Recent increases in the intensity of macroalgal blooms and calls for its cultivation for ocean carbon dioxide removal require greater understanding of the production and fate of this carbon. In addition to large amounts of biomass produced by macroalgae, a substantial amount of net primary production can be released in the form of dissolved organic carbon (DOC), some of which may resist remineralization and contribute to carbon export and sequestration. Here we characterize the DOC released by mature and senescent S. natans and show that its composition determines its bioavailability to natural bacterioplankton communities. S. natans derived DOC is enriched in carbohydrates and polyphenolic compounds and is low in hydrolyzable amino acids; reflected in its high carbon-to-nitrogen ratio. Remineralization bioassays demonstrate that the fraction of this DOC that resists biotic degradation over several months is driven, in part, by the total polyphenolic carbon portion of Sargassum DOM amended. Polyphenols released by S. natans are sensitive to light and have an absorption peak in the UV-C range around 270nm. We demonstrate that light can enhance the remineralization of these refractory compounds and quantify the formation of photooxidation products such as CO2. Our results demonstrate that DOC is an important component of S. natans ecology and contribution to carbon cycling. As polyphenols are ubiquitous among brown macroalgae, our results can be extended to environments or aquaculture dominated by other species. 

Primary Presenter: Chance English, University of California, Santa Barbara (cje@ucsb.edu)

Authors:

Chance English, University of California, Santa Barbara  (cje@ucsb.edu)

Rachel Parsons, Bermuda Institute of Ocean Sciences  (Rachel.Parsons@bios.edu)

Kevin Yongblah, Bermuda Institute of Ocean Sciences  (kevin.yongblah@bios.edu)

Craig Carlson, University of California, Santa Barbara  (craig_carlson@ucsb.edu)