Optimal nutrient availability could alleviate diatom Cylindrotheca closterium fouling during seedling cultivation of Sargassum hemiphyllum

Abstract

The common brown macroalga Sargassum hemiphyllum forms extensive beds in the intertidal zones of temperate coasts, which has important ecological roles for a healthy coastal ecosystem. Efficient production of its seedlings is in great need for both natural resource restoration and aquaculture of this species. Fouling by the diatom Cylindrotheca closterium is a main constraint for seedling cultivation of S. hemiphyllum. In this study, we investigated the interactions between S. hemiphyllum and C. closterium under different nutrient conditions. The two algae were incubated together (co-culture mode) or separately (mono-culture mode). NO3− and PO43− were used as nitrogen (N) and phosphorous (P) sources, respectively, and eight N concentrations were prepared from 0 to 300 μM with a N:P ratio of 10:1. The results showed that the growth of C. closterium was inhibited by S. hemiphyllum only when the N concentration was 5–50 μM, whereas the growth of S. hemiphyllum was inhibited by C. closterium regardless of N concentration in the culture medium. These results indicated that the inhibitory effects of the two algae on each other are species-specific and dependent on N availability. Inhibition was most evident under the nutrient-limited condition and was alleviated by sufficient N in the culture medium. At 5–25 μM N, co-culture decreased the tissue N and P contents and the maximal quantum yield of photosystem II (Fv/fm) of S. hemiphyllum, which resulted in N- and P-limited status and decreased growth. Meanwhile tissue N and P contents and Fv/fm of C. closterium were stimulated in co-culture mode, but its growth was still suppressed, suggesting an allelopathic effect from S. hemiphyllum. At 100–300 μM N, the tissue N and P accumulation, Fv/fm, and growth rate of co-cultured C. closterium were higher than the respective mono-culture values, indicating that it had a nutrient competitive advantage over the co-cultured S. hemiphyllum and so had an inhibitory effect on S. hemiphyllum. The competitive and inhibitory effects of C. closterium on S. hemiphyllum were relatively lower when the N concentration was in the 5–50 μM range. However, S. hemiphyllum grows well when N concentration is >25 μM and didn't suffer a N- and P- limitation. Thus, we suggest that the nutrient condition in the culture medium of S. hemiphyllum be set at 25–50 μM N with a N:P ratio of 10 to weaken the adverse effects of the fouling C. closterium.