Abstract
Photosystem II (PSII) herbicides have been detected in nearshore tropical waters such as those of the Great Barrier Reef and may add to the pressure posed by runoff containing sediments and nutrients to threatened seagrass habitats. There is a growing number of studies into the potential effects of herbicides on seagrass, generally using large experimental setups with potted plants. Here we describe the successful development of an acute 12-well plate phytotoxicity assay for the PSII herbicide Diuron using isolated Halophila ovalis leaves. Fluorescence images demonstrated Diuron affected the entire leaf surface evenly and responses were not influenced by isolating leaves from the plant. The optimum exposure duration was 24 h, by which time the inhibition of effective quantum yield of PSII (∆F/Fm’) was highest and no deterioration of photosystems was evident in control leaves. The inhibition of ∆F/Fm’ by Diuron in isolated H. ovalis leaves was identical to both potted and hydroponically grown plants (with leaves remaining attached to rhizomes), indicating similar reductions in photosynthetic activity in these acute well-plate assays. The sensitivity of the assay was not influenced by irradiance (range tested 40 to 400 μmol photons m-2 s-1). High irradiance, however, caused photo-oxidative stress in H. ovalis and this generally impacted in an additive or sub-additive way with Diuron to damage PSII. The bioassay using isolated leaves is more rapid, uses far less biological material and does not rely on specialised aquarium facilities in comparison with assays using potted plants. The development and validation of this sensitive bioassay will be useful to reliably screen and monitor the phytotoxicity of existing and emerging PSII herbicides and contribute to risk assessments and water quality guideline development in the future.
Highlights
Tropical seagrass significanceSeagrasses provide essential nursery and feeding grounds for commercially important fish, crustaceans and molluscs and form, almost exclusively, the diet of a number of macro-grazers, including the endangered dugong and green sea turtles [1]
A miniature 12-well plate phytotoxicity assay was developed to assess the exposure of seagrass to photosystem II (PSII) herbicides in the following way:
A series of preliminary experiments indicated that consistent maximum quantum yield measurements were only possible following careful pre-screening of the leaves
Summary
Tropical seagrass significanceSeagrasses provide essential nursery and feeding grounds for commercially important fish, crustaceans and molluscs and form, almost exclusively, the diet of a number of macro-grazers, including the endangered dugong and green sea turtles [1]. Seagrass promotes sediment stabilisation [1,2], nutrient cycling [3] and carbon sequestration [4]. Seagrass habitats are declining globally with approximately 110 km lost annually [6,7]. Effects of water quality on seagrass populations. The degradation of water quality factors in the dramatic decline of seagrass meadows around the globe [1]. Seagrass loss on the GBR is strongly influenced by long periods of severe light attenuation caused by suspended solids and phytoplankton that peak during these flood events [9]. Along with increased sediments and nutrients, several agricultural herbicides which target photosystem II (PSII) have been regularly detected in the catchment rivers and estuaries and within nearshore habitats of the GBR lagoon [10,11,12]
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