From Mars to Minas: Research Reclassifies Organisms That Thrive In Highly Saline Environments

[JORNAL DA USP – Universidade de São Paulo] Organisms that defy life by thriving in environments where most other living beings could not survive. To date, there is no scientific consensus regarding the criteria used to classify the various categories of extremophiles—organisms capable of surviving and growing under extreme physical or chemical conditions.

[Portuguese Version]

Seeking to overcome this limitation, molecular scientist Ana Paula Schiavo proposed a new classification system for halophiles, a subgroup of extremophiles that live in environments with high concentrations of sodium chloride, the most abundant salt on Earth.

This pioneering proposal to unify and standardize the classification of halophiles is one of the outcomes of the doctoral dissertation defended by the researcher at USP’s Institute of Chemistry (IQ), with funding from the National Council for Scientific and Technological Development (CNPq).

“It is a bold proposal that has the potential to become a milestone in science”, said microbiologist Rubens Tadeu Duarte, professor at the Federal University of Santa Catarina (UFSC). One of Brazil’s leading experts in the field, Duarte was not involved in the study.

Ana Paula Schiavo’s approach may also prove relevant to astrobiology, the field that investigates the origin, evolution, and distribution of life in the Universe. Mars, for example, is believed to contain subsurface saltwater lakes as well as intermittent surface brines near the equatorial regions during the summer.

A unified classification system based on objective criteria could facilitate communication between specialists studying halophiles, which require hypersaline conditions, and halotolerant organisms, which are capable of surviving under such conditions. Their ability to adapt to highly saline environments is being explored to better understand the limits of life in the Martian environment.

No consensus

The planet Mars has a surface exposed to intense ultraviolet radiation and high concentrations of salts that are harmful to life. Seeking to understand how microbial life might withstand such a hostile environment, the researcher chose to devote her doctoral research to the study of extremophiles. She conducted her search for species capable of simultaneously resisting ultraviolet radiation and hypersalinity in the iron-rich region of Diamantina, in the Brazilian state of Minas Gerais—an environment considered analogous to that of Mars.

In the course of her research, she encountered an unusual situation: the criteria used to classify extremophiles are largely arbitrary, and there is no consensus on an objective classification system. This lack of agreement within the scientific community regarding objective criteria has led to the coexistence of multiple classification systems, creating unnecessary communication barriers among specialists.

“The classification systems found in the extremophile literature fall short in terms of rigor. The situation is chaotic”, said Ana Paula Schiavo. She pointed out that there is not even agreement on what defines an extremophile. Nor are there clear boundaries separating the different categories of extremophiles, such as thermophiles and hyperthermophiles (which thrive at high temperatures), or acidophiles (which inhabit acidic environments) and alkaliphiles (which thrive in alkaline environments). “Different researchers classify different things”, summarized Rubens Duarte.

The researcher also found that many studies did not even measure the optimum growth point of the species correctly—that is, the conditions under which they exhibit their greatest growth potential given the available resources. This led her to compile a survey of 1,298 species described as halophilic or halotolerant in articles published between 1967 and 2021 in one of the leading microbiology journals, the International Journal of Systematic and Evolutionary Microbiology.

The survey revealed a recurring problem in the scientific literature: the conventional distinctions between “slight,” “moderate,” and “extreme” halophily are usually based on ranges of salt concentration in the environment inhabited by extremophiles, without any justification for the thresholds used to define these categories, resulting in inconsistent classifications.

“Under current classification systems, the minimum salt concentration required for organisms to be considered slightly halophilic is lower than the average salinity of seawater”, the researcher explained. She highlighted the inconsistency of this conclusion and asked: “How can the oceans, the largest biomes on Earth, be considered extreme environments?”

Statistical rarity

Ana Paula Schiavo’s research proposes to solve this problem by considering the statistical rarity of species found in nature, dividing organisms adapted to hypersaline conditions according to quartile metrics, a widely used statistical approach. The classification criterion is the species’ optimum growth across increasing salinity ranges.

The first and most significant boundary established by the study is the concentration of sodium chloride found in Earth’s oceans. Below this threshold are non-halophiles and organisms that can be classified as halotolerant (those capable of surviving at salt concentrations higher than those of seawater but whose optimum growth occurs at concentrations equal to or lower than that level). Above this threshold lies the second quartile, which includes slightly halophilic microorganisms. The third and fourth quartiles correspond, respectively, to moderately and extremely halophilic species.

“The novel contribution of this study is that it moves beyond historically used classification systems by proposing a classification based on analytical criteria”, said Fabio Rodrigues. The article is also co-authored by biologist Roberta Vincenzi, a postdoctoral researcher at USP’s Oceanographic Institute (IO).

Based on organisms collected in Minas Gerais, Ana Paula Schiavo’s research sought to standardize knowledge about microorganisms exposed to conditions similar to those found on Mars. The surface of the Red Planet is highly inhospitable to life as we know it. Exposed to extreme levels of ultraviolet radiation, the Martian regolith contains high concentrations of salts harmful to biological systems, such as magnesium and sodium perchlorates, as well as significant amounts of sodium chloride.

In her investigation of how resilient unicellular organisms can be under multiple extreme conditions, the researcher argues that if life ever existed on Mars, it may have persisted underground in aquifers that remain liquid because of the high abundance of salts, which lower water’s freezing point.

In addition to the doctoral dissertation, the new classification system has resulted in a preprint article entitled Proposal for a New Halophile Classification System Based on the Statistical Rarity Definition of Extremophiles. The manuscript is currently available while undergoing peer review at one of the leading journals in the field, Extremophiles.

*Danilo Albergaria is a Media Science Fellow at the São Paulo Research Foundation (Fapesp). Adapted for Jornal da USP

**Intern under the supervision of Simone Gomes

English version: Nexus Traduções, edited by Denis Pacheco

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