Test System Stability and Natural Variability of a Lemna Gibba L. Bioassay
Scherr C; S. Baumgartner; J. Spranger; M. Simond (2008)
Abstract
Background: In ecotoxicological and environmental studies Lemna spp. are used as test organisms due to their small size,
rapid predominantly vegetative reproduction, easy handling and high sensitivity to various chemicals. However, there is not
much information available concerning spatial and temporal stability of experimental set-ups used for Lemna bioassays,
though this is essential for interpretation and reliability of results. We therefore investigated stability and natural variability
of a Lemna gibba bioassay assessing area-related and frond number-related growth rates under controlled laboratory
conditions over about one year.
Methology/Principal Findings: Lemna gibba L. was grown in beakers with Steinberg medium for one week. Area-related
and frond number-related growth rates (r(area) and r(num)) were determined with a non-destructive image processing
system. To assess inter-experimental stability, 35 independent experiments were performed with 10 beakers each in the
course of one year. We observed changes in growth rates by a factor of two over time. These did not correlate well with
temperature or relative humidity in the growth chamber. In order to assess intra-experimental stability, we analysed six
systematic negative control experiments (nontoxicant tests) with 96 replicate beakers each. Evaluation showed that the
chosen experimental set-up was stable and did not produce false positive results. The coefficient of variation was lower for
r(area) (2.99%) than for r(num) (4.27%).
Conclusions/Significance: It is hypothesised that the variations in growth rates over time under controlled conditions are
partly due to endogenic periodicities in Lemna gibba. The relevance of these variations for toxicity investigations should be
investigated more closely. Area-related growth rate seems to be more precise as non-destructive calculation parameter than
number-related growth rate. Furthermore, we propose two new validity criteria for Lemna gibba bioassays: variability of
average specific and section-by-section segmented growth rate, complementary to average specific growth rate as the only
validity criterion existing in guidelines for duckweed bioassays.