Coupling effects of CO2-induced ocean acidification and nitrogen enrichment on growth, photosynthesis and nitrogen utilization of an invasive seaweed (Sargassum muticum)

Abstract

Sargassum muticum, an invasive seaweed, has colonized many parts of the world along the coast. Marine environment invaded by this species is aggravated the complexity by CO₂-induced ocean acidification (OA) and coastal eutrophication. However, the coupling effects of seawater acidification and eutrophication on this invasive species remain unclear. In this study, we cultured Sargassum muticum at two concentrations of pCO₂ (420 ppmv, LC and 1000 ppmv, HC) and nitrate (10 μM, LN and 200 μM, HN) for 16 days, to investigate the coupling effects of CO₂-induced seawater acidification and nitrate enrichment on growth and photosynthesis of Sargassum muticum. The results showed that high CO₂ increased the relative growth rate (RGR) of alga by 58.9% under LN condition, while such increment was not found under HN condition. Thus, the highest RGR was emerged in the HCLN treatment. The photosynthetic rate curve under different inorganic carbon concentrations (PC curve) presented that high CO₂ increased the maximum inorganic carbon utilization rate (V_max) by 8.1% under HN condition; while inhibited it by 29.8% under LN condition. The affinity to inorganic carbon, reflected by the half-saturation constant (K_0.5), was improved significantly by high CO₂ and/or high nitrate, compared with LCLN treatment. The photosynthetic rate curves under different irradiances (PI curve) suggested that the maximum photosynthetic rate (P_max) of alga was enhanced remarkably by high N, and kept unaffected by high CO₂. The lowest value of dark respiration rate (R_d) was found in HCLN treatment, and there was no significant difference among the other three treatments. Additionally, an increase chlorophyll a content caused by high N was only found in HC treatment. After 16 d culture, nitrate reductase activity (NRA) of algae in HN treatments decreased significantly, compared with those in LN treatments. Furthermore, high CO₂ enhanced NRA dramatically only in algae grown at LN level. Correspondingly, the lowest nitrate uptake rate (NUR) was found in LCHN treatment, and there was no significant difference among the other three treatments. In conclusion, our results showed that elevated CO₂ enhanced the RGR, and the coupling of high CO₂ and nitrate affected the photosynthesis and NUR, however did not synergistically promote growth of S. muticum. Therefore, we speculate that the future OA may exacerbate the invasiveness of S. muticum; nevertheless, the eutrophication of seawater would not amplify this effect.