Ocean warming interacts with elevated nitrogen to modulate carbon storage-excretion balance in Gracilariopsis...

Macroalgae farming represents a proposed approach for carbon dioxide removal (CDR), as macroalgae exhibit distinct advantages over other blue carbon taxa in cultivation scalability and cost effectiveness. Yet how future warming and coastal nitrogen enrichment will jointly affect macroalgal carbon fixation and sequestration remains poorly understood. Here, we investigated physiological and biochemical responses (growth, photosynthesis, pigment content, and carbon accumulation and excretion) of the commercially important seaweed Gracilariopsis lemaneiformis under a cross-factorial design of two temperatures (LT, 20°C; HT, 23°C) and two nitrate concentrations (LN, 8 μM; HN, 200 μM). By day 30, HN alone significantly increased growth, photosynthetic activity, pigments, phycobiliproteins, and particulate organic carbon/nitrogen (POC/PON) accumulation, but did not alter dissolved organic carbon (DOC) production rates or DOC/refractory DOC (RDOC) accumulation. Notably, HT alone had no significant effect on growth, POC accumulation or DOC/RDOC accumulation by day 30, but markedly promoted POC accumulation when combined with HN. These findings highlight a shift in carbon partitioning towards cellular storage (POC) rather than secretion (DOC) under future scenarios of combined ocean warming and elevated nitrogen levels. This study provides critical insights into using macroalgae farming as a potential CDR strategy through POC export to the deep ocean under concurrent climate change and coastal eutrophication.