Coccolithophores are microscopic, single-celled marine algae, part of the phytoplankton group, found in the upper, sunlit layers of oceans worldwide. They are distinguished by intricate shells made of calcium carbonate plates called coccoliths. These “armored algae” are a widespread component of marine ecosystems.
The Intricate Shells of Coccolithophores
Coccolithophores are defined by their outer covering, a coccosphere, made of individual calcium carbonate plates called coccoliths. Calcium carbonate is the same mineral found in everyday materials like chalk and limestone. These tiny plates are produced internally within the coccolithophore cell.
The process of forming these plates is called calcification, where the organism precisely controls the precipitation of calcium carbonate within specialized intracellular compartments. Once formed, these plates are then extruded to the cell’s exterior, assembling into a protective, spherical shield. The shapes and arrangements of coccoliths vary greatly among the roughly 200 known species, displaying remarkable diversity and complexity.
Primary Producers and the Marine Food Web
Coccolithophores function as primary producers within marine environments. They harness sunlight through photosynthesis, converting carbon dioxide and water into organic energy and releasing oxygen. This activity makes them significant contributors to ocean oxygen production.
They form the base of the marine food web, providing a foundational food source. Smaller marine animals, such as zooplankton and filter feeders, graze on coccolithophores. This transfers energy up the food chain, sustaining larger organisms like fish and other invertebrates.
Influence on Global Climate and Geology
Coccolithophores play a role in the Earth’s global carbon cycle. Through photosynthesis, they absorb carbon dioxide from the atmosphere, incorporating it into their organic matter. When these organisms die and sink, their organic carbon is transported to the deep ocean, a process known as the “biological carbon pump.”
The formation of their calcium carbonate shells, however, has an opposing effect, often referred to as the “carbonate counter-pump.” Calcification releases carbon dioxide into surface waters. The net impact on atmospheric carbon dioxide depends on the balance between these two processes and the ratio of inorganic to organic carbon exported to depth.
Massive blooms of coccolithophores also influence climate through the albedo effect. These blooms are bright due to their coccoliths’ light-scattering properties, reflecting sunlight. This reflectivity can locally cool the ocean’s surface by reducing absorbed solar radiation.
Coccolithophores also hold geological significance. When these organisms die, their coccolith shells sink to the seafloor and accumulate over millions of years. This deposition forms vast sedimentary layers, compacting into geological formations like chalk and limestone, famously exemplified by the White Cliffs of Dover.
The Threat of Ocean Acidification
Ocean acidification poses a significant challenge to coccolithophores, stemming from the increasing absorption of atmospheric carbon dioxide by the oceans. As the ocean absorbs excess CO2, it forms carbonic acid, leading to a decrease in the ocean’s pH, making the water more acidic.
A direct consequence of this increased acidity is a reduction in the availability of carbonate ions in seawater. Carbonate ions are a fundamental building block that coccolithophores use to construct their calcium carbonate shells. With fewer carbonate ions present, it becomes more difficult for coccolithophores to build and maintain their protective coccoliths.
The potential consequences for coccolithophores include thinner or malformed shells, slower growth rates, or even the dissolution of existing coccoliths. Such impacts could hinder their survival and disrupt their roles as primary producers and their influence on global carbon cycling and geology.