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Halobacterium: Mechanisms of Extreme Survival as a Solution to Waste Isaiah Stackleather Introduction Halobacteria is a class of archaebacteria that thrive in harsh environments with a unique capability to survive in hypersaline environments. Research conducted today shows that halobacteria diplay unique environmental response capabilities not only to high concentrations of salt, but also to desiccation, gamma irradiation, oxidative stress (the scarcity or overabundance of oxygen species such as O2 and H2O2), and microgravity [1,2,3]. In short, halobacteria are unique archaebacteria whose characteristic ability to survive in extreme environments is unlike that seen in most other microorganisms. his review aims to describe physiological mechanisms that halobacteria utilize survive and how these responses may provide solutions in ields such as waste management.
Environmental Stress Response Characterization Halobacteria can survive in many hypersaline environments. However, when it comes to environments of oxidative stress, microgravity, or gamma irradiation, little is known about their phenotypic response. he following section gives insight into the characteristic responses when exposed to certain extreme conditions. Desiccation Given the halobacteria’s known response to hypersaline systems, Kottemann et al. explored whether the wild-type halobacteria strain NRC-1 would respond in a similar way to desiccation, and the bacteria were, indeed, able to withstand high levels of desiccation. When placed in desiccation for twenty days, there was a 25% survival of viable cells with an almost full DNA recovery in two days [1]. herefore, Kottemann concluded that this ability to survive high levels of dryness occurred because of its capability to quickly repair double-stranded DNA breaks. hese results conirm the conclusions of Malcolm Potts, who suggested that halobacteria must either adapt to desiccation or already possess the capability to resist its harmful efects. NRC-1 colonies took refuge within salt crystals in order to protect themselves against desiccation [1]. his behavior, typical of haloarchaea, has even been observed in the viable mitochondrial DNA of haloarchaea found in 60,000 year old bones of deceased Aboriginals, demonstrating the efectiveness of this response [4]. Kixmuller et al. ofers an explanation for halobacteria survival within halite crystal deposits. Halobacteria thrive in environments with high concentrations of potassium
(K+) ions. However, desiccated environments, such as deserts, do not maintain very high K+ ion concentrations. heir survival is possible because of the kdpFABCQ operon, which regulates the expression of a gene that codes for the production of K+ ions, allowing the survival of halobacteria [5]. In non-desiccated regions, the function of this operon is not necessary. A knockout strain of the wild-type halobacteria, whose kdpFABCQ operon was nonfunctional, was exposed to a desiccated environment. he strain yielded a viable cell count 110 times less than that of the wild-type at the end of the desiccation period, conirming that in order for halobacteria to survive within halite crystals, the kdpFABCQ operon must be completely functional in order to create enough K+ ions for cell survival. Gamma Irradiation Kottemann et al. exposed halobacteria wild-type NRC-1 to high levels of gamma irradiation (up to 7.5 kGy, many times more than humans can withstand). In the presence of high gamma radiation, as also seen with desiccation exposure, wild-type NRC-1 experienced double-stranded breaks in their DNA, which were repaired after 48 hours of exposure. When subjected to the highest level of gamma radiation (7.5 kGy), NRC-1 did not yield a viable cell count. However, even with medium levels of gamma radiation exposure (2.5-4 kGy), NRC-1 managed to show resilience and maintained a 25% viable cell count [1]. From these results, Kottemann concluded that desiccation resistance and gamma irradiation resistance are related since NRC-1 can repair breaks in DNA caused by both stressors very eiciently. Furthermore, the natural pigmentation of NRC-1 and its habit of hiding within salt crystals, as seen previously, allows the bacteria extra protection from gamma irradiation. his experiment ofers insight into resistance of halobacteria to high levels of gamma irradiation. Oxidative Stress Oxidative stress, or an overabundance of toxic oxygen species, is another extreme environment in which halobacteria have been demonstrated to survive. Sharma et al. conducted an experiment on halobacteria wild-type NRC1, dealing with the transcription factor VNG0258H. As the concentration of reactive oxygen species within the surrounding environment increased, the expression of VNG0258H increased, allowing NRC-1 to survive oxidative stress. When the concentration of reactive oxygen speVolume 2 | 2012-2013 | 57
Street Broad Scientific cies was decreased, the level of expression of VNG0258H decreased as well, as is illustrated in Figure 1 [2].
REviEw growth. Researchers found that 1.5 M NaCl, 35° C, and a pH of 8.0 were the optimal conditions for haloarchaea in the removal of chromium waste, for it was at these levels that the maximum chromate removal was achieved, resulting in a inal concentration of chromate ions well below 0.04 mM. herefore, halophiles are very applicable to the ield of biohazard waste management, especially in regards to chromium waste.
Conclusion
Figure 1. his igure shows the relationship between oxygen species level over time and the expression of VNG0258H and aerobic and anaerobic genes. In both cases of high oxygen levels, VNG0258H had the highest levels of regulatory gene expression. From this, Sharma concluded that there was a relationship between VNG258H expression and NRC-1 resistance to oxidative stress. For further conirmation, Sharma created a knockout strain of NRC-1, without functioning VNG0258H transcription factors, and placed it in varying levels of oxidative species concentration. As reactive oxygen species concentration increased, the survival of the knockout bacteria decreased, conirming the necessity of VNG0258H in the regulation of oxidative stress. Microgravity Dronmayr-Pfafenhuemer et al. explored the response of halobacteria to simulated microgravity (gravitational force that is 100 times weaker than that of Earth’s) [3]. he survival of of the halobacteria species, Haloferax mediterranei, was tested in simulated microgravity with exposure to antibiotics. When placed in simulated microgravity, Haloferax mediterranei maintained a reasonable cell density after 6 days of exposure to the antibiotics. However, when exposed to normal Earth gravity, Haloferax mediterranei was only able to survive for a maximum of approximately 48 hours when exposed to the antibiotics. herefore, Dronmayr-Pfafenhuemer concluded that, when subjected to microgravity, the resistance of halobacteria to antibiotics and other environmental stresses increases.
Waste Management In the future, humans could take advantage of the ability of halobacteria to survive extreme environments, particularly in their waste removal capabilities. Amoozegar et al. explored the application of the response of a particular haloarchaea strain to toxic chromium waste [6]. Chromium creates very toxic saline waste, and since the waste has a high salt concentration, it is ideal for haloarchaea 58 | 2012-2013 | Volume 2
In conclusion, halobacteria are very complex organisms that are able to survive a wide range of environmental stressors, such as desiccation, high gamma irradiation, and microgravity. hese versatile organisms have a variety of mechanisms with which it adapts to stressful biological environments. Not only can these bacteria withstand extreme conditions, but their response mechanisms could also allow for their use in waste management. Further research includes the application of halobacteria to desiccated environments characterized by subzero temperatures, as well as environments not present on Earth, as seen on other planets. It is likely that other unique biological responses of halobacteria will be discovered and, therefore, may provide an untapped natural resource that could be put to work to beneit our society and environment.
References [1] Kotteman, M., Kish, A., Iloanusi, C., Bjork, S., & DiRuggiero, J. (2005). Physiological responses of the halophilic archaeon halobacterium sp. strain nrc1 to desiccation and gamma irradiation.Extremophiles, 9(3), 219-227. [2] Sharma K, Gillum N, Boyd JL, Schmid AK. (2012). he RosR transcription factor is required for gene expression dynamics in response to extreme oxidative stress in a hypersaline-adapted archaeon. BMC Genomics, 13,351-367. [3] Dornmayr-Pfafenhuemer M, Legat A, Schwimbersky K, Fendrihan S, Stan-Lotter H. (2011). Responses of Haloarchaea to Simulated Microgravity. Astrobiology, 11(3), 199–205. [4] Potts M. (2001). Dessication tolerance: a simple process?. TRENDS in Microbiology, 9(11), 553-559. [5] Kixmuller D, Greie JG. (2012). An ATP-driven potassium pimp promotes long-term survival of Halobacterium salinarum within salt crystals. Environmental Microbiology Reports, 4(2), 234, 241. [6] Amoozegar MA, Ghasemi A, Razavi MR. (2007). Evaluation of hexavalent chromium reduction by chromateresistantmoderately halophile.