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VOL. 76 (2), 189-208, 2010 THE IGNORED STOWAWAYS: WORLDWIDE...

contaminants (summarized in Table 2). A remarkable feature was
that mutation rate from TBT-sensitivity to TBT-resistance (µ = 7.07
× 10–8 mutants per cell per cell division) was two or three orders
of magnitude lower than those we have described for resistance to
other biocides in Dc1M strain. It may be that the acquisition of TBT-
resistance exemplified by the present work is intrinsically more rare
than other kinds of mutation. However, this aspect remains to be
investigated.

Table 2. Mutation rates (µ) in mutants per cell division of Dc1M strain from
sensibility to resistance against different lethal anthropogenic and natural
contaminants

Selective agent µ source

TBT 7.07 × 10–8 this paper
DCMU 2.1 × 10–6 Costas et al. (2001) (34)
TNT 1.4 × 10–5 García-Villada et al. (2002) (37)
Formaldehyde 3.6 × 10–6 López-Rodas et al. (2008c)
Tinto River 1.4 × 10–6 Costas et al. (2007) (49)
Geothermal waters 1.4 × 10–6 to 1.5 × 10–5 Costas et al. (2008) (47)
Mynydd Parys pond 1.6 × 10–6 López-Rodas et al. (2008a) (43)
Aguas Agrias stream 1.1 × 10–6 López-Rodas et al. (2008b) (48)
Vulcano Island pond 4.7 × 10–7 López-Rodas et al. (2009) (46)

Mutation from TBT-sensitivity to TBT-resistance occurs
recurrently. However, TBT-resistant cells have diminished fitness
compared to wild type TBT-sensitive cells. Consequently, the balance
between the recurrent appearance of mutants by rare pre-selective
mutation and their elimination by natural selection controls the
presence of TBT-resistant cells in the populations. As a result, an
equilibrium frequency of around 3 TBT-resistant mutants per each
10–4 wild type sensitive cells should maintain in Dc1M algal
populations non-exposed to TBT. Taking into account both the
relatively high number of resistance-mutants and the countless cells
comprising algal populations, it could be hypothesized that algal
colonization of antifouling paints should be almost instantaneous
because TBT-resistant cells are always present in microalgae

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