Survival of phytoplankton under prolonged darkness: Implications for the cretaceous-tertiary boundary darkness hypothesis
Abstract The fossil record indicates that there have been times when a larger than usual number of taxa have disappeared. In particular, 65 million years ago, at the Cretaceous-Tertiary boundary, many planktonic and other marine organisms became extinct. The hypothesis proposed by Alvarez et al. (1980), states that an asteroid hit the earth and produced a global dust cloud. This dust cloud prevented photosynthesis and thereby caused extinctions in the phytoplankton, which in turn affected organisms at higher trophic levels. Tests of this hypothesis involve the use of extant planktonic organisms. Two of these, Thoracospaera heimii (a dinoflagellate) and Ditylum brighwellii (a diatom) are considered very similar to species that crossed the Cretaceous-Tertiary boundary; others, Gonyaulax polyedra, Ensiculifera loeblichii, Scrippsiella trochoidea and Coccolithus pelagicus are organisms that first appeared in the Tertiary. Organisms were exposed under controlled laboratory conditions to environments resembling those that might have existed during the perturbation. Growth, survival and morphological changes were recorded. The results suggest that extended darkness is a viable hypothesis to explain the selective phytoplankton extinctions, especially if the dark period lasted perhaps three months or less. Although the experimental data cannot prove the cause of the extinctions, they can test the feasibility of the hypothesis. It is concluded that extinction hypotheses can be biologically modeled and that photosynthetic microorganisms are extremely useful tools in the modeling process.