SUBSEA Part 1

Underneath the Ocean’s canopy

Phytoplankton are arguably some of the most important organisms on Earth. Responsible for at least half of the planet’s oxygen production, phytoplankton fix inorganic carbon dioxide and transform it into organic carbon molecules, moving carbon from the atmosphere into the sea. They are the base of many oceanic food webs, providing sustenance for bacteria and zooplankton, which in turn feed a myriad of larger animals, including fish, sharks, and whales.

Phytoplankton require specific chemical conditions to exist, and when those conditions are just right, they bloom.

Much of what we know about where phytoplankton live in the ocean comes from satellites. Some satellites, like NASA PACE, are designed to capture fluorescence from chlorophyll — bright green patches at the surface of the Ocean — mapping the presence of phytoplankton over space and time on a global scale.

However, these satellites, quite literally, only scratch the surface. Satellite imagery cannot penetrate past the first few meters of the Ocean’s surface. And the Ocean is very deep.

Phytoplankton blooms are comparable to the layers of a rainforest. At the surface, the canopy is made up of tenacious microscopic organisms that thrive on sunlight. Yet 50 to 150 meters down, in the understory, photosynthesizing organisms can fix carbon under lower-light conditions.

While this canopy region in the Ocean is relatively well understood, the understory is vastly understudied, especially in the South Atlantic Subtropical Gyre.

Using a variety of instruments, including gliders, floats, sediment traps, and a wirewalker, Dr. Church and the SUBSEA team will investigate the understory and the chemical reactions occurring within it. These chemical reactions in the understory of the subtropical gyres are likely just as important as those in sunlit waters, and more research is needed to determine which reactions occur and how they sustain primary productivity.

Life requires chemistry

A biogeochemical cycle is the natural pathway by which a specific type of molecule (e.g., carbon dioxide, nitrate, phosphate) is recycled by biological processes. To be considered a biogeochemical cycle, inorganic matter is converted into organic matter, usually by plants. The organic matter is then recycled back to inorganic nutrients by bacteria and animals. The essential ingredients for life on our planet are constantly recycled between living and non-living forms.

Dr. Church and the SUBSEA will examine the biogeochemical exchange between the surface of subtropical ocean gyres and the subsurface. Because the surface waters of the gyres are nutrient-depleted, plankton living here use all available nutrients, transforming inorganic compounds into organic ones as soon as they enter the environment.

The subsurface, in contrast, is more nutrient-rich. When surface plankton die or defecate, organic matter sinks, providing nutrients for deeper ocean layers. The plankton at the surface are analogous to the leaves on a rainforest tree. As leaves fall from a tree, they travel to the forest floor, where decomposers consume them and transform them back into usable materials. In the oceans, decomposition occurs mostly as plankton particles sink. The organic material from the surface plankton is recycled into essential nutrients for life, fueling the nutrient demands of plankton growing in the subsurface.

The SUBSEA is interested in investigating the biogeochemical cycles of several molecules that are essential for life, including phosphorus, iron, oxygen, nitrogen, and carbon