The Lemna Bioassay: Contemporary Issues as the Most Standardized Plant Bioassay for Aquatic Ecotoxicology

Abstract

The Lemna bioassay is one of the most standardized higher plant bioassays for assessing the impacts of contaminants in aquatic environments. The simple anatomy and ease with which Lemna sp. can be handled makes them ideal test organisms. They have been used to predict the cytotoxic, cytogenetic, and mutagenic effects of several chemical pollutants including, inter alia, polycyclic aromatic hydrocarbons, heavy metals, metalloids, organometallic compounds, pesticides, pharmaceuticals, radionuclides, and pharmaceuticals. However, there is still some contention as to the exact scope of application and definition of the Lemna bioassay regarding its accuracy of prediction and toxicity assessment. In this article, we review some critical issues on the development and effective use of the Lemna bioassay.

KEY WORDS:

• bioaccumulation
• biphasic growth
• endpoints
• environmental pollution
• hormesis test organisms
• toxicity mechanisms

Notes

1. The work was a part of a larger DBU (Deutsche Bundesstiftung Umwelt) project, “Ecotoxicological test-batteries” involving the Technische Universitaet Aachen and private company LemnaTec GmbH. By the time of this review project, the final report was not yet available to the authors.

2. The toxic unit approach is mostly used to test the response addition model for the chemical mixtures. It is to compare the likely relative toxicity induced by metals. The approach has limitations, and should, therefore, be limited to assessment of multimetals that contribute the most to the general toxicity.

Most commonly used endpoints and their definitions are the following:    LD₅₀ is the dose of the toxicant mass per body weight that is lethal to 50% of the test organisms within a stated study period. Units used are ppm (mg kg−1), ppb (μg kg−1), and ppt (ng kg−1), and this endpoint is commonly applied where animals are test organism.    LC₅₀ is the concentration of a toxicant in an aqueous medium that is lethal to 50% of the test organisms within a defined study period. Units used are mg or μg per liter of solution. Units used are ppm (mg L−1) and ppb (μg L−1).    EC₅₀ is the effective concentration of toxicant in mg L−1 or μg L−1 that produces a specific measurable effect in 50% of the test organisms (e.g., measurable effect is lethality for vertebrates test organism, and a reduction in photosynthetic activity by 50% for the Lemna bioassay and algae, or reduction of fluorescence in fluorescence bacteria.    NOEL (NOEC) is the no observed effect level (concentration), or the level below which no adverse effects are observed. Note that this level depends strongly on the sensitivity of the techniques used to measure the effects.    LOEL (LOEC) is the lowest observed effect level, or the lowest level (concentration) at which adverse effects are observed. Note that this level (concentration) depends strongly on the sensitivity of the techniques used to measure the effects.    MATC is the maximum acceptable toxicant concentration and is a hypothetical threshold concentration that is the geometric mean between the NOEC and LOEC concentration.