Heterotrophic diazotrophic bacteria (HDB) are increasingly recognized as widespread and significant contributors to oceanic nitrogen fixation. Macroalgae are known to form stable associations with endophytic and epiphytic bacteria for their growth, development and defense. The red alga Kappaphycus alvarezii can grow under nutrient-limited conditions, suggesting that nutritionally beneficial microbial associations may help sustain its growth under these conditions. The present study investigated the presence of HDB associated with K. alvarezii by incubating surface sterilized thallus sections in nitrogen-limited media. The isolates were taxonomically identified using 16S rRNA gene analysis, and their diazotrophic potential was assessed by amplifying the nifH gene. Phylogenetic analysis revealed associations with diverse genera, including Yangia, Salipiger, Mangrovicoccus, Tritonibacter, Pseudooceanicola, Roseibium, Thalassospira, Oceanobacillus, Alteromonas, Marinobacter, Pseudomonas, Stutzerimonas and Tenacibaculum. Eleven percent of the isolates, including Salipiger sp., Yangia sp., Thalassospira sp. Mangrovicoccus ximenensis and Stutzerimonas stutzeri tested positive for the presence of nifH gene, indicating potential diazotrophic capability. Our findings indicate that K. alvarezii hosts a diverse assemblage of culturable bacteria with potential host-beneficial functions, highlighting the need for functional validation of their contributions to the nitrogen acquisition and fitness of K. alvarezii.
Emerson et al. (Fri,) studied this question.