Abstract
During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent primary producers in the oceans, succeeding an earlier biota in which green algae and cyanobacteria had been proportionally more abundant. This transition occurred during an interval marked by increased sulfate concentration in seawater. To test whether increasing sulfate availability facilitated the evolutionary transition in marine phytoplankton, the cyanobacterium Synechococcus sp., the green alga Tetraselmis suecica and three algae containing chlorophyll a+c (the diatom Thalassiosira weissflogii, the dinoflagellate Protoceratium reticulatum and the coccolithophorid Emiliania huxleyi) were grown in media containing 1, 5, 10, 20, or 30 mm SO(4) (2-) . The cyanobacterium and the green alga showed no growth response to varying [SO(4) (2-) ]. By contrast, the three chlorophyll a+c algae showed improved growth with higher [SO(4) (2-) ], but only up to 10 mm. The chlorophyll a+c algae, but not the green alga or cyanobacterium, also showed lower C:S with higher [SO(4) (2-) ]. When the same experiment was repeated in the presence of a ciliate predator (Euplotes sp.), T. suecica and T. weissflogii increased their specific growth rate in most treatments, whereas the growth rate of Synechococcus sp. was not affected or decreased in the presence of grazers. In a third experiment, T. suecica, T. weissflogii, P. reticulatum and Synechococcus sp. were grown in conditions approximating modern, earlier Paleozoic and Proterozoic seawater. In these treatments, sulfate availability, nitrogen source, metal availability and Pco(2) varied. Monospecific cultures exhibited their highest growth rates in the Proterozoic treatment. In mixed culture, T. weissflogii outgrew other species in modern seawater and T.suecica outgrew the others in Paleozoic water. Synechococcus sp. grew best in Proterozoic seawater, but did not outgrow eukaryotic species in any treatment. Collectively, our results suggest that secular increase in seawater [SO(4) (2-) ] may have facilitated the evolutionary expansion of chlorophyll a+c phytoplankton, but probably not to the exclusion of other biological and environmental factors.
Highlights
In the productive waters that bathe continental shelves, three groups of algae with chloroplasts containing chlorophyll a and chlorophyll c dominate photosynthesis: diatoms, dinoflagellates, and coccolithophorids
Dinoflagellates may have been the first of the major chlorophyll a+c phytoplankton to appear; purported dinocysts have been reported from Paleozoic rocks, and low abundances of dinosterane, the geologically stable form of dinosterol synthesized by dinoflagellates, have been reported from Cambrian deposits (Molodowan & Talyzina, 1998)
Our 1 mM to 5 mM [SO 4 2-] treatments probably bracket levels experienced by early cyanobacteria and green algae radiating in Neoproterozoic and earlier Paleozoic oceans; whereas the 10 mM to 20 mM [SO 4 2-] levels approximate sulfate abundances in the late Paleozoic and Mesozoic oceans that supported early dinoflagellates, coccolithophorids and diatoms (Fig. 1)
Summary
In the productive waters that bathe continental shelves, three groups of algae with chloroplasts containing chlorophyll a and chlorophyll c dominate photosynthesis: diatoms, dinoflagellates, and coccolithophorids. Despite their abundance and diversity, these taxa are relative newcomers to marine ecosystems (Falkowski et al, 2004). Both conventional fossils and preserved lipids suggest that in Proterozoic oceans cyanobacteria and other photosynthetic bacteria fueled ecosystems, whereas cyanobacteria and green algae were major primary producers in Paleozoic oceans (Knoll et al, 2007; Kodner et al, 2008; Johnston et al, 2009). The oldest coccoliths occur in later Triassic rocks, and coccolithophorids became widespread only during the Cretaceous Period (Bown et al, 2004)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
