Microbial Mat Dominated by Amphora spp. and Their Adaptative Strategies in an Arsenic-Rich Brackish Pond

Marine diatoms are essential members of both phytoplankton and phytobenthic communities, able to colonize submerged artificial and natural surfaces, contributing to benthic microbial biomass. Diatoms have developed different adaptative mechanisms to cope with various environmental stresses, including high concentrations of heavy metals. The aim of this study was to investigate the arsenic resistance of diatoms, isolated from microbial mats collected from an arsenic-rich brackish pond (Lake Mergolo della Tonnara, Italy), by evaluating (i) their ability to form biofilms in the presence of arsenite (AsIII) or arsenate (AsV), and (ii) the variations in the photosynthetic pigments’ contents (i.e., chlorophyll a and c) in their biofilms. The mats were dominated by members of the genus Amphora, and isolates were affiliated with species of A. capitellata, A. coffeaeformis, and A. montana. The strains grew better in the presence of AsV than AsIII, which is generally less toxic. After seven days of incubation, each strain exhibited a different ability to form biofilms on glass surfaces in the presence of arsenic (25 ppm), with A. montana strain 27 being the most effective (86%) in the presence of AsIII, and A. coffeaeformis strain 26 (74%) with AsV. Photosynthetic pigment levels (chlorophyll a and c) differed in each biofilm, being poorly reduced by AsIII in strain 27, and by AsV in strain 26, indicating a species-specific response to arsenic stress. Our results indicated that Amphora species thriving in this environment can form biofilms as an As-resistance mechanism, maintain their levels of photosynthetic pigments, and support the functioning of the pond ecosystem, with A. montana being favored in the presence of AsIII, whereas A. coffeaeformis 26 in the presence of AsV. As producers of biofilms, these strains could be useful to develop new strategies to remediate arsenic pollution.