Effects of stocking density and decreased carbon supply on the growth and photosynthesis in the farmed seaweed, Pyropia haitanensis (Bangiales, Rhodophyta)

Abstract

During seaweed mariculture the stocking density increases with growth season, which would potentially affect the inorganic carbon availability for photosynthesis. In the present study, the red macroalgal species, Pyropia haitanensis, collected from the cultivation site located at Nan’ao Island, Shantou, China (23o20’N, 116o40’E), was cultured under high and low stocking density level (5 and 1 g L−1) and two different carbon concentrations (ca. 390 and 20 ppm CO2 in air), to investigate how the stocking density and carbon supply affect the growth and photosynthesis of this alga. The relative growth rate (RGR), nitrate reductase (NR) activity, and photosynthetic rates of high stocking density-grown P. haitanensis thalli were decreased compared with low stocking density-grown thalli. The lowered carbon supply in culture inhibited the RGR, NR activity, photosynthetic rates, and photosystem II activity of the algae. It was shown that high stocking density exhibited similar negative effect compared with reduction of carbon supply on the photosynthesis of P. haitanensis. Moreover, high stocking density aggravated the effect resulting from decreased carbon supply on photosynthesis. We proposed that it is necessary to strive to maintain the relative low stocking density to obtain a sustained high rate of biomass increase in P. haitanensis mariculture.