Effects of plant growth regulators on physiological performances of three calcifying green macroalgae Halimeda species (Bryopsidales, Chlorophyta)

Abstract

The effects of various concentrations of four types of plant growth regulator (PGR) on the physiological performance and microbial populations of three calcifying Halimeda species were evaluated in a 42-day aquaria experiment. The results show that PGRs enhanced the growth and calcification rates at relatively low concentrations, while inhibiting the physiological activities of Halimeda at high concentrations. Net oxygen production rate and pigment concentrations were differed depending on the concentrations and types of PGRs. Carbonic anhydrase activity, total tissue carbon and nitrogen contents varied among different PGRs concentrations, which suggested that these physiological performances were enhanced under appropriate concentration availability of PGRs. Based on 16S rRNA high-throughput sequencing analyses, Alphaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria, belonging to the Proteobacteria phylum, were significantly higher in the PGRs treatment than those in control group. The microbial communities could be majorly divided into three groups corresponding to the Halimeda species, suggesting that physiological differences between Halimeda species may lead to secretion of specific types of biochemical compounds, thus selecting different microbial species at both the taxonomic level and functional level, which may contribute to carbon and nitrogen fixation. The results of this study indicated that it is both practical and efficient to amplify calcifying Halimeda biomass through the addition of PGRs, which could provide a theoretical basis for use of this technique in restoring and maintaining the ecological diversity and sustainable development of coral reef ecosystems.