Microbes come in many shapes and sizes, appearing as spheres, rods, and spirals under the microscope. Aside from these more commonly found microbial shapes, many microbes take on a more unique appearance. One of these microbes is the square-shaped Haloquadratum walsbyi, which was discovered in 1980 by A. E. Walsby. It became known as the “square bacterium,” but it turns out that this microbe was not a bacterium at all.
Not a Bacterium, But an Archeaon
Walsby stumbled upon Haloquadratum walsbyi because he was searching for bacteria containing gas vacuoles in saline pools. At the time, it was thought that gas-vacuole containing bacteria were found in all lakes. Walsby identified H. walsbyi from a coastal hypersaline pool near the Red Sea.
“These square bacteria are so thin and transparent and are so unlike any bacteria previously described that I would have overlooked them if they had not possessed gas vacuoles, and I had not been looking for different forms of gas-vacuolate organisms,” writes Walsby in his paper A Square Bacterium.
At the time of Walsby’s discovery, the classification of microorganisms was different from what we know today. While the three domains of life now include bacteria and archaea, it was just three years prior to Walsby’s discovery that Carl Woese and George Fox used ribosomal RNA sequencing to classify archaea separately from bacteria. H. walsbyi actually belongs to a group of microbes formerly called halobacteria that are now referred to as haloarchaea.
First Grown in the Lab Twenty Four Years After Discovery
It took scientists 24 years after its discovery to cultivate Haloquadratum in the lab. In fact, it was two independent groups of scientists that figured out how to grow it.
As with many slow-growing environmental microbes, the researchers needed to find a way for Haloquadratum to grow more quickly than other microbes in the pond sample. If the sample was grown in media that favored other species, these species would outcompete Haloquadratum and take over the culture. This is particularly important as Haloquadratum grows slowly, taking one to two days to double.
After many rounds of growth and tweaks to the conditions based on Haloquadratum’s affinity for salt and the nutrient needs of other haloarchaea, the researchers were finally able to grow Haloquadratum on its own.
Surviving in Hypersaline Environments
At high salt concentrations, cells are prone to drying out but Haloquadratum produces a giant protein to help it survive. This protein, or halomucin, forms a “capsule” of water around the cell. Aside from halomucin, Haloquadratum has other tricks up its sleeve to protect from desiccation. One of these is the synthesis of a polymer called poly-gamma-glutamate which also helps it maintain its square shape.
Living in hypersaline environments also means that dissolved oxygen and other nutrients are hard to come by. To counteract this, Haloquadratum cells are extremely flat. This increases its surface to volume ratio. The increased surface area means that Haloquadratum has more space for membrane transporters to take in nutrients from the environment. Haloquadratum has a wealth of membrane transporters ranging from the standard haloarchaeal transporters, bacteriorhodopsins that allow it to grow in response to light, and transporters not found in other archaea. It is because of this high surface to volume ratio that Haloquadratum has enough space for membrane components needed for viability and the ability to grow in response to light.
Further Reading
Cultivation of Walsbys square haloarchaeon. FEMS Microbiology Letters. 2004
Featured image: The outline of a Haloquadratum walsbyi cell revealed by polyhydroxybutyrate granules throughout the cell stained by Nile blue. Source: Zenke et al., 2015.
