Seaweed biogeochemistry: Global assessment of C:N and C:P ratios and implications for ocean afforestation.

ABSTRACT

Algal carbon-to-nitrogen (CN) and carbon-to-phosphorus (CP) ratios are fundamental for understanding many oceanic biogeochemical processes, such as nutrient flux and climate regulation. We synthesized literature data (444 species, >400 locations) and collected original samples from Tasmania, Australia (51 species, 10 locations) to update the global ratios of seaweed carbon-to-nitrogen (CN) and carbon-to-phosphorus (CP). The updated global mean molar ratio for seaweed CN is 20 (ranging from 6 to 123) and for CP is 801 (ranging from 76 to 4102). The CN and CP ratios were significantly influenced by seawater inorganic nutrient concentrations and seasonality. Additionally, CN ratios varied by phyla. Brown seaweeds (Ochrophyta, Phaeophyceae) had the highest mean CN of 27.5 (range 7.6-122.5), followed by green seaweeds (Chlorophyta) of 17.8 (6.2-54.3) and red seaweeds (Rhodophyta) of 14.8 (5.6-77.6). We used the updated CN and CP values to compare seaweed tissue stoichiometry with the most recently reported values for plankton community stoichiometry. Our results show that seaweeds have on average 2.8 and 4.0 times higher CN and CP than phytoplankton, indicating seaweeds can assimilate more carbon in their biomass for a given amount of nutrient resource. The stoichiometric comparison presented herein is central to the discourse on ocean afforestation (the deliberate replacement of phytoplankton with seaweeds to enhance the ocean biological carbon sink) by contributing to the understanding of the impact of nutrient reallocation from phytoplankton to seaweeds under large-scale seaweed cultivation.