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

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.

Mass and Fine‐Scale Morphological Changes Induced by Changing Seawater pH in the Coccolith Gephyrocapsa oceanica

“…These values reflect an integrated thickness of the total surface area of the coccoliths, and as such, these estimates may be influenced by changes of the constitutive crystallographic and ultrastructural parts forming the coccoliths (see below). From a methodological point of view (see section 2.3), coccolith surface area and thickness presented in this study for pH around 8 are in line with previous estimates inferred using another technique (SYRACO) and obtained from strain RCC 1314 grown in the same conditions also at the Oxford Biogeochemistry Laboratories (McClelland et al, ).…”

“…Lastly, the size normalized thickness or SNT (Table ; Parameter P9) is a dimensionless parameter that corresponds to a three‐dimensional aspect ratio of the tube, as calculated in McClelland et al () on the entire coccolith. It was calculated by dividing the tube height (P7) by the square root of the tube surface area (P5) to homogenize the 1‐D (height) and 2‐D (surface area) dimensions of the two variables (equation ).…”

Size-dependent response of foraminiferal calcification to seawater carbonate chemistry

“…Diverse small benthic foraminifera assemblages have also been found living in highly undersaturated oligohaline conditions (Flako-Zaritsky et al, 2011). Indeed, while biomineralisation pathways clearly differ, recent work favours a similarly positive response of calcification to increased aqueous pCO 2 with lower pH in the much smaller coccolithophores (Bolton et al, 2016;McClelland et al, 2016).…”

Carbon isotope ratios of coccolith–associated polysaccharides of Emiliania huxleyi as a function of growth rate and CO2 concentration

“…Another advantage is the potential to recover additional useful information about the coccolithophore community through parallel efforts. CAPs are extracted from fossilized coccoliths, so cell size and taxonomy information should be recoverable with coccolith calcite and CAP isotope values (Henderiks, 2008;Bolton et al, 2012;Bolton and Stoll, 2013;O'Dea et al, 2014;McClelland et al, 2016). The uronic acid contents of CAPs provide complementary information about the internal saturation state of the coccolith vesicle and, by extension, atmospheric CO 2 levels Rickaby et al, 2016).…”