Life driven by a chemical soup at the bottom of the sea

It is hard to believe, that there exists a thriving world of animals and microbes at the bottom of Earth’s oceans. No sunlight at all, enormous pressures that would immediately smush a person without protection and water temperatures close to the freezing point don’t seem to be a very homelike ambience for any living creature. However, my colleagues and I found evidence for a unique and diverse consortium of micro- and macroorganisms at certain spots at the sea bottom called hydrothermal vent fields, which represent flourishing islands in a barren desert. Hydrothermal vents occur where new oceanic crust – the outermost skin of our planet - is born or where old crust remelts and sinks back into the layer below the crust. In both cases liquid magma heats up the seawater in the oceanic crust until it gets so hot that it rises to the ocean bottom and emanates at the vents. But how can life survive down there? And what kind of life forms exists in this hostile deep?

The remotely operated vehicle (ROV) is able to dive down to 4000m water depth.
Two manipulator arms and several cameras and torches at the front side are necessary for
sampling at the sea bottom (courtesy of MARUM Bremen, Germany).

To answer these questions we traveled with a research vessel to a hydrothermal field in the middle of the Atlantic Ocean. Shortly after the vessel arrived at the vent field we skipped the remotely operated vehicle (ROV) we were using as our main sampling device. 2 hours later it reached the ocean bottom in 3000m water depth. It was an exhilarating moment, when the first light of the six cameras, which were mounted at the front side of the vehicle, fell on the mighty tower-like chimneys representing the vent orifices. Twirling swarms of various animals appeared on the screen on board of the vessel, where we watched the journey of the ROV. Thousands of shrimps, crabs and mussels were swimming, crawling and sitting around the huge edifices, which reach the size of multi-level buildings.

Hot hydrothermal vents at the ocean floor are often represented by remarkable chimney structures: the famous black smokers. The edifices consist of chemical compounds dissolved in the hot fluid and precipitating in contact with cold seawater
(courtesy of MARUM Bremen, Germany).

Around the wells, where the extremely hot and nutrient loaded fluid is outpouring into the cold seawater, we observed different types of bacteria devouring this invigorating soup. It seems that each group of microbes lives on different chemical compounds, which are dissolved in the fluid. To our surprise we found clues for the existence of a microbial chain similar to a “bucket brigade” consisting of distinct groups of sulfur bacteria. Each group recycles the waste product of another group – a system capable to solve many problems in the world, if adopted by man. In addition, the chemical composition of the chimney material revealed that next to eating all day long the microbes also assist in building up the impressive chimney structures, which in turn harbour these microorganisms – the germs build their own home!

The great abundance of microbes on and in these mini-skyscrapers attract the small animals like shrimps and crabs, which live on the bacteria. The small animals themselves represent the livelihood for predators like fishes. Hence, kilometers away from the next ray of light hydrothermal vents offer appropriate conditions for the development of a complete food chain being autonomous from the sun.

Our findings in the deep ocean opened our eyes once more for the intelligence of nature and the complexity of life on our planet. As we left this place in the middle of the Atlantic nowhere we believed devoutly, that many secrets at the dark ocean bottom are still waiting for being disclosed.

A paper referring to microbial activity in hydrothermal systems is available at doi:10.1016/j.chemgeo.2010.11.011