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. 

 

Map: The Deep Geological Cycle of Carbon

When I was a kid not so long ago in geologic time, my understanding of how diamonds are created went something like this.

In forests and swamps, large trees grew and died. The wood got buried under more wood and layers of sand and mud. The wood in the bottom layers under the influence of great pressure and higher temperatures got converted first to coal. As burial to greater depths continued, this coal turned first to graphite and then finally to diamond. The story of woody material turning eventually to coal and then to graphite was roughly correct, but this geological path doesn't lead to diamonds.

Most diamonds form at depths of about 150- 200 kilometers. Rarer varieties known as sublithospheric diamonds form even deeper down. The carbon required to make a diamond is transported from the earth's surface to those depths by a subducting plate. Subduction is the process whereby an oceanic plate made up of dense Mg and Fe rich rocks sinks into the mantle. The Marianna trench for example marks the place where the Pacific plate is sinking underneath the Japan Plate. Why can't coal then move into the mantle this way? It doesn't because coal deposits occur in continental settings where the crust is made up of much lighter rocks richer in Si, Al, Na and Ca. This continental crust is buoyant and does not subduct into the denser mantle. So there is no way for coal that began its journey in ancient river floodplain, bogs, and swamps to get directly transformed into diamonds.

The denser oceanic plate contains carbon from a variety of sources. The lower layers of the oceanic plate is made up of a Mg rich rock known as peridotite. Occasionally, faulting may bring this peridotite to shallower levels where it interacts with sea water and gets transformed into a rock known as serpentinite with calcium carbonate minerals also forming alongside. Carbon is trapped in minerals like calcite and dolomite. The upper layers of the ocean plate is made up of the volcanic rock basalt. It too interacts with sea water, with calcite precipitating in rock cavities. This becomes another source of carbon.

Then there is carbon which is part of the shells and skeletons of planktonic marine creatures. These tiny photosynthesizing organisms which live in the sunlight zones of the ocean precipitate a calcium carbonate skeleton. When the organism dies, these carbon containing skeletons sink and blanket the sea floor. This source of carbon is a relatively late addition in geologic history. Calcareous nanoplankton first appeared in the early Mesozoic, some 225 million years ago. Organic tissue of marine creatures can also get buried, making this another carbon source. And finally, carbon coated sediment washed into the ocean by rivers and then transported into abyssal depths by deep sea currents contribute some carbon to the oceanic plate.

Each oceanic plate has its own selection from this carbon menu, depending on its unique geologic history. For example, calcium carbonate starts dissolving below a depth known as the calcite compensation depth. Sea floor below this depth doesn't retain much skeletal debris. Or, oceanic crust that formed in the Cretaceous contains abundant calcite in veins and vugs, likely because the warmer Mesozoic oceans promoted calcite precipitation on the sea floor.

This beautiful map shows several subduction zones. In the map, SedCarb refers to skeletal carbonate, SedOrgC to organic carbon, AOC Carb to carbonate in altered oceanic crust, SerpCarb to carbonate in serpentinite rocks. The major source of carbon is identified by a particular geochemical signature. Mineral carbonate for example has higher amounts of the heavier isotope of carbon (C13), while carbon that makes up organic tissue is much richer in the lighter isotope (C12).

  Source: Terry Plank & Craig E. Manning: Subducting Carbon

As the oceanic plate subducts carbon begins to get removed from the plate. Some carbon is removed when the sediment and altered oceanic igneous rocks are scraped off and plastered on to the sea floor. Such deposits made from scraped off sea floor are called accretionary prisms. They often poke out above sea level to form island chains. The Andaman Islands is an example of an accretionary prism that is made up of mechanically removed slices of the subducting Indian oceanic plate.

At greater depths, sediments and oceanic crust begins to be metamorphosed under higher temperatures and pressures resulting in the loss of carbon dioxide and water. This carbon dioxide makes its way into the overlying mantle and gets incorporated into magma. The spectacular volcanic eruptions along Japan, Indonesia, Caribbean and the Western North American coastlines are a result of the rising and depressurization of such volatile bearing magma. Some of the carbon in the belched out carbon dioxide has come from the burning of skeletons of marine organisms in the deeply buried plate underneath these volcanic systems.

A quantity of carbon does remain in the downgoing plate and reaches depths of 150-200 kilometer or more. The igneous rocks of the subducted plate gets altered to a dense rock known as eclogite. And at these temperatures and pressures, diamonds may form within these eclogites along carbon dioxide or methane rich domains. The subducting slab is also releasing some trapped carbon along with other volatiles which infiltrate the surrounding mantle. Diamonds can form in such metasomatized or fertile regions of the mantle as well. The main host rock here is peridotite. Subducted carbon is one source of carbon for diamonds.

Geologists think that primordial carbon retained in the mantle from when the earth formed may also be finding its way into diamonds.

Multiple sources perhaps, diamond forming chemical reactions can be summarized simply as driven by the reduction of carbon sourced from either carbon dioxide or methane.

CO2 = C + O2

CH4 + O2 = C + H2O.

From eclogite and peridotite parent rocks, diamonds are transported to the surface by an unusual magma type known as kimberlite and even less commonly by lamproites. These magmas are rich in volatiles like water, carbon dioxide, fluorine and chlorine and are also rich in magnesium. They are generated at the base of thick continental plates generally during episodes of continental fracturing. The volatile rich magma physically disaggregates diamonds from their parent rocks and carry them as they ascend through deep continent penetrating cracks with amazing speed, traveling 200 kilometers in a matter of hours, bringing to the surface its tiny but dazzling prize. The block diagram below summarizes the geological environments of diamond formation and their ascent.

Source: Steven B. Shirey et. al., Diamonds and Geology of Mantle Carbon; modified from Tappert R and Tappert M 2011.

The famous Panna diamonds from Bundelkhand in Central India came to the surface in a kimberlite magma eruption around 1 billion years ago. It is possible that its source carbon was transported from the surface to diamond forming depths hundreds of millions of years earlier, perhaps during the convergence and assembly of an earlier supercontinent.

Diamonds are often older than their host kimberlites by hundreds of millions to billions of years. During diamond growth, other minerals get trapped inside them as micro-inclusions. Their composition is therefore a record of the fluid chemistry of the mantle and the carbon cycle as it existed in deep time, billions of years before present.

Diamonds are one component of the deep geological cycle of carbon. We are familiar with the exchange of carbon between the atmosphere and the biosphere. Carbon is transferred to and fro in this system on a timescale of days to years to hundreds of years, but not much more. Longer geological sequestration of carbon occurs at shallower levels of the crust too. Soil can store carbon for thousands of years. Carbon can get trapped for millions of years in carbonate minerals that make up limestone and also in coal and oil. It is this shallow crustal carbon cycle that we are breaking by burning limestone and fossil fuels.

The deep geological cycle can take carbon from the surface and keep it in the mantle for hundreds of millions of years. The mantle releases it through sustained volcanism thus modulating earth's climate on long time scales. And occasionally as a return gift it throws up a few diamonds as well.