Effect of hyposaline stress on the release of dissolved organic carbon from five common macroalgal species

Abstract

Macroalgae are important primary producers in coastal waters; they have high carbon sink potential and are often subjected to hyposaline stress in their natural habitats. The effect of hyposaline stress on dissolved organic carbon (DOC) release from macroalgae remains to be studied in depth. In this study, five common intertidal macroalgae in coastal waters of Fujian Province, China—Pyropia haitaneisis, Gracilaria lemaneiformis, Sargassum thunbergii, Enteromorpha prolifera, and Ulva lactuca—were used as research materials to investigate the effects of 6-h hyposaline treatments (5 PSU, 0 PSU) on the growth, DOC release rate, photosynthesis, respiration, and contents of carbon (C), nitrogen (N), and phosphorus (P). Our results showed that, although there were significant interspecific differences in the tolerance of the five species of macroalgae to low salinity, the DOC release rate of macroalgae increased overall with decreasing salinity, while the photosynthetic rate showed the opposite trend. Hyposaline treatments reduced the net photosynthetic rate of macroalgae, as the net photosynthetic rate of all five species decreased by more than 50% and 75% under the 5 PSU and 0 PSU treatments, respectively. The tissue C contents of P. haitaneisis, G. lemaneiformis, and E. prolifera increased significantly with decreasing salinity, by 6.90%, 40.15%, and 43.80% at 0 PSU, respectively. However, the tissue C contents of S. thunbergii and U. lactuca were not influenced or were slightly decreased by low salinity. These results suggest that short-term hyposaline treatment has a dual effect on organic carbon accumulation of macroalgae by inhibiting photosynthetic carbon fixation and increasing DOC release, and this in turn may have a large impact on the carbon cycle in macroalgae enrichment areas.