Ocean warming alters the responses to eutrophication in a commercially farmed seaweed, Gracilariopsis lemaneiformis

Abstract

The economically important marine red macroalga Gracilariopsis lemaneiformis was cultured for 2 weeks at two different temperatures, ambient and elevated CO2 levels, and two fertilizer levels. After the experimental period, growth, photosynthesis, and biochemical components were determined. Higher pCO2 significantly increased relative growth rates and nitrate reductase activity with the combination of higher temperature (HT) and/or N enrichment, whereas photosynthetic rates were rapidly reduced at the same condition. Both Q10 values for photosynthetic- (Pm) and respiratory (Rd) rates were higher in algae grown at 24 than at 20°C. This enhancement of Pm rate by HT was much more pronounced than that of the Rd rate. As a result, soluble carbohydrates produced by photosynthesis accumulated to higher levels at the higher temperature. Most of fluorescence variables and antioxidant enzyme activities (super oxide dismutase, catalase, peroxidase, malondialdehyde) did not differ among treatments. Additionally, higher pCO2 reduced chlorophyll a, carotenoids, phycoerythrin, and phycocyanin contents by 48.6, 41.7, 24.7, and 47.8% under LTLN condition, respectively, compared with the control treatment. We propose that high pCO2 would depress the carbon acquisition but increase N assimilation with higher temperature and/or coastal eutrophication, thereby exerting a positive effect on algal growth.