Chlorophyll fluorescence as a mechanistic bioindicator of photosynthetic inhibition in aquatic plants

This thesis describes an investigation of a chlorophyll fluorescence assay used to estimate the efficiency of photosynthetic electron transport in plants exposed to environmental stress from polycyclic aromatic hydrocarbons (PAHs) and solar ultraviolet radiation. A series of experiments were performed to validate pulse amplitude modulated chlorophyll fluorescence in aquatic plants as a rapid biological indicator of exposure to environmental contaminants and stressors. Chlorophyll fluorescence endpoints were measured in the macrophytes Lemna gibba and Myriophyllum spicatum upon exposure to creosote, a mixture of polycyclic aromatic hydrocarbons. In laboratory exposures, the concentration-dependent response of chlorophyll fluorescence was closely related to conventional endpoints such as frond production, shoot length, root length and leaf chlorophyll concentration. Chlorophyll fluorescence was similarly diminished in plants exposed to creosote in outdoor aquatic microcosms, at concentrations which inhibited seasonal plant population biomass. Chlorophyll fluorescence was therefore predictive of effects at higher levels of biological organization. To examine the applicability of this assay in the field, chlorophyll fluorescence was measured from natural assemblages of Lake Erie phytoplankton exposed to the PAHs anthracene and 1,2-dihydroxyanthraquinone. It was found that inhibition of photosynthetic electron transport could be detected with short exposures to low chemical concentrations.; The ability of chlorophyll fluorescence to detect inhibition of photosynthesis by sunlight (photoinhibition) was examined in Myriophyllum plants in microcosms screened to remove specific wavelengths of sunlight. The ultraviolet-B, ultraviolet-A and visible regions of sunlight had specific but minor effects on the various chlorophyll fluorescence parameters. No clear relationship between chlorophyll fluorescence and plant growth was found. However, when chlorophyll fluorescence was examined in Lake Erie phytoplankton exposed to the same sunlight treatments, a rapid and dose-dependent effect was observed. A nonlinear statistical model based on the cumulative energy received by the phytoplankton was used to estimate biological effectiveness coefficients for photoinhibition by each waveband. It was found that ultraviolet-B radiation was much more damaging than other wavelengths of sunlight. Chlorophyll fluorescence is therefore a suitable endpoint for detecting inhibition of photosynthetic electron transport in aquatic plants exposed to various environmental stresses, which is linked to effects at higher levels of biological organization.