What is hydrogen sulfide used for?

The discovery of the large, nitrate-storing sulfur bacteria


What are sulfur bacteria?

Many bacteria, like most higher organisms, breathe with oxygen and thereby oxidize ("burn") their food. But it is not only organic material, such as remains from the plant and animal world, that provides usable and high-calorie food. Many inorganic chemical compounds can also be used and can serve as staple food for special bacteria. The sulfur bacteria live on hydrogen sulfide, which they convert into sulfate. At the same time they breathe with oxygen. This gives them energy for their entire metabolism and growth.
Sulphate is one of the most important salts in seawater and is constantly penetrating the seabed. Beneath a millimeter-thin cover layer that covers the surface of the sea floor like a breathing skin, the sea floor is an oxygen-free world, but not dead. This is the world of bacteria that break down organic remains of dying algae after they have sunk to the sea floor. Many of these bacteria breathe with sulfate, which is converted into hydrogen sulfide.

So there is a constant change between sulphate and hydrogen sulphide, which is one of the most important ecological material cycles in the sea and ensures that the organic material is converted back into inorganic minerals and plant nutrient salts. Enormous amounts of hydrogen sulfide are produced in the sea floor. For higher organisms like fish or crustaceans it is crucial that it is broken down again in the sea floor before it gets into the sea water. Because hydrogen sulfide is highly toxic, just as toxic as cyanide, but it is a natural toxin. That is why there are many bacteria in nature that have specialized in breaking down hydrogen sulfide. The most famous of them breathe with oxygen. They occur almost everywhere, in the sea, in lakes, in the ground or in bioreactors and sewage treatment plants where wastewater is treated.

In our coastal waters in particular, where the discharge of nitrogenous nutrient salts has increased sharply in recent decades, the increased nitrate content leads to increased algae production, which in turn causes the organic algae residues to sink more strongly to the sea floor. Above all, this stimulates sulphate respiration, and the production of hydrogen sulphide becomes even greater, until it can hardly be oxidized with the simultaneously decreasing oxygen content in seawater. Then the sulfur bacteria take over the stage and spread a white sheet over the black sea floor. This is a thin layer of thread-like sulfur bacteria called Beggiatoa, to which the refraction of light in countless sulfur droplets in their cells gives them a white glow. They are the last barrier against the escape of hydrogen sulfide from the sea floor, and they use the oxygen in the sea water to do this. This was their known function until our discovery of nitrate vacuoles in sulfur bacteria.
Why giant bacteria?

Most bacteria are one to a few micrometers (thousandths of a millimeter) in size. There are good reasons for it. Bacteria can only take up their food from their aqueous environment in the form of dissolved substances. If the bacteria become too big, the uptake is too slow because the cells have no internal transport system (e.g. circulation) and they starve. Therefore, large bacteria are a rarity in the wild. Bacteria as large as the recently discovered thiomargarita (Schulz et al., Science, April 16, 1999) should not actually be able to exist. This is only possible because they have a special cell structure.

The active part of the cell of these large sulfur bacteria consists of a thin shell of cytoplasm that encloses a large, spherical vacuole. The cytoplasmic layer is only a few micrometers thick, which corresponds to the size of normal bacteria. In this way, the thiomargarita ensure the transport of substances in the cell despite their size. The vacuole is like a water-filled balloon - a very unique structure among bacteria. It serves as a storage tank for nitrate, which these bacteria use to breathe. As a respiratory agent, nitrate is in principle just as good as oxygen, but it has the great advantage that it can be stored behind a cell membrane, while oxygen inevitably escapes as a gas. Like a diver with compressed air bottles, the thiomargarita can breathe with their nitrate store for months before they need a new supply from the outside. With this nitrate, the bacteria oxidize hydrogen sulfide, which is constantly produced in the surrounding seabed, in which they are immobile. They then alternately need nitrate again, and are dependent on seawater containing nitrates penetrating the seabed through the currents and pumping activity of Atlantic waves.
Thiomargarita namibiensis:
The sulfur globules are clearly visible.
The Thiomargarita also have close relatives who are movable threads instead of immobile balls, e.g. the Thioploca. These form the largest visible bacterial communities in the world on the Pacific coast of South America. Here there is a natural fertilization of the sea, as nitrate-rich deep water from the Pacific is driven up by the SE wind, but also a constant lack of oxygen at the bottom of the water column. Thioploca form mats on the sea floor over a coastline of 3000 km. The mats are braided together from slimy sheaths of the thioploca and reach a mass of 1 kg fresh weight per square meter. Many filaments of bacteria live together in each shell, which extends into the sea floor like a vertical tunnel. This causes the bacteria to slide up and down constantly. The up to 7 cm long, thin bacterial filaments stretch into the sea water, where they take nitrate out of the water for their breathing. With their storage tanks full, they slide down through their tunnels into the seabed, where other bacteria cause an intensive production of hydrogen sulfide. This hydrogen sulphide is their food and is completely converted by the thioploca.

We first discovered this way of life four years ago on a research trip off the Chilean coast. But after the bacterial vacuole was recognized as a nitrate store, we and other researchers found this phenomenon in other bacteria as well, and always in large sulfur bacteria, all of which are closely related to one another. At the hot springs in tectonic spreading zones on the deep sea floor, thick mats of free-living, thread-like sulfur bacteria of the genus Beggiatoa also live. Its individual cells are much smaller than those of the thiomargarita, but the multicellular bacterial filaments are large enough (a tenth of a millimeter in diameter and a few centimeters long) that they could be seen with the naked eye from the window of the deep-sea submersible, ALVIN.

Nitrate-storing sulfur bacteria are not only found in locations that are exotic for us. After its discovery in Chile, we also looked for this property in the native Beggiatoa in the Baltic Sea and actually found it. So it seems like a widespread adaptation that could presumably be found anywhere in the oceans now that we know what we are looking for. Characteristics are cell size, glowing sulfur granules and an apparently empty cell interior. The widespread distribution of this group of bacteria also means that their metabolism plays a correspondingly important role in ecological material cycles. Due to the oxidation of hydrogen sulfide with nitrate in these bacteria, the material cycles of sulfur and nitrate are coupled in a way, the consequences of which we cannot yet overlook.




Bo B. Jørgensen
Max Planck Institute for Marine Microbiology