Sunday, November 30, 2014

Life, uh, Finds a Way

A few hundred years ago we believed that nothing could live in the deep sea. Even as our understanding of the world grew, in some ways we lost our imagination. We began to understand that the deep was incredibly cold, very low in oxygen, and subject to astonishing pressures. So we assumed that this environment was just too inhospitable . Boy were we ever wrong.


A mile and a half down (2560m), on the vent field of an active volcano.
Courtesy NOAA Ocean Explorer via Flickr

More and more, we're seeing that life not only exists in the darkness, but thrives. A few weeks ago we talked about animals up in the water that survive with little light, but now we're headed to the bottom. The above image comes from a deep sea hydrothermal vent community, which in the last few years has become relatively well known. Thanks to programs like Blue Planet and Planet Earth, there is beautiful footage of these ecosystems readily available to the public.

But hydrothermal vents are a small part of the ocean bottom. Other incredibly diverse ecosystems exists, and with the prevalence of the internet, researchers are starting to show them as they're discovered. Some of the coolest environments being found are cold seeps.

There are several types of cold seep ecosystems, but what defines them all, is gasses escaping from underground into the water. Seeps have been found all over the world, from as shallow as 15m (easily diveable) to over 7000m (not easily anything), and even in inland bodies of water. Generally seeps are found on the edges of continental slopes where the earth's crust is bending and folding. Unlike hydrothermal vents cold seeps are not due to magma heating seawater and expelling it back out of the ground. Seeps form in places where a lot of plant and animal matter has settled to the bottom, become buried, and decayed. As that material breaks down it makes a lot of methane gas. Then as the ground bends it squeezes the gas, pushing it closer to the surface through the soft sediments on top. My apologies for being crass, but yes, the earth does fart.


A stream of bubbles escapes from the ground at a cold seep
Courtesy Deepwater Canyons 2013 - Pathways to the Abyss NOAA-OER/BOEM/USGS

The gas alone is not enough to establish an ecosystem; some organism needs to harvest the gaseous bounty to start a food chain. On land and in the surface waters, plants convert the sun's energy into chemical energy using CO2. At cold seeps bacteria and archaea convert the energy in the methane (CH4) coming from underground, and sulfate in seawater, to a type they can use. In a spectacular bit of symbiosis the archaea manipulate the methane and the bacteria manipulate the sulfate, then they use one another's products to complete their energy conversion. The exact nature of this back and forth isn't well understood, but scientists do know it produces hydrogen sulfide.

The hydrogen sulfide is then used by bacteria living inside animals to produce even more energy. These animals (some types of mussels, clams, and tube worms) have little to no digestive system. They get their energy straight from their symbiotic bacteria. At many cold seep sites large mats of bacteria and archaea are surrounded by these animals. Sometimes they even form a bulls-eye of different colors radiating out from the center.


Bacterial mats (white) and mussels (brown/orange) thriving at a cold seep
Courtesy NOAA Photo Library via Flickr

These large sedentary animals draw in small animals that take shelter in the jumbled chaos of their shells. Predators of those small organisms then come hunting. Many animals also feed directly on the bacterial mats, so there don't even have to be larger organisms around them. Entire ecosystems develop from the toots of the planet. Some of the animals that take advantage of cold seep environments are even ones that we eat, like sablefish and crabs. King crabs have even been observed feeding on bacterial mats, leaving, and coming back only after enough time has passed for the mats to regrow!

All of this biological activity helps contain methane (a potent heat trapping molecule) in the sea, keeping the temperature of the earth from rising even more. Also many seeps found in very deep water have low enough temperatures and high enough pressures for solid ice crystals to encase methane molecules. These "methane hydrates" also help keep carbon out of the atmosphere. You can see the biological and geological carbon traps interacting in this adorable video below.



Cold seeps are a good number of deep sea ecosystems, but there's more talk about, and even more to discover. Clearly the ocean bottom isn't quite so lifeless as we once believed. And probably it's even more full of life than we currently understand. Jurassic Park's Ian Malcom knew what he was talking about.

References:

Levin, Lisa, "Ecology of Cold Seep Sediments: Interactions of Fauna with Flow, Chemistry and Microbes", Oceanography and Marine Biology: An Annual Review, 2005, 43, 1-46, Taylor & Francis

Niemann et al., "Methane-Carbon Flow into the Benthic Food Web at Cold Seeps- A Case Study From the Costa Rica Subduction Zone", PLOS ONE, Oct. 2013, DOI: 10.1371/journal.pone.0074894

"Discovery of a New Chemosynthetic Community" NOAA Ocean Explorer

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