The Boiling Hot Deep

Yes, I just used the term ‘boiling hot’ to describe the deep ocean. And no, it is not a typo. While the vast majority of the deep ocean is indeed very cold, that is not true for its entirety. Along major ocean basins lie long series of underwater mountains, or ridges. Along these ridges scientists have discovered a unique geological formation now known as hydrothermal vents. They are one of my absolute favorite marine ecosystems! Since this summer has been so crazy busy and stressful, I thought it was a good time for a fun post. A post about something wildly cool just for the heck of it.

Deep-sea hydrothermal vents are extremely unique ecosystems, in that they are simultaneously one of the harshest environments to live in, and yet they are rich in biological complexity and diversity. These vents act as little havens for communities of bacteria, archaea (a type of singled celled organism), crustaceans, bivalves, various worms and more in the otherwise near barren sea floor.

So, what exactly are the deep-sea hydrothermal vents? How do they occur? Well, it’s not as complicated as you might think. We all know that the earth’s surface is made up of multiple separate puzzle pieces of crust that ‘float’ on top of the fluid magma layer (called the mantle) hundreds of feet down towards our planet’s core. These pieces are called tectonic plates, and they are constantly moving and shifting about the globe due to the movement of the magma filled mantle layer they sit atop of.

The places two or more plates meet are known as boundaries. There are three ‘types’ of boundaries. The first type of boundary is when two or more plates converge, or push, into each other. This can result in the formation of continental mountain ridges and deep-sea trenches. Notable examples include the Rocky Mountains and Marina’s Trench – both were formed due to convergent plate interactions. The second type of boundary is when two or more tectonic plates move side-by-side each other. In many cases this results in earthquakes. A big example of this is the infamous San Jose Faultline in California. ‘Faultline’ is another word for this type of tectonic plate boundary. And then there is the third type of boundary, where two plates are diverging, or moving away, from each other. These are often characterized by volcanic activity, especially when the boundary is underwater.

The aforementioned underwater mountain ridges with hydrothermal vents are located at divergent tectonic plate boundaries, where the seafloor is spreading apart and magma flows out. Cold sea water seeps into the seafloor through little cracks and holes between sediment particles, hits the nearby layers of hot basalt (the type of rock oceanic crust is made up of – in this case melted into magma), and then due to its now super-heated status, vents back out. Hot rises, remember? The water that flows out of the vents is as I said, super-heated (upwards of 400 °C/752°F!!!), and super rich with nutrients it picked up from the magma. This includes copper, aluminum, iron, cobalt, manganese, lead, zinc, silica, sulfides (especially H₂S), and dissolved hydrogen and methane. When the hot water hits the cold seawater above the seafloor sediment, the pH and temperature abruptly changes, causing the metal sulfides to rapidly precipitate and form big black plumes of particles (hence their common name “black smokers”). As you can imagine, this unique geochemical position of deep-sea vents therefore greatly impacts the community makeup of nearby organisms.

The reason the biological communities around vents are able to survive is because the primary producers (the organisms that convert energy and raw materials into sugars/food) have uniquely adapted metabolisms that produce food via chemosynthesis. On land and in shallow surface waters primary producers (plants, algae, bacteria) undergo photosynthesis – they use the energy from the sun to produce food. At the depths these hydrothermal vents can be found at, there is no sunlight. They have permanent darkness. To survive without the sun’s energy, these microbes developed the ability of chemosynthesis. They manufacture food by harnessing the energy in the chemicals that are found in the black ‘smoke’ spewing from the vents, primarily the H₂S. Their metabolisms are so efficient and nutrients are so abundant around the vents that their growth rates are comparable to organisms in tropical environments, which are well known to be been rich areas of diversity.

Many new, unseen before species have been discovered in these communities. For example, one study alone isolated 9 species and 18 different strains of flagellated protists (single celled organisms). Many of these microorganisms live as symbionts with larger species, some are free floating, and others form mats along the bottom. It is through the chemosynthesis of the bacteria and archaea that the larger organisms are able to survive. Take the tube worms for example (these are the large, red and white tubes commonly seen in pictures). The microorganisms live in their tissues, similar to how zooxanthellae algae live in coral tissue. The worm gets 100% of its food from these microscopic symbionts. In fact, they don’t even have mouths or digestive tracks! In other cases, small organisms like plankton eat the mats, beginning a food chain that includes the larger crustaceans, snails, bivalves, octopuses and specialized fish.

To date, there have been over 500 different species of eukaryotes identified around vents thus far. That means there is 500 different species NOT counting bacteria or archea (Eukaryotes have organelles inside their cells, like mitochondria and a nucleus. Bacteria and acrchea do not have these and are known as prokaryotes. Counting those, that number would be SIGNIFICANTLY higher). Despite the fact that these organisms can send out larvae great distances to establish themselves at other vents, each vent occupies such a narrow range of habitat, far from other vents, that each one shows high levels of endemism – distinct species originally from that location. Between the Atlantic and the Pacific, only about 40% of species can be found in communities in both basins.

Another unique characteristic of vent biological communities is the high turnover and succession rates of the organisms that live there. Frequent volcanic eruptions likely ‘resets’ the vent community often. As a result, many of the organisms have adapted to colonize and become established very quickly. For example, only two years after the Cleft Segment along the Juan de Fuca Ridge in the Eastern Pacific began smoking, the microorganisms, tubeworms and their associated fauna had become fully established. However, by the six-year mark, the community had shifted from being dominated by tubeworms to being dominated by limpets, a relative of the snail with flattened shells. Additionally, the hotter the vent, the more the community nearest the vent is dominated by hyperthermophilic (heat tolerant) archaea as opposed to bacteria.

However, much of the diversity and community structure of these habitats is still unknown. Discoveries are being made every year! In March 2017, researchers found evidence of what they think could be the oldest lifeforms on earth. Fossils of microorganisms were found in sediment from a hydrothermal vent dating back 4.280 billion years ago! Who knows what 2018 will bring? The deep sea is basically the Wild West of earth’s habitats. ‘We know more about the surface of the moon than the bottom of the ocean’ isn’t a cliché for no reason! These unique hot spots of life is a perfect example of why I love studying the oceans. There is always something new to learn!

What’s something you find endlessly interesting?

Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; slack, John F.; Rittner, Martin; Pirajno, Franco; O’Neil, Jonathan; Little, Crispin T. S. (2 March 2017). Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature. 543: 60–64.

Thornburg, C., Zabriskie, M., & McPhail, K. (2010). Deep sea hydrothermal vents: potential hot spots for natural products discovery? Journal of Natural Products, pp 489–499.

Tunnicliffe, V. (1991). “The Biology of Hydrothermal Vents: Ecology and Evolution”. Oceanography and Marine Biology: An Annual Review. 29: 319–408.