Differential growth and physiological responses of Sargassum fusiforme and epiphytic Ulva lactuca to culture densities and interspecific competition

Abstract

The commercial seaweed Sargassum fusiforme is adversely affected by the increasing presence of epiphytic macroalgae, mainly Ulva species, during seawater cultivation, especially at high culture densities. In the present study, comparative studies of S. fusiforme and Ulva lactuca were carry out to examine their physiological responses to densities between monoculture and bi-culture. The results showed that, both species of algae showed significant reduction in growth when the density of the cultures (mono-and bi-culture) was increased, although U. lactuca exhibited stronger growth and photosynthetic rate than S. fusiforme in both cultures. Specifically, for S. fusiforme, its maximum quantum yield of PSII (Fv/Fm), light harvesting efficiency (α), soluble carbohydrate (SC) content, and the ratio of SC/SP were all significantly decreased by high density, whereas its nitrate reductase (NR) activity, chlorophyll a (Chl a) and soluble protein (SP) contents were significantly increased in the monoculture. High density also decreased the photosynthesis and contents of SC and SP of U. lactuca in the monoculture. Moreover, the SC contents and NR activities of both S. fusiforme and U. lactuca significantly increased whereas their SP contents decreased in the bi-culture regardless of the culture densities. U. lactuca was dominant because of its potent neighboring shading and possible allelopathic effects on S. fusiforme. Its nutrients uptake was also more rapid than that of S. fusiforme in bi-culture. The interspecific competition between the two algae became intense with the increasing density. Thus, for the maricultivation of S. fusiforme, a moderate culture density would be preferable because of the higher photosynthetic efficiency and growth rate of S. fusiforme.