I am biased, but I am fascinated by the microbial world and some of the more unusual environments that some microbes can survive in, by this I don’t mean ‘the ocean’ but more of the extreme places that these microbes survive and often thrive.

In this post, I wanted to explore some of these microbes, where they can survive, and in some cases how they are actually managing to do it given all the environmental stresses.

As with most of my posts, this article is a combination of my ramblings and understandings of the topic, it should be recognized that this is a huge and evolving area that people spend their lives researching and understanding, so at best I will only be able to touch in the surface of this topic and provide very basic introductory information. In many ways, this post is about cementing my knowledge (as the best way to confirm what you know is to share with others).

So what are Extremophiles and Extremotolerant Microbes?

In microbiology we use two terms to describe microbes that can survive in extreme environments, these are an extremophile which can be considered as a microorganism that thrives under extreme environments and extremotolerant microbes which don’t thrive under extreme conditions but simply survive.

In this post, I am largely focussing on extremophiles rather than some of the tolerant species.

What are examples of Extreme conditions?

There are many environments that can be extreme, and some are more easily recognized than others. Extreme environments can include extreme temperatures, but can also include high salt environments.

In addition to thriving in one extreme environment, an organism can also be a polyextremophile meaning that it can thrive in multiple extreme environments (i.e. a thermoacidophile can grow in hot and acidic environments).

A list of some of the extreme conditions, and the name we apply to organisms that grow in those conditions is provided below:

• Halophiles – grow in high salt conditions.
• Thermophiles – grow in temperatures between 41 – 122 Deg C.
• Acidophiles – grow under highly acidic conditions (pH 5.0 and below).
• Psychrophiles – grow in low temperatures ranging from -20 to 20 degrees C.
• Alkaliphile – grow in alkaline environments with a pH from 8.5 – 11.
• Piezophiles – have a maximum growth rate at high pressures (above 10 MPa).
• Xerophiles – grow in conditions with a low availability of water (water activity – Aw). Water activity measures humidity above a substance relative to humidity above pure water (Aw = 1). Xerophiles can survive with water activities below 0.8.
• Hyperthermophiles – grow in temperatures from 60 Deg C upwards (many above 100 Deg C).
• Oligotrophs – grow in an environment with very low levels of nutrients.
• Radioresistance – capable of surviving where doses of acute ionising radiation needed to achieve 90% reduction are greater than 1000 gray (Gy).
• Osmophile – grows in environments with high osmotic pressures (for example high sugar concentrations).
• Metallotolerant -grows in environments with a high concentration of heavy metals.
• Capnophile – grows in the presence of high concentrations of CO2.
• Thermoacidophile – both thermophillic and acidophillic. These are given their own category as it is unusual due to the adaptions required for an organism to be both thermophillic and acidophillic.

Examples of Microbes that can survive in Extreme conditions, and how they Survive?

Extremophiles are fascinating microbes as they have often developed very specialised systems to allow them to survive in extreme conditions and environments as discussed above.

A great example of Extremophiles are the microorganisms that grow in the Grand Prismatic Spring in Yellowstone National Park. The temperature of this spring is around 70 Deg C, these are thought to include Cyanobacteria (Calothrix spp, Synechoccus spp, Phormidium spp), and other bacteria (Chloroflexus spp, Thermus aquaticus).

These many microorganisms are pigmented with different colours and produce microbial mats which result in the colours of the spring. Whilst a full rundown of how these microorganisms survive is outside of the scope of this post, thermophiles as an example have adapted with increased stability of their proteins and cell membranes, and adaptation of genetic material to better withstand higher temperatures.

Another example of an extremophile is Methanopyrus kandleri which is a heat loving and salt loving species of Archea which was discovered in a volcanic vent in the ocean. The temperature in these environments is typically above 70 Deg C and there is a high osmotic concentration.

Although there are many more Extremophiles, I wanted to give another example of a different type. In this case the bacterial species Deinococcus radiodurans which is a polyextremophile (and is listed in the Guinness book of records as the toughest bacteria).

The species Deinococcus radiodurans is an extremophile that can withstand drought conditions, dehydration, vacuum, acid and most importantly radiation levels much higher than other bacterial species. For context, an X-Ray leads to a dose of about 1mGy (1/1000 of a Gy), radiation levels of around 5 Gy can kill a human, 200-800 Gy can kill E.Coli, 4000 Gy will kill the radiation resistant tardigrade yet this extremophile can withstand acute radiation levels of 5000 Gy with no loss in viability, or acute doses of up to 15,000 Gy with a 37% viability (which is crazy).

Again I won’t go into all of the adaptations, but this microbe has developed mechanisms of radiation resistance which include having multiple copies of its genome, and rapid DNA repair mechanisms.

The importance of Extremophiles?

Extremophiles are important for many reasons, one of which is for research and industrial applications. Because Extremophiles secrete enzymes out of the cell, and these enzymes are often more stable (to things like temperature, pH, salinity) it makes the enzymes more useful in industrial processes. Some of these enzymes we use across multiple industries from food production, forensics, to medicines.

Extremophiles can also be used in an environmental context, some microbes are being suggested as potential options for remediating and treating heavy metal as well as organic pollutants. A study identified that a Geobacillus SH-1 strain (isolated from a deep oil well) was able to completely decompose hydrocarbon n-alkanes (C12 – C21) in 8 days.

Extremophiles are incredible due to the harsh environments in which they can survive and often thrive, from high temperatures, to the acid mine environments. Survival in these extreme environments does require many adaptations, but, It goes to show that life can often find a way to survive in the most extreme of environments, and gives us a greater understanding of the bounds of life.