Like green plants, photosynthetic bacteria harness the energy of sunlight to use carbon dioxide to make carbohydrates. While higher plants and photosynthetic protists produce oxygen as a by-product of photosynthesis, the photosynthetic Eubacteria do not. These bacteria also have different photosynthetic pigments than green plants and cyanobacteria. These pigments, called bacteriochlorophylls, have an absorption spectrum in the infrared range. Their ability to harness the energy of infrared light allows photosynthetic Eubacteria to occupy niches that other organisms cannot. This means that they often dwell in surface sediments at the bottom of shallow ponds or estuaries where they utilize the radiant energy that passes through the green algae and plants above them.
The purple non-sulphur bacteria are a particularly important group of photosynthetic Eubacteria in the Arctic. These species vary in shape – spiral organisms belong to the genus Rhodospirillum, rods to the genus Rhodopseudomonas, and spheres to the genus Rhodomicrobium. All of these organisms are found in mud and in lakes and ponds with high concentrations of organic matter.
There are other groups of photosynthetic Eubacteria in the Arctic that manufacture carbohydrates from inorganic materials. These bacteria do not use light energy, but instead exploit the energy released by oxidizing certain substances in their environment: sulphur bacteria oxidize hydrogen sulphide; iron bacteria complete the oxidation of iron compounds; and nitrifying bacteria oxidize ammonia-producing nitrates.
Species of purple sulphur bacteria, such as Chromatium, are prevalent in many meromictic lakes in the Arctic. The saline water at the bottom of these lakes creates an anoxic layer in which hydrogen sulphide accumulates. Purple sulphur bacteria are the only organisms able to tolerate these anoxic conditions. Conversely, they are unable to tolerate free oxygen.
Nitrifying bacteria are of particular importance in arctic terrestrial and aquatic ecosystems as many species occur in cold environments. Nitrifying bacteria are often the most important producers of nitrates in these environments, a nutrient of critical importance to higher plants. Nitrifying bacteria are also found in the upper water column of arctic lakes and ponds. In many cases, they directly support the other primary producers in these habitats and are the limiting factor in phytoplankton growth.