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 plants and other algae and cyanobacteria living in the surface waters. These organisms use the sun’s energy to create food for themselves and for other animals around them.

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

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 sunlight, convert these into organic matter—in this case a carbohydrate , glucose (C6H12O6)—and oxygen (O2).

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. As tiny plants and animals known as phytoplankton and zooplankton living in the sun-lit upper ocean die, they drift down towards the bottom as detritus . Deep-sea corals living on the ocean floor filter these small bits of organic matter out of the water as food. In this way, deep-water coral retain a connection to the upper ocean even though they live in perpetual darkness. Their cousins in shallow water also survive by means of photosynthesis, but in a way that involves some help, as we’ll see in another Hot Topic.

Organisms that form the basis for an entire community like this 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 rather than from sunlight through a process known as chemosynthesis .

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

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

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 explode.

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