Abstract
Abstract. The Cretaceous ocean witnessed intervals of profound perturbations such as volcanic input of large amounts of CO2, 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.
Highlights
Trace metal concentrations influence the productivity and species composition of marine algae communities (Bruland et al, 1991; Sunda and Huntsman, 1998)
We address the following questions: (i) does coccolithophore growth change in response to increasing trace metal concentrations? (ii) Does coccolith size and morphology, as well as coccolithophore size, change in response to high and anomalous trace metal concentrations? (iii) Do trace metal combinations, which mimic oceanic anoxic events (OAEs) conditions, lead to a uniform response among species or to speciesspecific responses on morphological features? (iv) Do coccolith morphometrical features have a potential to serve as a proxy to reconstruct paleo-ocean trace metal concentrations?
Zn, Ni and V concentrations were added in low (L), medium (M), high (H ) and extreme treatments because of their high concentrations identified in the Aptian OAE 1a (Erba et al, 2015) and Cenomanian–Turonian OAE 2 (Snow et al, 2005; Table 1)
Summary
Trace metal concentrations influence the productivity and species composition of marine algae communities (Bruland et al, 1991; Sunda and Huntsman, 1998). The geological record offers the opportunity to investigate past case histories marked by profound changes in the ocean, such as volcanic injection of large amounts of CO2, ocean anoxia, eutrophication and introduction of biologically relevant metals (e.g., Larson and Erba, 1999; Erba, 2004; Jenkyns, 2010; Erba et al, 2015). These events can be seen as “natural experiments” useful to decrypt the ecosystem response to major perturbations at timescales longer than current modifications. Finding out how the changes in seawater composition affect marine biota requires the integration of a long-term and large-scale geological perspective that has been recognized as an essential ingredient for more coherent
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