"Dark energy" in the deep sea

Symbionts allow life far away from sunlight

Black Smoker in the deep sea © Marum, University of Bremen
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All life on earth and in the sea depends on the light of the sun - at least that's what most of them learned in school. But now it is clear that even in several thousand meters of water depth and thus far away from the sunlight still life exists. The microorganisms feed in the dark of high-energy substances that flow from the Earth's interior. This is made possible by symbiotic bacteria that oxidize methane and sulfides and thus make them usable as nutrients.

For long stretches, the bottom of the deep sea appears like a desert, because only very little biomass comes down from the upper layers of the ocean. Almost exactly 30 years ago, the oceanographers were surprised when they discovered the first hydrothermal springs with their extraordinary life forms in 2, 500 meters of water depth off the Galapagos Islands. So deep is not enough sunshine. Almost 400 degrees of hot water flows out of chimney-like chimneys bringing with it minerals and nutrients. Many of these minerals precipitate and form meter-high vents. The black color of the plume of smoke is due to the precipitated iron sulfide and gives this chimney the aptly named "Black Smoker".

Enigmatic deep-sea life

The scientists took stock and came to a surprising result: a whole zoo of exotic creatures had settled there. But what do these organisms feed on? After all, there are no photosynthetic organisms down there that could produce biomass. Living beings have only one source of food at their disposal, they must draw all their energy and nutrients from what the hydrothermal wells provide: hydrogen sulphide, sulphides and other salts of iron, manganese and copper, as well as methane, nitrogen and phosphorus compounds.

Particularly noteworthy are the highly developed creatures such as snails, shells and tube worms, which live there in large numbers. Unlike their cousins ​​in shallow water, they have significantly reduced their intestinal tract; the meter-long tube worms have even neither mouth nor intestine outlet. Only the microbiological examination showed it. Inside, special microorganisms settle, which can use the coming of the hydrothermal sources of methane or sulfide energetically and supply their respective host. Only thanks to this symbiosis with the methane oxidizing and sulfide oxidizing bacteria can these animals live there.

The deep sea mussel Bathymodiolus under the microscope. The specific probes can be used to detect the methanotrophs (left) and the methylotrophs in the gill tissue of the mussel. Max Planck Institute for Marine Microbiology

Outsourcing: No mouth, no intestines, but symbionts

However, these symbioses are not the sole specialty of the deep-sea dwellers. In the 1980s, when the symbioses in the deep sea began to be analyzed, other researchers also looked more closely and found their way into shallow coastal sediments. At the Max Planck Institute for Marine Microbiology in Bremen, for example, the symbiosis research group headed by Dr. Ing. Nicole Dubilier intensively with the various symbiont-host relationships. Dr. Dubilier is an expert on a close relative of the earthworm. These gutless Oligochaeten live in shallow water sediments and have managed in the course of their development, thanks to microbial symbionts of their digestive and excretion tract to get rid. display

With genomics the secret on the track

However, using conventional culture techniques, the microorganisms could not be studied because they are inseparable from their host. Only the analysis of the ribosomal 16S RNA as a marker led to the goal. If one has these RNA sequences, one can color and differentiate the different symbiont cells in the host tissue with color-coded probes. Only this so-called fluorescence in situ hybridization (FISH) brought the scientific breakthrough.

The molecular biologists, however, pursued yet another new path - metagenomics. The researchers first isolated the total genetic material, ie a mixture of host and symbiont DNA, then chopped them up and cloned these fragments into special cloning vectors. Using a newly developed mathematical algorithm, the scientists around Nicole Dubilier then compared the frequency of the "letters" of the four DNA building blocks G, A, T and C in the sequences and were able to assign these to the various organisms. The surprise was great, as the researchers used this approach to detect four different types of microorganisms in the oligochaete. With the reconstructed genomes, they found out how these microorganisms affect each other and the host metabolism.

Targeting deep-sea sources

QUEST - all-rounder of the deep sea RCOM

Now Dr. Dubilier apply this methodology to the deep sea dwellers at the hydrothermal wells. Her group investigates deep-water communities in the SPP1144 Priority Program funded by the German Research Foundation (DFG). However, in contrast to shallow water sampling in several thousand meters depth is a great challenge. Unmanned remote-controlled robots deliver data via cable to the control room on board the research vessel. There, the researchers and technicians sit in front of the monitors. Using the joystick, they control the gripping arms in order to set down measuring instruments accurately and to take the samples. So far, they have been able to examine the mussel Bathymodiolus at five different deep-sea locations.

Nicole Dubilier and her young research group at the Max Planck Institute for Marine Microbiology in Bremen are optimistic: "Classic microbiology does not go far with these multi-symbiont host systems. Modern methods such as fluorescence in situ hybridization and 16S rRNA analysis are in demand. This will give us reliable data quickly, leading to verifiable models. "


DFG priority program 1144 "From the mantle to the ocean: energy, matter and life cycles at spreading axes"

Census of Marine Life: Biogeography of Deep-Water Chemosynthetic Ecosystems

Bremen Max Planck Institute for Marine Microbiology

(Manfred Schlösser, MPI for Marine Microbiology, 06.07.2007 - AHE)