The Effects of Environmental Contaminants on Freshwater Cyanobacteria

Abstract

Cyanobacteria are photosynthetic prokaryotes with a cosmopolitan distribution and are key contributors to Earth’s primary production. In favourable conditions, some taxa have the ability to form dense algal blooms. When blooms are created by toxin-producing strains they can become a public health threat and may lead to deaths of wild and domestic animals. Different species and strains respond differently to environmental conditions and contaminants, and this thesis investigates the impacts of natural (microcystin, a hepatotoxic cyanotoxin) and anthropogenic (glyphosate) contaminants on cyanobacteria.

Some non-microcystin-producing cyanobacteria have the ability to sequester microcystin, one of them being the strain Cuspidothrix issatschenkoi CAWBG02, and the reason for this ability is unknown. This strain was supplemented with microcystin in a range of environmental conditions (temperature/light intensity combinations) to determine any impacts on fitness by measuring photosynthetic performance, growth, and pigment content (chlorophyll-a and phycocyanin). Microcystin supplementation at a concentration of 25 μg L-1 did not have an effect on growth or pigment content in C. issatschenkoi, and photosynthetic performance was also largely unaffected. Based on the results from this thesis, C. issatschenkoi does not acquire competitive advantages from microcystin sequestration due to the lack of effects on growth and photosynthetic functioning. Previous research has suggested that microcystin is used to suppress competitors, and a 33% decrease in maximum quantum yield was detected in one environmental condition (low temperature, intermediate light), however, there was no significant difference in all of the other conditions used. Therefore, the results from this study do not support the idea that microcystin is used to suppress competitors. Microcystin sequestration may be more beneficial to non-producing-strains within the same species as the toxin-producer (e.g. non-toxic M. aeruginosa).

Glyphosate and MCPA are two of the most commonly used herbicides in New Zealand and globally. These herbicides enter waterways through leaching and come into contact with non- target species. To determine whether these herbicides have an effect on freshwater cyanobacteria in New Zealand, a screening assay was carried out on fourteen strains. Further, the impacts on pigment content (chlorophyll-a and phycocyanin), growth, and photosynthetic performance in two toxic (Nodularia spumigena, Microcystis aeruginosa) and two non-toxic (Dolichospermum lemmermannii, Microcystis wesenbergii) strains were quantified. Growth rates were not impacted by MCPA in any of the fourteen cyanobacterial strains. Glyphosate impacted growth in seven of the fourteen strains, and response differed between strains even within a single species. In general, strains employing a colonial growth strategy were more tolerant to glyphosate in the screening assay. N. spumigena and M. wesenbergii were unaffected by glyphosate at the concentrations used in the functional analysis, and D. lemmermannii had decreased photosynthetic performance from glyphosate. Intermediate concentrations of glyphosate resulted in promotion of PSII functioning in M. aeruginosa, and high concentrations inhibited maximum quantum yield. The lack of significant impacts on M.wesenbergii and N. spumigena, as well as negative effects of PSII functioning in D. lemmermannii, suggest that glyphosate contamination in freshwaters may benefit toxic M. aeruginosa more than competing toxic and non-toxic species or strains.