The overgrowth of epiphytic Ulva prolifera during seedling cultivation of Sargassum hemiphyllum can be mitigated by regulating nitrogen availability

Abstract

Sargassum hemiphyllum is a common species in many countries of the world. Its culture is both economically and ecologically important. Ulva prolifera is an annoying macro-epiphyte during seedling cultivation of S. hemiphyllum and becomes a bottleneck for the successful production of Sargassum seedlings. The present study aimed to develop an effective control method of U. prolifera. The performances of the two macroalgae cultivated under co-culture and mono-culture mode using NO3− or NH4+ as nitrogen resource were evaluated to test whether the U. prolifera could be inhibited through manipulation of nitrogen availability. Increasing NO3− and NH4+ concentrations accelerated the growth and tissue N accumulation in U. prolifera better than in S. hemiphyllum. However, the maximum growth rate of U. prolifera at infinite tissue N contents was about 70% higher than that of S. hemiphyllum, and the critical N content (NC) of U. prolifera was about twice that of S. hemiphyllum, indicating a higher risk of N limitation for U. prolifera than for S. hemiphyllum. In co-culture mode, when cultivated with 5 and 10 μM NO3− or 5 μM NH4+, the tissue N content of S. hemiphyllum was comparable to or higher than its NC, whereas the tissue N content of U. prolifera was much lower than its NC; the growth rate, chlorophyll a and soluble protein contents of U. prolifera were lower than its corresponding values in mono-culture mode. The inhibition effect of S. hemiphyllum on U. prolifera was highest in the cultivation medium with 5 μM NO3−. This suggested that the competitive advantage of epiphytic U. prolifera can to some extent be mitigated by manipulating nitrogen availability in cultivation medium. We recommend using NO3− as the N resource, and set the N concentration at approximately 5–10 μM in cultivation medium.