Impact of trace metal concentrations on coccolithophore growth and morphology: laboratory simulations of Cretaceous stress

Abstract

Abstract. The Cretaceous ocean witnessed intervals of profound perturbations such as volcanic input of large amounts of CO 2 , anoxia, eutrophication and introduction of biologically relevant metals. Some of these extreme events were characterized by size reduction and/or morphological changes of a few calcareous nannofossil species. The correspondence between intervals of high trace metal concentrations and coccolith dwarfism suggests a negative effect of these elements on nannoplankton biocalcification processes in past oceans. In order to test this hypothesis, we explored the potential effect of a mixture of trace metals on growth and morphology of four living coccolithophore species, namely Emiliania huxleyi, Gephyrocapsa oceanica, Pleurochrysis carterae and Coccolithus pelagicus. The phylogenetic history of coccolithophores shows that the selected living species are linked to Mesozoic species showing dwarfism under excess metal concentrations. The trace metals tested were chosen to simulate the environmental stress identified in the geological record and upon known trace metal interactions with living coccolithophore algae.Our laboratory experiments demonstrated that elevated trace metal concentrations, similarly to the fossil record, affect coccolithophore algae size and/or weight. Smaller coccoliths were detected in E. huxleyi and C. pelagicus, while coccoliths of G. oceanica showed a decrease in size only at the highest trace metal concentrations. P. carterae coccolith size was unresponsive to changing trace metal concentrations. These differences among species allow discriminating the most-(P. carterae), intermediate-(E. huxleyi and G. oceanica) and least-tolerant (C. pelagicus) taxa. The fossil record and the experimental results converge on a selective response of coccolithophores to metal availability.These species-specific differences must be considered before morphological features of coccoliths are used to reconstruct paleo-chemical conditions.

Can morphological features of coccolithophores serve as a reliable proxy to reconstruct environmental conditions of the past?

“…Indeed, many studies on geological records calibrated biomineralization responses of ancient species to environmental drivers with experiments with modern species (e.g. Bornemann et al, 2006;Erba et al, 2010;Suchéras-Marx et al, 2010;Linnert et al, 2014;O'Dea et al, 2014;Lübke et al, 2015;Faucher et al, 2017a;Faucher et al, 2017b). This builds on the concept that coccolithophores conserved a certain response to certain environmental parameters over geological timescales.…”

Calcareous Nannofossil Size and Abundance Response to the Messinian Salinity Crisis Onset and Paleoenvironmental Dynamics

“…However, our results show that the CN size reduction occurred during period characterized by high CN productivity, thus indicating optimal environmental condition for calcareous nannoplankton to growth rather than stressed environment, at least during the MSC onset bioevent. Relevant size decreases were recorded during Oceanic Anoxic Events (Erba et al, 2010;Lübke & Mutterlose, 2016) in relation with abrupt increase in CO 2 emissions (and the consequent pH decrease in surface water) (Erba et al, 2010), toxic metal increase (Faucher, Hoffmann, et al, 2017), or with an increase in continental runoff and the associated increase in water turbidity (Lübke & Mutterlose, 2016). Generally, the amplitude of the CN size reduction during the OAEs is comparable to that recorded during the MSC CN bioevent (Erba et al, 2019).…”

Strain-specific morphological response of the dominant calcifying phytoplankton species Emiliania huxleyi to salinity change

“…Coccolithophore cell, and by inference coccosphere, size is known to vary with the cell division cycle (e.g., [57]) and environmental conditions including nutrient availability, temperature, trace metals and CO 2 (e.g., [7,[82][83][84][85]). Cell volume in algae and other plants also responds to salinity changes by regulating cell turgor pressure to accommodate for changing ion gradients across cell membranes [86,87].…”