Haloarchaea

Class of salt tolerant archaea

Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class of prokaryotic organisms under the archaeal phylum Euryarchaeota, found in water saturated or nearly saturated with salt. Halobacteria are now recognized as archaea rather than bacteria and are one of the largest groups. The name 'halobacteria' was assigned to this group of organisms before the existence of the domain Archaea was realized, and while valid according to taxonomic rules, should be updated. Halophilic archaea are generally referred to as haloarchaea to distinguish them from halophilic bacteria. These microorganisms are among the halophile organisms, that they require high salt concentrations to grow, with most species requiring more than 2.0M NaCl for growth and survival. They are a distinct evolutionary branch of the Archaea distinguished by the possession of ether linked lipids and the absence of murein in their cell walls. Haloarchaea can grow aerobically or anaerobically. Parts of the membranes of haloarchaea are purplish in color, and large blooms of haloarchaea appear reddish, from the pigment bacteriorhodopsin, related to the retinal pigment rhodopsin, which it uses to transform light energy into chemical energy by a process unrelated to chlorophyll based photosynthesis. Haloarchaea have a potential to solubilize phosphorus. Phosphorus solubilizing halophilic archaea may well play a role in P (phosphorus) nutrition to vegetation growing in hypersaline soils. Haloarchaea may also have applications as inoculants for crops growing in hypersaline regions.

Detailed phylogenetic and comparative analyses of genome sequences from members of the class Haloarchaea has led to division of this class into three orders, Halobacteriales, Haloferacales and Natrialbales, which can be reliably distinguished from each other as well as all other archaea/bacteria through molecular signatures known as conserved signature indels. However, Haloarchaea can grow up to saturation (about 37% salts). Optimal growth also occurs when pH is neutral or basic and temperatures at 45°C. Some haloarchaea though can grow even when temperatures exceed 50°C. The pigmentation will become enhanced when oxygen levels are low due to an increase in a red pigmented ATP. Some even older ones from more than 250 million years ago have been reported. Haloarchaea is also used to treat water that is high in salinity. This is due to its ability to withstand high nutrient levels and the heavy metals that may be present. The number of solutes causes osmotic stress on microbes, which can cause cell lysis, unfolding of proteins and inactivation of enzymes when there is a large enough imbalance. Haloarchaea combat this by retaining compatible solutes such as potassium chloride (KCl) in their intracellular space to allow them to balance osmotic pressure. Retaining these salts is referred to as the “salt in” method where the cell accumulates a high internal concentration of potassium. Because of the elevated potassium levels, haloarchaea have specialized proteins that have a highly negative surface charge to tolerate high potassium concentrations. Haloarchaea have adapted to use glycerol as a carbon and energy source in catabolic processes, which is often present in high salt environments due to Dunaliella species that produce glycerol in large quantities. Early evolution of retinal proteins has been proposed as the purple Earth hypothesis. Recently, haloarchaea was sent 36 km (about 22 miles) up into Earths atmosphere, within a balloon. The two types that were sent up were able to survive the freezing temperatures and high radiation levels. This only further extends the theory that halophiles could exist on Mars.

Certain types of haloarchaea have been found to produce carotenoids, which normally has to be synthesized using chemicals. With haloarchaea naturally producing it, there is now a natural way to synthesize carotenoids for medical use. Haloarchaea has also been proposed to help meet the high demand of carotenoids by pharmaceutical companies due to how easy it can be grown in a lab. Genes in Haloarchaea can also be manipulated in order to produce various strands of carotenoids, further helping meet pharmaceutical companies needs.

Haloarchaea have been proposed that certain types can be used to make biodegradable plastics, which could help decrease plastic pollution. Haloarchaea are able to produce polyhydroxyalkanote (PHA), polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV), when exposed to certain conditions. For large scale production of these bioplastics, haloarchaea is favored due to the low cost, fast growth, and lack of need to sterilize area due to the salty environment they prefer. They are also a cleaner option for bioplastics due to them not needing chemicals for cell lysis and have a higher recyclability of the process. Certain types of haloarchaea have also been found to poses denitrifying characteristics. If haloarchaea are complete denitrifiers, they could aid salt marsh and other salty environments by buffering these areas of nitrate and nitrite. This could help animal diversity and decrease pollution in these waterways. However, when tested in the lab, haloarchaea have been found to be partial denitrifiers. This means that if haloarchaea are used to treat areas that are high in nitrite and nitrate, they could contribute to nitrogen contaminates and cause an increase in ozone depletion, furthering climate change. The only type of haloarchaea that has been found to reduce nitrogen pollution to atmospheric nitrogen has been Haloferax mediterranei. This shows that haloarchaea may be contributing to nitrogen pollution and isn't a suitable solution to reducing nitrate and nitrite within high salinity areas.