Measuring the influence of nutrients and river water on the photosynthetic efficiency of Didymosphenia geminata using pulse amplitude modulated fluorometry

Over the past three decades the freshwater diatom Didymosphenia geminata (Lyngbye) M. Schmidt has expanded its range globally. In some rivers D. geminata has become invasive, forming expansive and thick polysaccharide-dominated mats. Techniques to maintain and study D. geminata in the laboratory are limited. In this study, a 96-well plate format assay using pulse amplitude modulated (PAM) fluorometry was developed to study D. geminata under controlled conditions. The PAM assay and morphological assessments were used to investigate the addition of sodium nitrate (NaNO3) to a previously developed D. geminata-specific growth medium (Didymo Medium; DM). Addition of low concentrations (ca. 0.003–0.018 µM) enhanced cell survival and health. Central-composite design (CCD) experiments coupled with response surface methodology were then used to investigate optimal concentrations of six key chemicals in DM (magnesium sulphate, calcium chloride, mono-potassium phosphate, sodium metasilicate pentahydrate, ferric sodi...