Nitrate modulates high CO2 effects on carbon partitioning in Gracilariopsis lemaneiformis: Trade-offs between particulate and dissolved organic carbon sequestration.

Intensive studies have explored the feasibility of using high CO2 concentrations to culture microalgae for carbon capture. Compared to microalgae, macroalgae are easier to be harvested, thereby reducing culture cost. However, little is known regarding the possibility of using macroalgae to fix and sequester high levels of CO2. In this study, Gracilariopsis lemaneiformis was cultured under low (8 μM) and high (200 μM) nitrate concentrations to clarify the physiological and molecular responses of carbon fixation and sequestration to a 10 % high CO2 concentration (HC). HC, inducing low pH, inhibited specific growth rate (SGR) and net particulate organic carbon (POC) production. High nitrate (HN) promoted maximum photochemical efficiency, content of photosynthetic pigments and particulate organic nitrogen (PON), but could not relieve the negative effect of HC and instead led to a further decrease in SGR and net POC production likely by exacerbating cellular C: N imbalances. HC resulted in increased dissolved organic carbon (DOC) and refractory dissolved organic carbon (RDOC) accumulation while HN inhibited the accumulation under HC. Transcriptomic analysis revealed that HC downregulated 56 % of differentially expressed genes (DEGs). Downregulated genes included those involved in the TCA cycle, Calvin cycle, nitrogen assimilation, protein processing and degradation, consistent with HC-inhibited growth. Although 41 % of DEGs were upregulated by HN, the combination of HN and HC led to 78 % DEGs being downregulated. The findings in this study indicate that high CO2 (10 %) reduces net POC production but enhances DOC and RDOC accumulation, with these effects being modulated by nitrate availability. When using macroalgae to capture flue gas-level CO2, attention should be paid to the appropriate concentrations of CO2 (<10 %) and nitrate (<200 μM). This research provides crucial insights into using CO2 in flue gas to culture macroalgae for carbon sequestration.