Tuesday, July 6, 2021

Coccolithophore Life Cycles and Calcite Morphology

Our world is full of examples of biological processes leading to exquisite geological products. And none more so than the one observed in the Coccolithophores. These are single celled marine algae. They produce crystals of calcite (CaCO3), which they use to create a shell around their tissue. The shell is called a coccolith. The amount of calcium carbonate used up in these shells is enormous. About 10% of global carbon is fixed in coccolithophores, making them an important carbon sink. 

The shapes of these calcite crystals vary enormously according to species, but also, as I found out in a recently published paper, on life cycle stages of the organism.

Coccolithophores have haploid (one set of chromosomes) and diploid (two sets of chromosomes) life cycles. In a haploid life cycle stage relatively simple rhombic crystals are produced in a vesicle inside the cell. The entire shell (holococcolith) is made up of an aggregation of such rhombic crystals. The diploid life cycle stage produces more complex mineral forms. Here too, the crystals are produced inside a vesicle or a compartment inside the cell, but scientists find that the development of shape may be mediated by silicon. The resulting shell (heterococcolith) is intricately shaped, made  up of a variety of crystal shapes in different species. The functional role of the shells could be varied. They may be providing mechanical stability, helping in maintenance of buoyancy, or in scattering harmful ultraviolet light in the upper column of the ocean.

Take a look at this magnified pictures of holococcoliths (a and c) and heterococcoliths (b and d). Scale bar: a and b = 5 micrometer. c = 500 nanometers. d = 1 micrometer.

Source: Role of silicon in the development of complex crystal shapes in coccolithophores: Gerald Langer et. al. 2021.

The prevailing thinking has been that the holococcoliths and heterococcoliths represent two independent origins of calcification. However, this study finds  that the calcite production sites in both life cycle stages are intracellular, and they likely use the same cellular mechanisms to transport ions, maintain calcium carbonate saturation levels, and to modulate the shape of the growing crystal by suppressing and enhancing specific growth directions. 

Based of this similarity in basic processes the researchers propose that the last common ancestor of this algal group must have had the ability to produce both holo and heterococcoliths. Holococcoliths being simpler represent the ancestral form of biomineralization in these algae. Initially, both haploid and diploid life cycle stages would have produced only holococcoliths. The haploid life stage retained this form of calcification. Subsequently, the diploid phase gained additional functionality to produce more complex crystals. Heterococcoliths thus evolved later in this ancestor,  recruiting silicon to mediate, in not yet fully understood ways, the production of varied crystal shapes. 

These algae acquired the ability to calcify around 250 million years ago. Interestingly, the simpler holococcoliths appear in the fossil record a good 37 million years later than the heterococcoliths. Scientist think that this could be an artifact of poor preservation of the simpler more fragile holococcoliths.

A parallel development in the marine realm has also had an impact on coccolithophores and other biomineralizing species. Another group of algae known as the diatoms started proliferating in the oceans in mid late Mesozoic by around 200-150 million years ago. Diatoms use silicon to produce beautiful skeletons. They progressively became efficient removers of silica from sea water. In the mid Mesozoic, large reefs built by the silica secreting sponges were common in the shallow marine settings. By late Mesozoic -Early Cenozoic times silica sponge communities shifted to deeper water and to higher latitudes, an ecologic displacement, some scientists think, forced by silica limitation in shallow tropical waters.  

By Cenozoic period diatoms had become the dominant silicon extractors from the upper layers of the ocean. So much so, that this diversion of silicon by diatoms impacted  Coccolithophores too. Many species stopped using silicon to mediate crystal growth, instead evolving alternate pathways to build their calcium carbonate shells. 

I love stories of the intricate interplay and feedbacks between evolution and geology. This is a theme I keep returning to. 


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