Evolutionary Rates in the Haptophyta: Exploring Molecular and Phenotypic Diversity

Abstract

Haptophytes are photosynthetic protists found in both freshwater and marine environments with an origin possibly dating back to the Neoproterozoic era. The most recent molecular phylogeny reveals several haptophyte “mystery clades” that await morphological verification, but it is otherwise highly consistent with morphology-based phylogenies, including that of the coccolithophores (calcifying haptophytes). The fossil coccolith record offers unique insights into extinct lineages, including the adaptive radiations that produced extant descendant species. By combining molecular data of extant coccolithophores and phenotype-based studies of their ancestral lineages, it has become possible to probe the modes and rates of speciation in more detail, although this approach is still limited to only few taxa because of the lack of whole-genome datasets. The evolution of calcification likely involved several steps, but its origin can be traced back to an early association with organic scales typical for all haptophytes. Other key haptophyte traits, including the haplo-diplontic life cycle, are herein mapped upon the coccolithophorid phylogeny to help navigate a discussion of their ecological benefits and trade-offs in a rapidly changing ocean.

Inferred nutrient forcing on the late middle Eocene to early Oligocene (~40–31 Ma) evolution of the coccolithophore Reticulofenestra (order Isochrysidales)

“…We propose, however, that the adaptive morphology of coccoliths, not only cell size, should be considered to decipher the long- (and short-) term evolution of the species of the RGE lineage, and of coccolithophores in general. Our interpretation also echoes the view that the diversity of trophic strategies among coccolithophores is vastly underestimated (Segev et al 2016; Taylor et al 2017; Godrijan et al 2020, 2022; Henderiks et al 2022) and that this diversity underpins the biodiversity of coccolithophores.…”

“…Following the Cretaceous/Paleogene mass extinction, the coccolithophores underwent bursts of diversification, reaching their highest diversity during the early Eocene climatic optimum (EECO). Subsequently, global cooling and associated oceanic eutrophication fueled the diversification and spreading of opportunistic (r-selected) species, leading to an early Oligocene bottleneck of the entirety of coccolithophore diversity (Bown et al 2004; Aubry and Bord 2009; Hannisdal et al 2012; Henderiks et al 2022). While the major K-selected taxonomic groups became sequentially restricted, species of the genus Reticulofenestra increased in abundance, leading to their dominance in the Oligocene coccolithophore communities (Aubry 1992).…”

Diversity and Evolution of Mamiellophyceae: Early-Diverging Phytoplanktonic Green Algae Containing Many Cosmopolitan Species

“…The estimation of the timeline of speciation from molecular evolution rates poses significant challenges, and unlike diatoms [18], haptophytes [19] and dinoflagellates [20], which have fossil record from the mid-Triassic (240 Myr ago) [21], the lack of fossil data precludes the use of calibration points within Mamiellophyceae and, more generally, within the phylum Chlorophyta [22]. The divergence time between the Monomastigales and the Mamiellales (Table 1) has thus been estimated from the divergence of chloroplast genes to be 1190 [23] and 478 [24] Myr ago, with external calibration points, such as the root of Viridiplantae at 970 Myr and the root of the land plants at 475 Myr ago [24].…”

An uneven distribution of strontium in the coccolithophore Scyphosphaera apsteinii revealed by nanoscale X-ray fluorescence tomography

“…Firstly, they are phylogenetically distant and might have different minor element fractionation mechanisms. 16 Secondly, S. apsteinii has an atypically high Sr/Ca ratio (and Sr partitioning coefficient, D Sr ) compared to placolith bearing species such as E. huxleyi, Coccolithus braarudii, and Helicosphaera carteri. 7,12,17,18 This high Sr/ Ca ratio is also observed in C. crassus, the fossil example of uneven Sr distribution.…”