Light Snacks and Food Chemistry:
Photosynthesis and Chemosynthesis

The ocean floor near cold seeps and hydrothermal vents is home to an amazing variety of animals, including clams, shrimps, tubeworms, crabs, and many others. When these communities were first discovered living deep below the surface of the water, no one understood how life could exist so far from the reach of sunlight.

At one time, scientists thought that all food at the bottom of the ocean ultimately had to originate with algae , other plants, and cyanobacteria living in the surface waters. These organisms use the sun’s energy to create food for themselves and for the animals around them.

This process is known as photosynthesis . It describes how these organisms take inorganic carbon in the form of carbon dioxide (CO₂), combine it with water (H₂O), and, using energy from sunlight, convert them into organic matter—in this case a carbohydrate , glucose (C₆H₁₂O₆)—and oxygen (O₂).

CO₂ + 6H₂O + Sunlight -> C₆H₁₂O₆ + 6O₂

Some animals consume plants and other photosynthesizers then use the organic material they produce to grow and make other, more complex molecules, like proteins and starches. Animals consume photosynthesizers to survive and are eaten by still other animals to form a food chain.

Even at the bottom of ocean far from sunlight, many animals are still part of the photosynthetic food chain. When tiny plants and animals known as phytoplankton and zooplankton living in the sunlit upper ocean die, they drift down towards the bottom as detritus .

Organisms such as photosynthesizers, which form the basis for an entire community, are also called primary producers . Primary production from photosynthesis occurs wherever there is sufficient sunlight—on land, in shallow water, even inside and below clear ice. But who are the primary producers where there is no light?

It turns out that there are a great many bacteria and other microorganisms living in special places on the seafloor that support entire ecosystems using a different source of energy. These microorganisms get their energy from chemicals flowing out of the seafloor, through a process known as chemosynthesis .

Many bacteria found at cold seeps make organic matter and harvest energy by oxidizing chemicals known as hydrocarbons (including methane and many of the compounds that make up crude oil) or sulfides (including the rotten-egg-smelling hydrogen sulfide). In fact, some of the first organisms to arrive at a newly formed cold seep are bacteria, which form large mats on the seafloor. Animals such as shrimp, crabs, and fish follow soon after to feed on the mats and form the next link in the deep-sea food chain.

These communities form remarkable concentrations of life on the ocean floor. Seep ecosystems teem with fish, crabs, mussels, tubeworms, and many other animals, all of them thriving far from sunlight. As vibrant as these communities are, they were first found only in 1984 in the Gulf of Mexico. Very little is known about the individual species and the ecosystems found there.

Many bacteria found at hydrothermal vents also perform chemosynthesis. Some get their energy by oxidizing hydrogen sulfide to sulfate or elemental sulfur, using either oxygen or nitrate as an electron acceptor. Others gain energy by oxidizing hydrogen, iron, or methane. They use that energy, and carbon dioxide or methane from the water, to produce organic matter.

Chemosynthetic bacteria have also been found living in the ocean on whale carcasses and shipwrecks. It seems that almost anywhere there is an energy source to support primary producers, life will take hold and thrive.

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