Is it safe for everyone to take a hot spring bath in the same pool in winter?

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

As winter quietly approaches, hot springs have become the first choice for many people to relax. Soaking in a steaming hot spring pool, you can enjoy the cozy feeling after the cold is dispelled. However, are you curious, is it really completely safe to bathe in a shared hot spring pool?

Hot springs are formed by high-temperature water that naturally gushes out from underground. Although high-temperature environments can indeed kill most microorganisms, some special microorganisms have the ability to adapt to high temperatures and can even survive and reproduce in such environments.

In recent years, scientists have conducted extensive research on hot springs and the microbial communities in hot springs. For example, hot springs in Yellowstone National Park in the United States and geothermal areas in New Zealand have been thoroughly investigated to reveal their microbial diversity. Similarly, in my country, scientists have also conducted systematic research on hot springs in Yunnan, Tibet, Hainan and other regions, providing us with valuable data for a more comprehensive understanding of hot spring ecology.

Hot springs in different geographic locations have unique microbial communities

The microbial structure and diversity of hot springs are deeply influenced by physical and chemical properties such as temperature and pH.

The three major hot springs in Haikou area - Rongyu Hot Spring on Haidian Island, Happy Farm Hot Spring on the Crater and Haichangliu Hot Spring on the West Coast - are all alkaline water environments. Studies have shown that physical and chemical factors such as temperature, pH, phosphorus, nitrate nitrogen and magnesium have significant effects on fungal and bacterial communities in Haikou hot springs. Different combinations and levels of these factors jointly shape the structure and diversity of hot spring microbial communities, providing important clues for our understanding of hot spring ecosystems.

Physicochemical properties of different hot springs in Haidian Island, Haikou, Hainan

(T: temperature; TP: total phosphorus; NN: nitrate nitrogen; Ca: calcium; Mg: magnesium; Se: selenium; Cd: cadmium; Pb: lead)

(Image source: Reference 1)

The number of fungal and bacterial communities in three different hot springs in Haikou area

(Image source: Reference 1)

Some thermophilic microorganisms have shown amazing adaptability to survive in different geothermal areas. In 2021, researchers from the University of Auckland in New Zealand conducted an in-depth analysis of water samples from six hot spring pools in the Taupo Volcanic Zone and found that a microorganism called Acidithiobacillus is widely distributed in various geothermal environments. The pH of these hot springs ranges from an extremely acidic 1.00 to a neutral 7.50, and the temperature span is even more amazing, from 17.5°C to 92.9°C, showing that this microorganism is highly adaptable to extreme environments.

pH, temperature ranges, and water chemistry of 6 geothermal environments

(Image source: Reference 2)

Why can they survive in hot springs?

By analyzing the genome sequence of Acidithiobacillus, the researchers discovered the genomic adaptability of some thermophilic microorganisms in high temperature environments.

1. High GC content: We know that the genomes of all organisms are composed of nucleotides, including adenine (A), guanine (G), cytosine (C), and thymine (T). Purines and pyrimidines are connected by hydrogen bonds. The genome of Acidithiobacillus has a high GC content. GC base pairs are more stable than the two hydrogen bonds of AT base pairs due to their three hydrogen bonds, which makes GC-rich DNA more stable at high temperatures.

2. High proline content: The proteins of organisms are composed of 20 kinds of amino acids, among which proline is an amino acid that can increase the thermal stability of proteins because it can reduce the conformational freedom of the protein backbone. Simply put, if the protein is compared to a chain that can bend freely, then ordinary amino acids are like flexible links, and proline is like a rigid link in the chain, which limits the bending of the chain in certain directions. This restriction helps the protein form a specific three-dimensional structure. Therefore, this characteristic may help Acidithiobacillus maintain the structure and function of its proteins in high temperature environments.

3. Genome simplification: The study found that the average genome size of Taiwan hot spring strains is only 2.1±0.3Mbp, which is significantly smaller than the Acidithiobacillus reference genome (3.3±0.4Mbp), and there is a phenomenon of genome simplification. Prokaryotes that adapt to high temperatures have a phenomenon of genome simplification, which helps microorganisms to more efficiently utilize the resources needed for genome replication in resource-limited environments, thereby improving their adaptability in high temperature environments.

4. pH homeostasis mechanism: Acidithiobacillus has a variety of enzymes and proteins related to pH homeostasis, such as amino acid decarboxylase, Na+/H+ antiporter, and proton pump ATPase. Amino acid decarboxylase catalyzes the decarboxylation of amino acids to produce carbon dioxide and the corresponding amines. This process consumes hydrogen ions in the cell, thereby helping to reduce the acidity in the cell. Na+/H+ antiporter regulates pH by exchanging sodium ions in the cell with protons outside the cell. Proton pump ATPase provides energy by hydrolyzing ATP to pump protons from the cell to the outside of the cell, thereby directly reducing the acidity in the cell.

Is it safe to take a hot spring bath in winter?

From a microbial perspective, current research has not found that thermophiles are directly harmful to the human body, so from the perspective of thermophiles alone, hot springs are generally safe. Thermophiles are a type of extreme microorganisms that are mainly adapted to high temperature environments, and the water temperature of hot springs is usually between 30-40°C, which is far lower than the optimal growth temperature of thermophiles (usually above 50°C). Therefore, the activity of thermophiles under common hot spring conditions is limited to a certain extent.

However, the sanitation of hot spring water is key to safety. If the hot spring water source or the water quality in the pool is polluted, some pathogens may grow, such as the common E. coli or Legionella. Although the normal water temperature of hot springs (30-45℃) has a certain inhibitory effect on some pathogens, it is not enough to completely kill high-temperature resistant microorganisms. These pathogens can survive at relatively low temperatures and may pose a threat to human health, especially in open wounds or when immunity is low.

Therefore, when choosing a hot spring, you should give priority to places with strict water quality management and regular testing. Even so, you should avoid completely submerging your head in the water when soaking in a hot spring, and wash your body promptly after soaking in the hot spring to reduce the risk of potential infection . Hot springs are a healthy way to relax in winter, but you can enjoy this pleasure more safely if you understand their hygiene management status and take appropriate protective measures.

In addition to high temperature resistance, what other extreme microorganisms are there?

Extremophiles are microorganisms that can grow in environments that are intolerable or even fatal to other terrestrial organisms. They can survive in a variety of extreme ecological niches, including extreme heat, extreme cold, high salt, strong acid and alkali, high pressure and radiation. Some microorganisms can even grow in environments that were previously considered unsuitable for life, such as toxic waste, organic solvents, and heavy metals.

The diversity of Earth's extreme environments

(Image source: Reference 3)

1. Temperature

The temperature range on the Earth's surface is extremely wide, spanning extreme hot and cold environments, from the extremely cold regions of Antarctica (-98.6°C) to deep-sea hydrothermal vents (495°C).

The most cold-resistant microorganism known so far is Deinococcus geothermalis DSM 11300, which can survive in an environment of -25°C, and the most heat-resistant microorganism is Geogemma barossii strain 121, which can survive in an environment of 130°C. In addition, some hyperthermophiles that usually grow at high temperatures above 80°C, such as Methanopyrus kandleri strain 116, must also grow under high pressure conditions because high pressure can keep water liquid at higher temperatures.

2. Acidity and Alkalinity

Extreme pH environments also pose a huge challenge to microbial survival. The lowest known pH record is from Iron Mountain in Shasta County, California (pH 3.6), while the highest pH record is from Lake Gorka in Chrzanow, Poland (pH 13.3). However, no microorganisms have been found to grow in these environments.

The most acid-resistant microorganism known to date is Picrophilus oshimae, which can survive in an acidic environment of pH 0. How acidic is a pH 0 environment? Well, the pH of 98% concentrated sulfuric acid commonly used in industry is close to 0.

The most alkali-resistant microorganism, Serpentinomonas sp. B1, can grow stably in an alkaline environment of pH 12.5, demonstrating the amazing adaptability of microorganisms to extreme environments.

3. Salt

There are many types of salt environments on Earth, ranging from oceans (salinity of about 3-4%) to hot springs (salinity up to 10.5%) to soda lakes (salinity up to 37.1%), covering a wide range from low salt to high salt. Microorganisms are generally adapted to grow in environments with salinity between 0% and 35%.

Among them, the microorganism that is currently known to tolerate the highest salinity is the Halarsenatibacter silvermanii strain SLAS-1T from Lake Searles in California, USA, and its optimal growth salinity is 35% sodium chloride (NaCl). At room temperature, the saturated solubility of table salt (NaCl) in water is about 36%-37%. 35% NaCl is close to saturation, that is, if you want to know how salty the water is for this super salt-tolerant bacterium to live in, you can try saturated salt water.

4. Pressure

In deep-sea environments, scientists have discovered a variety of barophilic microorganisms that can survive extremely high pressures. The current known record holder is Thermococcus piezophilus, a thermophilic archaeon that can survive pressures up to 125 MPa.

A pressure of 125 MPa is equivalent to a weight of about 1,250 kg per square centimeter, which is similar to a small car pressing on an area the size of a thumb.

5. Radiation

Microorganisms are exposed to radiation including ultraviolet rays, X-rays, gamma rays, etc. Radiation damages microbial cells through direct or indirect effects (such as the generation of reactive oxygen species), damaging DNA, proteins, lipids and RNA. Radiation-resistant microorganisms, such as the radiation-resistant thermophilic archaeon Thermococcus gammatolerans EJ3, can resist gamma rays up to 30 kGy.

Life limits determined by pure culture of extremophiles

(Image source: Reference 3)

Summarize

Extreme microorganisms demonstrate the ability of life to adapt to extreme environments, from high-temperature hot springs to deep-sea high pressure. Through their unique genomic characteristics and metabolic mechanisms, they can survive in environments that are unbearable for ordinary people.

Although there are some thermophilic microorganisms in hot springs, they usually do not pose a threat to the human body at common hot spring water temperatures. The safety of hot springs mainly depends on water quality management, so it is important to choose a hot spring venue with good sanitation. Taking appropriate protective measures can greatly reduce potential health risks.

(Note: Latin parts in the text should be italicized)

References:

1. Shu Wei, Tian Xiaoyu & Zhao Hongwei. (2020). Analysis of fungal and bacterial diversity and environmental influencing factors in hot springs in Haikou, Hainan. Journal of Microbiology (09), 262+2000-2011. doi:10.13343/j.cnki.wsxb.20200145.

2. Sriaporn, C., Campbell, KA, Van Kranendonk, MJ, & Handley, KM (2021). Genomic adaptations enabling Acidithiobacillus distribution across wide-ranging hot spring temperatures and pHs. Microbiome, 9(1), 135.

3. Merino, N., Aronson, HS, Bojanova, DP, Feyhl-Buska, J., Wong, ML, Zhang, S., & Giovannelli, D. (2019). Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context. Frontiers in microbiology, 10, 780.