Information about Halobacteria

Halobacteria: Introduction and Research Applications

By Eric Grewal

Life is endemic throughout planet Earth. Over 1 million species are known to inhabit our world, and leagues more are yet to be discovered (1). In fact, certain organisms even thrive in harsh conditions including ocean trenches, acid pools, and deserts, devoid of most other life. Known as extremophiles, these organisms are mostly bacteria and archaea and have adapted over thousands of years to survive in their respective ecosystems.

One notable group of extremophiles is Halobacteria. Halobacteria are microorganisms that require high salt concentrations in order to survive and are known to reside naturally in habitats such as salt lakes and salt marshes. The term “halobacteria” is a misnomer, as halobacteria are not bacteria but actually are members of the domain Archaea (2).

Halobacterium salinarum is a unicellular organism that is surrounded by a single cell membrane. Each organism is rod-like in shape and has mechanisms for movement such as the use of gas vesicles and flagella. Halobacteria reproduce quickly through prokaryotic binary fission and are known to proliferate in aquatic ecosystems that experience sudden increases in salt. This organism is able to survive in environments of high salt concentrations thanks to large amounts of salts, ions, and other amino acids within its membrane that maintain osmotic equilibrium with the high-salt conditions outside of the cell, as well as the various membrane channels that allow for the exchange of ions through active transport (3). Halobacteria are known to grow optimally in temperatures of 37-42 °C and salt (NaCl) molarities of 4.3 M (4). H. salinarum exists in various strains, the most extensively studied of which is Halobacterium sp. NRC-1 (5).

H. salinarum has three primary methods of energy production, which include light-driven and light-independent mechanisms. Notably, H. salinarum is known for the production of bacteriorhodopsin, a light-sensitive protein that is produced by the organism in high-light environments. Bacteriorhodopsin is a membrane protein that acts as a proton pump, using light energy to create chemical gradients that can be used to produce energy (6). Production of this protein through the expression of the Bat gene is amplified by exposure to light, which causes increased phototropic growth (7). The formation of bacteriorhodopsin is also accompanied by the production of gas vesicles, small structures filled with gas that allows the organisms to move to the top of a solution in order to maximize oxygen and light exposure. The vesicles, composed of single layers of protein, create Halobacteria’s characteristic color, which is pink in the case of Halobacterium sp. NRC-1. Retinal pigments in the membrane of halobacteria also define its color. Alternatively, Halobacteria produces energy through the fermentation of arginine in anaerobic environments of low light and can also metabolize sugars through oxidation of amino acids into sugars as part of the trichloroacetic acid cycle in aerobic conditions (8).

Halobacteria play a prominent role in the environment, as they serve as the primary food source for filter feeders such as brine shrimp and contribute largely towards the biomass of aquatic ecosystems. The ecological effects of Halobacteria can be observed throughout the food chain; for example, they are responsible for the pink color manifested in flamingoes that consume brine shrimp.

It has also been posited that Halobacteria are some of the oldest organisms to exist on the planet currently, based on fossil data and archaean genome analysis. Some researchers have predicted the presence of Halobacteria or similar organisms in extra-terrestrial ecosystems such as that of the planet Mars, because of its resilience and multiple metabolic mechanisms.

H. salinarum has been extensively studied through genomics, metabolomics, and systems approaches in order to understand more about its gene networks. In fact, the genome of Halobacterium sp. NRC-1 has been sequenced and has been found to contain over 2.57 million base pairs, 2,674 mapped genes, one large chromosome and two mini-chromosomes (9). Halobacteria serve as good model organisms because although their gene regulatory networks are simple, their overall genetic processing methods are similar to those of humans. Thus, Halobacteria experiments may yield gene regulation information relevant for applications and further research in eukaryotes. Additionally, Halobacteria genomes can be manipulated in order to upregulate, downregulate, or even knockout certain genes, which adds to their value as model organisms.  Halobacteria also have an interesting ability to repair DNA in the case of radiation and UV exposure, which is currently being explored in systems biology laboratories.

Halobacteria hold a robust promise in the classroom due to their high durability and simple maintenance requirements. These archaeans can be grown from liquid culture, colony-to-liquid cultures, and even cell colonies (cell growth resource), so studies on the growth of halobacteria can easily be performed in a high school or college lab facility. Systems experiments and perturbations are also viable in halobacteria, thanks to the number of phenotypes that can be affected and exhibited by changes in the environment. Temperature, oxygen, salt concentration, and light exposure are just a few of the factors that can affect the growth of halobacteria and the expression of various genes, such as those that dictate pigment. Therefore, Halobacteria have been used in a variety of curricula that use it as a dynamic model specimen.

Since the sequencing of its complete genome, Halobacterium salinarum has become one of the most popular model organisms in the fields of microbiology, systems biology, and genomics (10). New research continues to yield more insight about its gene regulatory networks, metabolome, and its role in ocean ecosystems, and further studies of its proteome and genome may lead to novel research applications of this resilient extremophile.



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