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Desulfacinum hydrothermale
Scientific classification
Domain:	Bacteria
Phylum:	Thermodesulfobacteriota
Class:	Syntrophobacteria
Order:	Syntrophobacterales
Family:	Syntrophobacteraceae
Genus:	Desulfacinum
Species:	D. hydrothermale
Binomial name
Desulfacinum hydrothermale Sievert and Kuever 2000

Desulfacinum hydrothermale is a thermophilic sulfate-reducing bacterium from the genus Desulfacinum which has been isolated from submarine hydrothermal vents and sediments collected in Milos, Greece

Overview:

Desulfacinum hydrothermale is a thermophilic (heat-loving) sulfate-reducing bacterium (a tiny, single-celled organism) that grows best at high temperatures. It is prevalent in hydrothermal vent ecosystems, which are extreme deep-sea environments where hot, mineral-rich water escapes from the cracks in the ocean floor. The bacterium was first discovered in anoxic (oxygen-free) marine sediments and thrives in acidic, high-temperature, and strictly anaerobic conditions. While many strains exist, strain MT-96T is the most commonly observed and analyzed.

Taxonomy:

Desulfacinum hydrothermale belongs to the group of Syntrophobacteraceae, which includes bacteria that live in partnership with other microbes in places where there's no oxygen. Based on evolutionary relationships, it shares phylogenetic similarity with Desulfacinum infernum, which is found in deep oil reservoirs. A study by Baena et al. (2011) characterized the taxonomy of Desulfacinum hydrothermale and its relationship to similar thermophilic bacteria. In this study, D. hydrothermale was closely related to Desulfosoma caldarium, sharing several traits such as thermophilic growth and sulfate-reducing capabilities. Researchers use a specific type of genetic marker called the 16S ribosomal Ribonucleic Acid (16S rRNA), a gene used to build microbial phylogenies, the researchers found a 93% sequence similarity between D. hydrothermale and D. caldarium, both of which thrive in high-temperature environments where sulfate is abundant.

Discovery:

Desulfacinum hydrothermale was isolated from submarine hydrothermal vents. These vents were located near Milos Island in Greece . In the early 1990s, scientists Silke M. Sievert and Heribert Cypionka co-discovered the bacteria in shallow waters. At the same time, Friedrich Widdel and Friedhelm Bak studied the cultivation conditions (lab-based methods used to grow microorganisms) for sulfate-reducing bacteria, including Desulfacinum hydrothermale in 1992. They cultivated and isolated the bacteria using acetate (a simple organic molecule) and introduced sulfide to establish the anaerobic environment necessary for the bacteria to thrive. These cultures were incubated for approximately three weeks at temperatures ranging from 35 °C to 80 °C in 2 °C to 4 °C intervals to assess optimal growth conditions. Finally, researchers tested growth in a mineral medium, a solution of essential nutrients, and determined that D. hydrothermale could grow in a pH range of 5-9.

Morphology:

Desulfacinum hydrothermale is a Gram-negative, non-spore forming (it does not produce protective spores during harsh conditions), oval-shaped bacterium. According to Sievert and Kuever, the cells of this bacterium typically measure 0.8–1.0 μm in diameter and 1.5–2.5 μm in length. They also observed that the bacteria become less motile. D. hydrothermale grows best in high-temperature environments of up to 64 °C, where oxygen is minimal, sulfide concentrations are high, and the pH is low (acidic).

Genomics:

Genome sequencing

Since its first genetic sequencing (reading the complete DNA code) in 2018, researchers have identified over 3,300 genes in D. hydrothermale. A computer-based analysis known as metabolic reconstruction, which utilizes a database called BioCyc, showed that these genes are organized into approximately 143 metabolic pathways, which are sequences of chemical reactions essential for cell survival and growth. These pathways include an estimated 826 enzymatic reactions, which are chemical processes sped up by special proteins called enzymes.

GC content

The genome of D. hydrothermale is approximately 2.5 megabases (Mb) in size. The bacterium's DNA has a guanine-cytosine (GC) content of approximately 59.5%. This means that 59.5% of its DNA base pairs are made up of the nucleotides guanine (G) and cytosine (C). GC content measures DNA stability and composition; higher levels indicate greater stability. D. hydrothermale was measured using high-performance liquid chromatography (HPLC). This laboratory technique separates and analyzes parts in a mixture, in this case, genomic DNA, which refers to an organism's complete set of genes.

Protein-coding capacity

D. hydrothermale has strong biochemical abilities, allowing it to conduct various chemical reactions. Its genome includes genes for hydrogenase-related proteins, which are enzymes that help the bacterium use hydrogen gas for energy in a process called hydrogen metabolism.

Metabolism

Anaerobic energy production

D. hydrothermale is an anaerobic chemoorganoheterotroph, meaning it lives without oxygen and obtains energy and carbon by breaking down organic compounds. When oxygen is unavailable, it uses sulfate as a terminal electron acceptor, necessary for energy-producing reactions, to produce hydrogen sulfide.

Autotrophy and hydrogen oxidation

This bacterium can also oxidize (chemically react with) hydrogen to produce energy while creating its own organic compounds from carbon dioxide, known as autotrophic hydrogen oxidation.

Environmental chemical cycling

Its ability to survive at high temperatures and in anoxic marine sediments is called metabolic flexibility. This flexibility helps it participate in biogeochemical cycling, the natural movement and transformation of elements like sulfur within an ecosystem.

Environmental Impact

Role in hydrothermal systems

Recognizing the importance of Desulfacinum hydrothermale is essential due to its unique role in deep-sea hydrothermal ecosystems, particularly within geothermally heated marine sediments, where hot fluids from within the Earth alter the seafloor environment. This thermophilic, sulfate-reducing bacterium significantly influences sulfur cycling by converting sulfate into sulfide, creating sulfide-rich fluids typical of hydrothermal systems.

Carbon cycling

According to Galushko and Kuever (2019), D. hydrothermale can degrade a wide range of organic molecules, including long-chain fatty acids, fat molecules found in oils and lipids. This bacterium is a major contributor to the carbon cycle, the process of exchanging carbon between living organisms, the atmosphere, oceans, and the Earth’s crust. It can also grow autotrophically, making its own food using hydrogen and carbon dioxide for energy and carbon dioxide as a carbon source. It has high metabolic flexibility and can survive under extreme conditions using different energy sources.

Industrial and scientific significance

Zeng et al. (2021) note that these characteristics help D. hydrothermale support microbial life in low-oxygen, high-temperature environments, such as hydrothermal vents. This bacterium also has potential for industrial applications, including bioremediation, where microbes clean up pollutants such as sulfate, and bioenergy production, where bacteria help generate usable energy. Its adaptations also offer insight into early Earth environments and support astrobiological research, which includes interest in extraterrestrial hydrothermal systems, which are hot, mineral-rich environments that may exist on other planets or moons.