Biodiversity and toxin production of cyanobacteria in mangrove swamps in the Red Sea off the southern coast of Saudi Arabia

This study investigated the diversity and toxin production of cyanobacteria forming microbial mats on the soil surface and mangrove pneumatophores in three different mangrove swamps in the Red Sea off the southern coast of Saudi Arabia. In total, 34 species belonging to 15 genera and five families were recorded. Benthic mats had higher numbers of species (25-31) than pneumatophore mats (10-12). Cyanobacterial biodiversity (H) and species evenness (E) were greater in benthic mats (H = 2.52-2.83 and E = 0.79-0.83) than in pneumatophore mats (H = 1.7-1.82 and E = 0.73-0.74). The results showed high similarity of species between different mangrove sites (67.6%-91.7%) and low similarity between benthic and pneumatophore mats at the same site (32.3%-35.75%). During this study, only nine cyanobacterial species were successfully isolated and tested for their toxicity. The extracts of these species exhibited toxicity to Artemia salina (LC50 = 0.3-5.1 mg l(-1)). Based on the results of ELISA and HPLC analyses, four species were found to produce microcystins (683-974 mu g g(-1)) and three species produced saxitoxins (58.9-93.8 mu g g(-1)). The presence of toxic cyanobacteria and their toxins in mangrove swamps may pose a risk for marine animals because these toxins can accumulate in aquatic food webs with a potential for transfer to higher trophic levels.