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

Abstract

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 sodium ethylenediaminetetraacetic acid (EDTA) and NaNO₃). An optimised DM recipe is provided. The PAM assay was also used to analyse the influence of maintaining D. geminata cells in river waters sourced from locations with and without the diatom. A NaNO₃ spiking experiment was undertaken using water from one location. The maximum quantum yields of cells maintained in all river waters remained relatively constant and higher than those maintained in DM or Milli-Q water for the seven day test period. The results of the NaNO₃ river water spiking experiment provided contrasting results to the culture medium trials, with minimal impact on photosynthetic efficiency. These data, coupled with the results of the CCD experiment, suggest complex interactions among nutrients that have varying effects on D. geminata cell health. Together with microscopical observations, the 96-well plate PAM assay provides a useful tool for improving knowledge of D. geminata biology and growth requirements.

Keywords:

• diatom
• freshwater algae
• growth medium
• invasive species
• photosynthetic activity

The authors thank Cathy Kilroy (National Institute of Water and Atmospheric; NIWA) and Rosemary Bird (Ministry for Primary Industries) for helpful discussions and advice, Eric Goodwin (Cawthron Institute) for statistical advice, and NIWA field staff for collection of water samples.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/0269249X.2016.1182074

Additional information

Funding

This work was supported by the New Zealand Ministry of Primary Industry [grant MAF Agreement Number 17496].