Monteiro, F. M., Bach, L. T., Brownlee, C., Bown, P., Rickaby, R. E., Poulton, A. J., … & Gutowska, M. A. (2016). Why marine phytoplankton calcify. Science advances, 2(7). DOI: 10.1126/sciadv.1501822.
In many ways, coccolithophores are just like any other type of phytoplankton—they are single-celled, microscopic algae that use sunlight for energy. Unlike other phytoplankton, coccolithophores surround themselves with plates called coccoliths, which are made of calcium carbonate (the same material mollusks use to build shells). At times, coccolithophores can become so dense in the water that their chalky, white plates can be seen from space (Fig. 1).
What makes coccolithophores so important is their role in the global carbon cycle. Carbon dioxide is naturally absorbed from the atmosphere into the ocean and coccolithophores use carbon dioxide as building material for their plates. When coccolithophores die or stick together, they sink out of the surface water, transporting this carbon to the deep ocean.
Despite their important contribution to carbon cycling in the ocean, we do not fully understand why coccolithophores are the lone phytoplankton group to calcify. Monteiro et al. (2016) compiled information on the diversity and physiology of coccolithophores and their coccoliths to determine reasons for calcification. A better grasp on the role of calcification in coccolithophores under present conditions may help us understand how they may respond to future climate change.
As a result of its commonality in the ocean, Emiliania huxleyi (see title figure) has become the “poster child” for coccolithophores. While E. huxleyi may be the most abundant species, it is not the only coccolithophore. In fact, a spectacular array of diversity exists between coccolithophore species, especially in the size and architecture of their surrounding coccoliths (Fig. 2). The number of coccoliths per cell can vary from as few as six to several hundred and be arranged in single or multiple layers. The coccoliths themselves range from simple disk-like shapes to more elaborate spine- or trumpet-shaped projections (Fig. 2). Why do coccolithophores have these diverse and elaborate plates?
The researchers found several possible benefits for coccolithophores to calcify. The more promising explanations are described below.
The two main ingredients any phytoplankton cell needs to undergo photosynthesis are carbon dioxide and sunlight. Calcification may help coccolithophores obtain more of these essential ingredients (Fig. 3A). When coccolithophores calcify, they are thought to naturally concentrate carbon dioxide around the cell, making it more readily available for photosynthesis. In addition, coccoliths scatter light and can be arranged around the cell to maximize sunlight collection, which may come in handy if coccolithophores are deeper in the ocean or during a cloudy day.
Calcification could be a means of capturing more sunlight or a way to protect against it. Sunlight can become damaging to phytoplankton that are directly at the surface and calcification may help to protect coccolithophores from this potential photodamage (Fig. 3B). Coccoliths can literally act as a sunshade, deflecting harmful light or ultraviolet (UV) rays before they reach the interior cell or act as a sponge to absorb light. Ultimately, how coccolithophore species use their coccoliths depends on where they are in water column (e.g. shallow vs. deep).
Protection from predators:
The most compelling reason for coccolithophores having calcified plates was not involved with photosynthesis, but rather predation. According to Monteiro et al. (2016), the coccosphere (combination of all coccoliths) likely exists as a form of armor for the inner cell, shielding potential invaders from sneaking within the plates or ingesting the cell whole (Fig. 3C). To give you an idea of how well this protection works, consider the smallest possible invader in the ocean – the marine virus. For a virus to infect a coccolithophore host, it needs to make its way past the coccosphere and to the inner cell. In some cases, holes between and within coccoliths are so small (<200 nanometers) and packed with carbohydrates that even viruses cannot squeeze through.
But, what if the potential grazer was much larger than the coccolithophore? Some coccolithophores can avoid being eaten by plankton grazers by enlarging their coccosphere or adopting elongated or spiny plates (see Fig. 2 for examples). Better yet, coccolithophores can extend or modify their coccoliths in real-time during an attack. Grazers may also choose to avoid ingesting coccolithophores altogether because their carbonate plates have low nutritional value and aren’t worth the effort.
Discussion and significance:
Coccolithophores likely first adopted calcification as a means to protect themselves against predators though other benefits, such as those relating to sunlight, have helped to shape the diversity in coccoliths seen today. The variability in coccolith size and shape is thought to be driven by the particular habitats occupied by different coccolithophore species. For instance, coccolithophores who reside in shallow water will want to have plates that shield from overexposure to light, whereas deep-dwelling species will want plates that are arranged to funnel any possible sunlight.
Coccolithophores are important in both their role as primary producers at the base of marine food webs and for their increased contribution to the carbon cycle via the sinking of heavy coccoliths. How may these important phytoplankton species respond to a changing climate?
Changes in seawater chemistry associated with ocean acidification pose the scariest threat to coccolithophores. Over time, a more acidic ocean (lower pH) may place stress on the ability of coccolithophores to build their carbonate plates. If coccolithophores cannot efficiently build their plates, they may become more susceptible to predation and harmful sunlight exposure. Furthermore, as calcification is tightly linked to photosynthesis, a decrease in calcification may lower coccolithophore growth and in turn alter rates of carbon cycling in the ocean. To better assess these potential impacts under future climate scenarios, we need more information on the physiology and tolerance of a wider range of coccolithophore species.