We humans enjoy living in a comfortable environment, not too cold, not too warm. This we define as "normal" and anything that deviates from it we define as "extreme". Nonetheless, there are many organisms that thrive under conditions we would just experience a quick death. There is evidence that the first organisms that appeared on our Earth, which were rather prokaryotic (that is cells that lack organised nucleus and their chromosome is "floating" around their cytoplasm), flourished under environments we would consider to be hellish [Matthew S. Dodd et al., Evidence for early life in Earth’s oldest hydrothermal vent precipitates, Nature 543, 60–64 (2017)]. Compared to them, we are living in an "extreme" environment of very low temperatures filled with the very reactive, highly oxidative and dangerous oxygen. Nevertheless, there is an astonishing variety of prokaryotic organisms that can survive in "extreme" environments and these are called "extremophiles". A famous example of an extremophile, is the halophilic Proteobacterium GFAJ-1, which is a resident of the Mono Lake in California and thrives under extreme concentrations of arsenic. The story that surrounds its discovery is a textbook example (just my two pennies), of how the scientific methodology works to, eventually, find the truth about the mechanisms of life. This story started with a group of researchers claiming that this bacterium uses arsenate (AsO43-) instead of phosphate (PO43-) to build its macromolecules [Wolfe-Simon, F. et al., A bacterium that can grow by using arsenic instead of phosphorus. Science 332, 1163–1166 (2010)]. However, soon other researchers realised that GFAJ-1 has actually evolved mechanisms to absorb even the tiniest amounts of (PO43-) from its environment and survive [Elias, M. et al., The molecular basis of phosphate discrimination in arsenate-rich environments. Nature 491, 134–137 (2012)]. Anyway, to focus on the title of this post, there is a group of extremophile archaea (organisms that resemble both Bacteria and Eukaryotic cells in different aspects of their biology), that live in pods with extremely high concentrations of salt and are,amazingly, square!
These remarkably square-shaped cells were discovered by Professor Anthony Edward Walsby in 1980 in a brine pool on the Sinai Peninsula and they were named after him, Haloquadratum walsbyi. The study of H. walsbyi was hampered for many years, while researchers were trying to figure out a way to grow it in the laboratory. Finally the magic recipe was announced in 2004 [Bolhuis, H. et al., Isolation and cultivation of Walsby’s square archaeon. Environ Microbiol 6, 1287–1291.(2004), Burns D. G., et al., Cultivation of Walsby’s square haloarchaeon. FEMS Microbiol Lett 238, 469–473.(2004)] paving the way for their study. The genome of H. walsbyi was published 2 years later [Bolhuis H.,The genome of the square archaeon Haloquadratum walsbyi : life at the limits of water activity. BMC Genomics. Jul 4;7:169 (2006)] revealing some of their quirky properties and components, like the huge secretory protein (9159 amino acids!!), Halomucin (UniProt Code: Q18DN4). H. walsbyi uses this protein to protect itself from desiccation stress, caused by the very low availability of free water due to the extreme concentrations of salt (NaCl) and MgCl2 found in the ponds where it lives.
The peculiar morphology of these cells most probably arose from their need to have a large surface-to-volume ratio, so they can absorb easily the scarce free nutrients from their environment. This pressure led them to become square and extremely thin, achieving the one of the highest surface-to-volume ratios. Their flat and square shape is very intriguing. It would be very interesting to determine the exact molecular mechanism that helps them maintain this shape. Furthermore determining the molecular details of the mechanism of their division could be another very interesting research topic. However, these endeavours would not be trivial as this organism is not studied extensively and basic molecular tools (e.g. methods to transform H. walsbyi cells with engineered genes) that are taken for granted for other microorganisms, such as Escherichia coli, are just lacking. Nevertheless, studying the peculiarities can always lead to stunning discoveries.