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Castle and Rodgers (2009) propose that toxins produced by photosynthetic microbes (“algae”) were factors in the five major Phanerozoic mass extinctions.
Although toxins have undoubtedly caused metazoan mortality throughout the Phanerozoic, the data presented by Castle and Rodgers (2009) do not constrain the timing, scale, impact, or longevity of any such events and do not support the suggestion that microbially produced toxins significantly affect extinction rate.
To demonstrate a causal relationship between the production of toxins and extinction events, a reliable high-resolution proxy for toxin production is required; such a proxy must indicate an increase in toxin production at, or immediately before, each mass extinction.
Castle and Rodgers (2009) propose stromatolite abundance as such a proxy. Unfortunately, its accuracy as a measure of algal abundance is questionable. The increased abundance of certain algae inhibits, rather than promotes, stromatolite growth (Macintyre et al., 1996). Furthermore, the link between toxin production and stromatolitic organisms is only tentatively supported (Burns et …