Comparative effects of herbicides on photosynthesis and growth of the tropical microalga Nephroselmis pyriformis

PSII inhibiting pesticides have been discovered in sediments along the Queensland coast. To predict flow on effects to benthic marine communities, it is important to evaluate effects on phytobenthic organisms. Nephroselmis pyriformis was isolated from the benthos of Rowes Bay, Townsville, Australia (Latitude 19 15.0’ S, Longitude 146 50.0’ E) and taken into monoclonal culture at NQAIF (NQAIF 117). Cultures were maintained at 24°C in Guillard's (f/2) marine medium at a 12:12 hour light:dark cycle with an irradiance of 43 μmol photons m-2 s-1 for at least six months prior to experimentation. Dose response experiments were conducted for the PSII inhibiting herbicides diuron, atrazine, and hexazinone for chronic (3-5 days, diuron: 0.1-73nM; atrazine: 14-3,000nM; hexazinone: 2-2,600nM) and acute (6 min; diuron: 0.43-82nM; atrazine: 2.6-750nM; hexazinone-6 min: 1.2-584nM, hexazinone-240min: 0.3-73nM) exposures. Chronic effects of these herbicides were measured using percent growth (OD750nm) and percent PSII yield (Mini-PAM) as endpoints, while acute effects measured percent PSII yield with an imaging PAM (I-PAM). 3-day percent growth EC50s were 25±SE2, 160±SE12, and 33±SE1 nM for diuron, atrazine, and hexazinone, respectively. 3-day percent PSII yield EC50s were 25±SE1, 130±SE6, and 24±SE0.5 nM for diuron, atrazine, and hexazinone, respectively. 6-min percent PSII yield EC50s for acute exposures were, 8.9±SE0.2, 66±SE2.1, and 72±SE3.7 nM for diuron, atrazine, and hexazinone, respectively, while the 240-min percent PSII yield EC50 for hexazinone was 9.5±SE0.2 nM. Acute and chronic toxicity measurements were extremely sensitive for all endpoints and effects were recorded at environmentally relevant concentrations for diuron. Chronic exposure results showed that except for diuron, the 3-day percent PSII yield was more sensitive than percent growth. However, growth of N. pyriformis partially recovered after 5 days without exchange of media, while percent PSII yield did not. This may indicate that N. pyriformis could rescue growth by switching to heterotrophic nutrient acquisition