% more DOC overall than Zostera. The quality of that carbon also changed substantially as the fragments aged: the proportion that was readily available to microbes initially increased, peaking around the onset of physiological decline, before falling sharply as the plants senesced.

Across the experiment, microbial communities consumed roughly 40–45% of the released DOC, while about 60% resisted microbial degradation. This recalcitrant DOC represented approximately 13% of the original carbon biomass of Fucus and 11% of Zostera, pointing to a potentially important pathway through which carbon fixed in productive coastal habitats can persist after plant material is exported away from the habitat.

The study highlights that understanding coastal carbon cycling requires looking not only at how much carbon macrophytes produce, but also at what happens to that carbon after plants fragment, senesce and are transported away from the coast.

Reference

Brinch-Iversen L, Glud RN, Middelboe M, Attard K (2026). Release and bioavailability of dissolved organic carbon derived from Fucus serratus and Zostera marina. Marine Ecology Progress Series. Article