How does life thrive when the deep sea is dark?

On land, life is almost completely dependent on photosynthesis. Plants utilize carbon dioxide and sunlight to produce organic matter. But not only the plants themselves benefit; they serve as the bottom of the food chain.

Animals with a plant-based diet gain their energy from eating plants, and they in turn serve as energy source for meat-eating animals on top of the food chain. So, if there is no light, there is no food, and thus no life dependent on photosynthesis. Aside from life on land thriving due to sunlight, what happens in places completely drenched in darkness?

The oceans present a habitat almost entirely governed by darkness. Only the uppermost layer of the ocean is penetrated by sunlight and in the top 200 m photosynthesis is possible:

In shallow, coastal areas, a range of marine organisms are capable of photosynthesis, taking on the role of plants on land. Far from any coastlines, phytoplankton – tiny marine organisms doing photosynthesis – are the only photosynthetic source of organic matter.

Thus, in the upper part of oceans marine life thrives, and organic matter produced through photosynthesis feeds a large variety of organisms. Below 200 m and especially in the deep sea on the other hand, marine life is less teeming. Down there it is dependent on organic matter from carcasses sinking down through the water column.

Carcasses are leftovers of dead marine animals that depended on photosynthetically produced organic matter. Scavenging falling organic matter is not the only way to survive in the deep sea though; organisms independent of sunlight and carcass-scavenging live in so-called seafloor hydrothermal systems.  

The diversity of marine life in these systems is surprising. One can find larger animals ranging from octopuses over fish, crabs, shrimp, and tube worms down to small crustaceans, as well as tiny microbes.

It’s generally the big that eat the small, with octopuses being the top predator in the deep sea. With that in mind, it is not surprising that microbes end up at the bottom of the food chain in these systems. But from what do the microbes gain their energy from to produce organic matter in these seafloor hydrothermal systems, independent of sunlight? 

These systems are essentially hot springs on the seafloor, and they provide the basis for life in the deep sea independent of sunlight. Hot springs can only form when there is a heat source nearby to heat up water. Mid-ocean ridges are a prime candidate for such conditions:

There, tectonic plates move apart below the oceans, forming new seafloor, and magma occurs comparatively shallow. Seawater can flow through cracks deep into the rocks below the seafloor and is heated by magma below. The heated seawater chemically reacts with the rocks below the seafloor and turns into hydrothermal fluids carrying dissolved elements from the rocks. The fluids with temperatures of up to almost 400 °C rise upwards and where they reach the seafloor, fantastic structures called hydrothermal vents are produced that consist of minerals forming from the dissolved elements.

Those systems were unknown until their discovery in the late 1970’s (1), and aside from producing mineral towers on the seafloor, the fluids contain hydrogen sulfide, hydrogen, and methane. These chemical compounds are the key to how microbes produce organic matter in seafloor hydrothermal systems and sustain life in the dark completely independent of other life forms and sunlight. 

Specific microbes have adapted a process based on what is provided by the hot springs: chemical compounds. Most of those microbes are bacteria, and the pathway they are putting to use is called chemosynthesis. Organisms capable of this process are called chemoautotrophs; they can oxidise hydrogen sulfide, hydrogen, and methane readily available from the hydrothermal fluids.

The oxidation reactions create an energy surplus that the bacteria then use to produce organic matter from carbon dioxide in the seawater. These chemoautotrophs form thick bacterial mats on the hydrothermal vents where they have unlimited access to the material they need. And these bacterial mats serve as sole food source for the crustaceans, that then in turn are eaten by larger organisms, which then are also eaten by larger organisms.

Ultimately, marine life in seafloor hydrothermal systems in the deep sea is diverse and thrives thanks to tiny, industrious microbes resorting to chemosynthesis in the complete absence of sunlight at depths of up to several kilometers below sea level.