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Algal testing
Algae toxicity test (EU Annex V C3, OECD 201, 1984a)
TESTING STRATEGIES
The algal growth inhibition test measures the inhibition of growth during the exponential phase
under optimum standard conditions of light, temperature and nutrient concentrations. The test
produces an EC50 that can be considered equivalent to a short-term L(E)C50. Often both ErC50
(estimated from specific growth rate) and EbC50 (estimated from biomass growth) are available,
however the latter should not be used. The reason is that direct use of the biomass concentration
without logarithmic transformation cannot be applied to an analysis of results from a system in
exponential growth. Where only the EbC50 is reported, but primary data are available, a reanalysis
of the data should therefore be carried out to determine the ErC50.
It is sometimes seen also when test was done according to standard test guidelines, that the
exponential growth ceased in the control before the end of the test period. Likewise it may be
seen that the validity criteria of the test were not fulfilled (pH increase etc.) or growth of the
algae in the exposed concentrations was increased (due to e.g. loss of test substance from the test
system) at the end of the test. In such cases only data from the part of the test where exponential
growth and the validity criteria for the controls as well as for the exposed groups occurred
should be used. In many such cases this may be achieved by excluding data from the last test day
from the calculation of ErC50 and NOEC or ErC10. (Nyholm, 1985; Nyholm and Källqvist,
1989; Ratte, 1998; Weyers & Vollmer, 2000; Källquist; 1999, 2000; Weyers et al., 2000). If only
EbC50 is reported and no primary data are available, it should be considered to perform a new
algae study to obtain a valid ErC50 and NOEC or ErC10.
The algal growth inhibition test is not only a multi-generation test but also provides a measure of
sub-lethal effect - reduction in population growth. It can therefore be considered a true chronic
test, albeit of short duration. The NOEC may therefore be used in the assessment strategy, but
with some modification compared to NOECs from long-term chronic tests with fish or Daphnia
(i.e. availability of a NOEC for algae alone is not used as a justification deviating from using the
lowest L(E)C50-value from short-term studies).
6.3.2.3 Decision table for further testing
The decisions to be made in respect of further testing requirements are detailed in Table 34.
Although the basic criteria outlined above must always be taken into account, common sense
must also be applied when considering individual situations. Decisions taken in respect of
further testing will be different depending on species sensitivity. In all cases, the algal study
from the base set is first considered as a short-term study and the EC50 used for calculation of
the PNECwater. However, the algal study is technically a multi-generation test and thus, if there
are other long-term NOEC data, the algal NOEC can be considered as a long-term NOEC in the
revised assessment. Generally, this algal NOEC would not be used unsupported by other longterm
data.
Chapter 4 (Use of (Q)SARs) gives full details on the use within the testing strategy of the QSAR
estimates for substances with a non-specific mode of action and for estimating long-term fish
and Daphnia toxicity.
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