Microbial diversity of halotolerant and halophilic endophytes from the mangrove Avicennia germinans located at the solar salterns of Cabo Rojo, Puerto Rico

Abstract

The relationship between plants and their environment has been described as a continuum, in which changes of a variable may have an effect on another. Ecological shifts in temperature, humidity, and soil chemistry can alter the microbiome associated with superficial or the internal plant structures. Halophytes are plants that require the abiotic stress of salinity for optimum growth and development. Previous studies have highlighted the differences in their unique microflora in contrast with mesophilic plants. Avicennia germinans, also known as black mangrove, is an ecologically important halophyte found at the intertidal areas of tropical and subtropical regions. This study was targeted at evaluating the biodiversity of the prokaryotic halophilic or halotolerant endophytes in different strata in Avicennia germinans, as well as their community shifts based on salinity gradients using culture-independent techniques. These analyses can increase our understanding on the associated microbiota and metabolic processes that help the black mangrove thrive in such harsh conditions in their ecosystems, as there are no studies that assess the endophytic prokaryotic diversity in this plant species. Two mangrove patches located at the Solar Saltern System of Cabo Rojo, Puerto Rico, denominated in this study as HS and Sal, were selected based on their variable water source (saline pond vs. hypersaline pond). Using classical culture techniques and phylogenetic analyses, one archaeal and eight different bacterial genera were recovered as endophytes of black mangroves. Diversity among sampling sites was determined using Shannon, Simpson and Jaccard diversity indexes. Based on the taxonomic analysis of three strains, this study proposes a bacterial and an archaeal new species and one new bacterial genus. Culture-independent analyses of prokaryotic endophytes of black mangrove retrieved 16,285 OTUs total, of which 9,445 OTUs were obtained from HS samples and 6,840 OTUs from Sal samples. A total of five phyla were identified in both sampling sites (HS and Sal). On a genus level, 58 different genera were retrieved, of which only 39 are shared among both sampling sites. Shannon, Simpson and Jaccard indexes were calculated to determine the diversity, evenness and similarities between samples. Highly represented genera were consistent in both samples, raising the question of the ecological role these organisms play in the development and physiology of the plant.