8.8: Biological Sources and Sinks

Numerous biological processes alter the concentration of substances in the ocean. The first involves microscopic drifting photosynthetic microbes, the phytoplankton, introduced in Chapter 4. In the presence of sufficient sunlight and appropriate biologically important nutrients, phytoplankton carry out photosynthesis, the light-driven manufacture of organic compounds from inorganic carbon.

Different types of photosynthesis exist, with different pathways for different elements—like oxygen. So we have to be specific when talking about photosynthesis in the ocean. Photosynthetic reactions that yield oxygen are called oxygenic photosynthesis—oxygen-yielding photosynthesis. Contrast that with anoxygenic photosynthesis, which does not yield oxygen. Both use carbon dioxide, but only oxygenic photosynthesis releases oxygen as a byproduct.

Organisms that carry out oxygenic or anoxygenic photosynthesis are autotrophs, organisms capable of using external energy sources to manufacture their own cellular components from inorganic materials. Through photosynthesis autotrophs turn inorganic carbon—namely carbon dioxide—into organic carbon, the stuff of which all life is made. To be classified as a true autotroph, an organism must get at least 50 percent of its cellular carbon from carbon dioxide, according to microbiologists (e.g., Schönheit et al. 2016).

The autotrophs with which you are most familiar are plants—the green, multicellular organisms with leaves, trunks, and roots that you eat, smoke, pluck, sit under, weave into clothing, and use to build houses. Phytoplankton are autotrophs too and carry out photosynthesis at rates roughly equivalent to land plants.

Hard as it may be to fathom, phytoplankton supply about half of the oxygen in the atmosphere. Put another way, every other breath you take comes from phytoplankton. Other photosynthetic oceanic autotrophs, such as seaweeds, seagrasses, mangroves, and corals (with their photosynthetic symbionts), also contribute to the production of oxygen, about 3 percent globally (e.g., Malone et al. 2017). Of course, they also transform dissolved inorganic carbon into organic carbon. Because all of these autotrophs use light energy, they may be referred to as photoautotrophs, organic matter–making organisms that use sunlight as an energy source.

Another type of autotroph is the chemosynthetic autotroph, or chemoautotroph. Like plants and phytoplankton, chemoautotrophs fix inorganic carbon into organic carbon. However, they don’t require light, and they don’t produce oxygen. Instead, chemoautotrophs use chemical energy, such as methane or hydrogen sulfide. This process, called chemosynthesis, removes these dissolved gases from the water column and uses them as an energy source. While we commonly associate chemosynthesis with hydrothermal vents, exploration of the seafloor in recent decades has revealed several types of chemosynthetic communities, including whale falls, wood falls, and cold seeps (e.g., Dubilier 2008).

In addition to photosynthesis and chemosynthesis, a third biological process changes the concentrations of gases in the ocean. Respiration, the metabolic breakdown of organic matter to obtain energy, occurs in all living organisms. Sparing you the details, respiration produces carbon dioxide as a byproduct. Knowing whether the ocean acts as a net sink or source for carbon dioxide hinges on our understanding of respiration in the ocean.

Oxygen may be affected by respiration too. Aerobic respiration—the most common form—consumes oxygen as part of the metabolic process. Thus, aerobic respiration acts as a source of carbon dioxide and a sink for oxygen. Anaerobic respiration—breakdown of organic matter in the absence of oxygen, which occurs primarily within the seafloor—releases carbon dioxide but has no effect on oxygen concentrations.

A fourth process involves removal of dissolved substances as organisms construct their cellular materials. In making their cell walls, different groups of phytoplankton may use silica or calcium carbonate. Australia’s Great Barrier Reef is the ultimate expression of the way in which organisms used dissolved materials to build homes. Of course, the different varieties of mollusks, from clams and oysters to any of the prized varieties of seashells, represent sinks for dissolved elements. In truth, nearly every organism in the ocean at one time or another may serve as a source, sink, or both.