[Originally posted to MBA Student Oceanography Club (SOC)]In our February meeting, we talked a little bit about some often overlooked, but really important creatures, including foraminifera, coccolithophores and zooxanthellae. These tiny organisms, along with a few others, have an outsized influence on the biology and the geology of our planet, so I thought I would share a little more about them. To appreciate the magnitude of their effects you have to remember that these organisms are major constituents of the plankton, and as such fill the surface waters covering over 70% of the earth’s surface. This means that in aggregate, these tiny organisms play an important role in capturing sunlight, cycling nutrients, driving the make-up of the atmosphere and creating geological formations. For example, phytoplankton, a mix of photosynthesizing algae and bacteria, account for as much as 50% of the world’s primary productivity, more than rainforests, grasslands, marshes or coral reefs. Small shell-forming organisms are also a major force in the earth’s carbon cycle, where atmospheric carbon, such as the greenhouse gas CO2, is absorbed by the oceans and ultimately locked up in marine sediment and rock. I’ll focus on a few major groups of single celled algae and animal-like protists that help make this planet a comfortable place for us to live.
Animal-like Marine Protists
Foraminifera (often called forams for short) are single celled protists that form shells, and can be found living on the bottom of the sea or drifting in the plankton. Planktonic forams almost all form calcium carbonate shells by pulling carbon out of the water. When the forams die, these shells fall out of the water and accumulate in huge numbers on the ocean floor where, along with coccoliths (see below), they contribute to the formation of carbonate rocks (i.e. limestone and chalk). This is an important process driving the earth’s carbon cycle, and leads to the long-term storage of carbon in the earth’s crust.
Radiolarians are similar to forams, but most form their beautiful shells from silica. Radiolarians can be very common in the tropics where their fallen shells cover the bottom in what is commonly called radiolarian ooze, which can harden into siliceous rocks (i.e. chert and flint).
Plant-like Marine Protists
Coccolithophores create calcium carbonate plates (coccoliths) that form an armored surface, they therefore store carbon in a manner similar to foraminifera. However, because they are able to photosynthesize, they have an even greater effect on atmospheric carbon by directly consuming CO2. Coccolithophores can form huge blooms that are easily visible from space (the milky green water in the photo shows such a bloom off the coast of Alaska), and help drive the exchange of gases between the ocean’s surface and the atmosphere.
Diatoms form silica shells, and are probably the most common component of the phytoplankton. As a result, diatoms are one of the most important sources of photosynthesis on the planet; it has been estimated that 20-30% of atmospheric oxygen is produced by diatoms, as much as the world’s
combined tropical rainforests. This photosynthesis also takes up lots of CO2, and the heavy silica shells quickly drag this carbon to the bottom of the ocean.
Dinoflagellates are a diverse group of marine protists, that unlike the others, do not form hard mineral shells (though some have hard, cellulose coverings). Their inclusion among the plant-like protists is a bit arbitrary, as about half of the dinoflagellates do not photosynthesize, but many of the best-known examples do. These include species that give rise to red tides and other harmful algal blooms (like the red tide off the coast of California pictured above), species that create the glow in bioluminescent water, and the zooxanthellae that live symbiotically within the tissues of a number of other organisms, including corals, jellyfish, clams, sea slugs and even foraminifera and radiolarians.