Alive in the Dead Sea

Shoreline of the Dead Sea 

Microbes exist everywhere in the world, even in places that seem uninhabitable. Even after you use the hand sanitizer that’s on your desk to clean your hands, some microbes find ways to survive. How do microbes do that? How do they survive in places and situations in which we cannot imagine life being? To better understand how it’s possible, we need to learn more about the lives of microbes.

Microbial life belongs in all three kingdoms of life, bacteria such as E. coli that is found in our gastrointestinal tracts, archaea that are normally found in more extreme parts of the planets such as underwater sea vents, and eukaryotes such as yeasts. Though these categories help us differentiate the different organisms we find, they all have commonalties.  Life requires nutrition, and in order to utilize nutrients they need a way to break them down or metabolize them. Metabolism is a certain set of chemical transformations that sustain life in cells. This done by converting food/fuel/nutrients into energy to drive and run cellular processes. Bacteria that are normally used as study subjects in labs, such as E. coli, uses carbon that is broken down from glucose molecules that are ingested by a host. Yeasts will also use sugars for nutrients. There are some microbes that have been found to use other sources for nutrients, such as heavy metals and sulfates.

Environments normally dictate what nutrients will be available for microbes to use, so under certain circumstances, microbes have to make do with what is available. An example of this is the Dead Sea (Pictured above), one of the saltiest bodies of water in the world. It has a salinity of around 342 g/kg (grams of minerals per kilogram of water) or 34.2%, which is nearly 10 times as salty as ocean water. Because of the high salt (various minerals such as sodium, chloride, magnesium, and sulfur) content, there are no macroscopic organisms such as aquatic animals or plants present in the waters. This is often associated with the name “Dead Sea”. The sea is fed with underwater vents or springs which have been found to be the home to some microbes, mostly bacteria with some archaea (Figure 1) (Ionescu et al, 2012). There are different ideas on how these microbes got to these springs, but it is understood how they survive in these harsh conditions. So that does lead to the question, what nutrients exist around these vents, and how do these microbes utilize them?

Figure 1: The spring system found in the Dead Sea is shown above.

Current research has found that the microbes found in the springs of the Dead Sea actually metabolize the minerals and metals that are found in the waters, such as sulfate and iron. Organisms that are able to oxidize sulfur or iron are called chemolithotrophs. Chemolithotrophs are given this name due to their ability to acquire energy from oxidation of inorganic compounds, known as electron donors. They are not the only microbes known to do this, there are many bacteria and archaea in hydrothermal vents found in the depths of the ocean. Acidithiobacillus is a genus of bacteria found in deep sea vents that is known to reduce iron, which is how they get energy (Figure 2) (Valdes et al, 2008). When the bacteria has oxidized the minerals and metals, some are useful. In Figure 2 D, the effects of bleaching for byproducts can be seen. Like the bacterial colonies found in hydrothermal deep sea vents, those in the Dead Sea springs are found in biofilms. A biofilm is comprised of bacteria that stick together, making it easier for them to convert nutrients into energy and therefore survive. Bacteria in biofilms can be considered more tenacious because they work together as community of cells rather than just a single cell (Donlan, 2002). These microbes have found their way to these springs one way or another, and because the amount of work that it takes to metabolize what they have available, they do not thrive. Since there is not much more down there but them, they have evolved ways to metabolize the minerals they grow around.

   

Figure 2: A) The figure above shows how a single cell of biofilm uses minerals surrounding it for energy, it is metabolizing the FEII, as well as the sulfates. B) The metabolic cycles in aerobic and anaerobic conditions. C) Water contaminated with FEII and other minerals from acid mine runoffs. D) The effcts of bleaching to obtain minerals and metals. (Valdes et al, 2008).

The electron transport chain in humans, the process that creates a proton motor force to start the production of a cell’s energy ATP, is started with the reduction of NADH. NADH is an electron donor that is produced from glycolysis which is the metabolic pathway that converts glucose. The use of minerals such as iron for energy sounds much different than the use of sugars that we use, but there are some similarities. FEII is oxidized to FEIII on the outer membrane of Cytochrome C, an electron carrier, so an electron is taken. The electron is moved to a complex named Rusticyanin, which shuttles the electron into the electron transport chain (Ehrenreich, 1994). If you are still interested in how ferrous iron is used for energy or want to get a better understanding on how the process all works, check out this video from Shomu’s Biology.

There are colonies of these iron-reducing bacteria found in the northern spring systems in the Dead Sea, but there are bacteria colonies with a different appetite in the southern systems. The Dead Sea has a higher sulfur content than it does iron, so it makes sense that the south spring systems are abundant with sulfur oxidizing bacteria, such as Epsilonproteobacteria. It may make sense that different bacteria are found in the north and south springs due to the mineral compositions. Interestingly, biofilms that are found in neighboring springs are composed of different bacterial species (Figure 3). This shows that microbes in these underwater springs cannot leave their home spring, due to harsh environmental factors and lack of available nutrients (Ionescu et al, 2012).

                                            

Microscopic image of Epsilonproteobacteria

Figure 2: The above map shows the different spring systems discussed. Springs 10-12 are the southern springs, while springs 1-5 are northern (Ionescu et al, 2012).

Metabolism is crucial for all organisms, and as seen, organisms will find anyway to complete the process. Bacteria have found many useful nutrients to keep their chemical reactions running and will continue to do so. If they can feed on metal, one may ask if there is any limit to what they can eat.

References

Donlan, R. 2002. Biofilms: Microbial Life on Surfaces. Emerging Infectious Diseases. 8:881-890

Ehrenreich, A. Widdel, F. (1994). Anaerobic Oxidation of Ferrous Iron by Purple Bacteria, a New Type of Phototrophic Metabolism. Applied and Environmental Microbiology. 60:4517-4526

Ionescu, D. Siebert. C Polerecky, L. Munwes, YY. Lott C, et al. (2012) Microbial and Chemical Characterization of Underwater Fresh Water Springs in the Dead Sea. PLOS ONE 7(6): e38319. https://doi.org/10.1371/journal.pone.0038319

Valdés, J., Pedroso, I., Quatrini, R., Dodson, R., Tettelin, H., & Blake, R. et al. (2008). Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications. BMC Genomics, 9(1), 597. http://dx.doi.org/10.1186/1471-2164-9-597

Picture Sources

http://alittleadrift.com/visit-jordan-dead-sea/

https://fr.wikipedia.org/wiki/Epsilonproteobacteria

Ionescu, D. Siebert. C Polerecky, L. Munwes, YY. Lott C, et al. (2012) Microbial and Chemical Characterization of Underwater Fresh Water Springs in the Dead Sea. PLOS ONE 7(6): e38319. https://doi.org/10.1371/journal.pone.0038319

Valdés, J., Pedroso, I., Quatrini, R., Dodson, R., Tettelin, H., & Blake, R. et al. (2008). Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications. BMC Genomics, 9(1), 597. http://dx.doi.org/10.1186/1471-2164-9-597