Calcification response of a key phytoplankton family to millennial-scale environmental change.

H L O Mcclelland,N Barbarin,M Hermoso,L Beaufort,P Ferretti,M Greaves,R E M Rickaby

doi:10.1038/srep34263

Abstract

Coccolithophores are single-celled photosynthesizing marine algae, responsible for half of the calcification in the surface ocean, and exert a strong influence on the distribution of carbon among global reservoirs, and thus Earth’s climate. Calcification in the surface ocean decreases the buffering capacity of seawater for CO2, whilst photosynthetic carbon fixation has the opposite effect. Experiments in culture have suggested that coccolithophore calcification decreases under high CO2 concentrations ([CO2(aq)]) constituting a negative feedback. However, the extent to which these results are representative of natural populations, and of the response over more than a few hundred generations is unclear. Here we describe and apply a novel rationale for size-normalizing the mass of the calcite plates produced by the most abundant family of coccolithophores, the Noëlaerhabdaceae. On average, ancient populations subjected to coupled gradual increases in [CO2(aq)] and temperature over a few million generations in a natural environment become relatively more highly calcified, implying a positive climatic feedback. We hypothesize that this is the result of selection manifest in natural populations over millennial timescales, so has necessarily eluded laboratory experiments.

Highlights

An apparent dichotomy exists between the consensus view of phenotypic plasticity as observed in short-term experiments and the theoretical result of long-term evolutionary adaptation
Studies to date have yielded contradictory results regarding the response of calcification to environmental change on geological timescales[14,16], but this disagreement is, at least in part, a result of the lack of consistency between the parameter measured, and how this is inferred to represent “calcification intensity”
In order to solve this problem, we have developed a procedure for size-normalising coccolith mass, by correlating an index based on coccolith morphometry with the molar ratio of particulate inorganic to particulate organic carbon (PIC:POC) of the biomass

Summary

Introduction

An apparent dichotomy exists between the consensus view of phenotypic plasticity as observed in short-term experiments and the theoretical result of long-term evolutionary adaptation. Experiments lasting hundreds of generations have shown that asexual coccolithophore populations have the potential to adapt in culture[12,13] and large-scale surveys have revealed trends across spatial environmental gradients in nature[14,15]. The fossil record, by contrast, is an archive of information about ancient natural coccolithophore communities that responded to real environmental changes over geological timescales. The challenge is to extract meaningful information from this resource. Isolated coccoliths in deep-sea sediment are often the only remnants of ancient coccolithophores to survive geological time, so inferences about the physiology of coccolithophores that lived in the past must come from this evidence alone

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Conclusion