Walsby's Square Archaea! Haloquadratum walsbyi

Procrastination and overwhelming itch to get back to blogging win over the more pressing obligations tonight. Fuck'em, it's Friday night, I can write about protists if I feel like it. Moreover, I can even write about non-protists, especially those I've been meaning to write about for a month now. Square Archaea!

Despite their awesome morphological diversity, seldom do cells take the shape of a flat square. Or any other flat geometric shape. In fact, there are reasons for this – the cell cytoplasm is generally hypertonic relative to its surroundings (the cell contains more solute), so there is considerable osmotic pressure against the cell membrane, kind of like an inflated balloon. The optimal favoured geometry in this situation is a perfect sphere, so without cytoskeletal or cell membrane alterations, the cell swells up into a ball. Plant mutants deficient in wall cellulose tend to have very bubbly cells, as do plants treated with cellulose-inhibiting drugs, due to turgor pressure from within. Thus, it would take a lot of structural effort for a cell to take on a very flat shape, with a high length x width : height ratio.

Normally. Except when they do just that: brought to us from a very salty pool on the Sinai Peninsula is Walsby's square archaeon, a thin rectangular sheet of cell:

Left: Walsby's square archaeon, Haloquadratum walsbyi. A – phase contrast of archaean with conspicuous gas vesicles; B – TEM detail of gas vesicle; C – darkfield micrograph of large cell – their unusual flexibility means they're seldom found unfolded. (Bolhuis et al. 2004 Env Microbiol) Right: Original images of the square archaean, from Walsby 1980 (Walsby 2005 Tr Microbiol).

These flat ~1.5 x 1.5 µm cells are only 0.1µm thick at most. The cell periphery is lined with conspicuous gas vesicles of undetermined function; they were previously thought to be involved in buoyancy, but experimental data cast doubt on that idea. Instead, they may participate in positioning cells parallel to the surface in order to optimise light exposure for their photoactive pigments (Oren et al. 2006 Saline Syst, OA). They may also play a role in buffering changes in turgor pressure (Walsby 2005). Some cells are unbelievably square, but most tend to be rectangular, due to growth. One can't really divide into squares unless some weird four-way cell division process is employed, so the non-square delinquents have a reason for their geometric imperfections.

Left: Cryo TEM of H.walsbyi, showing prominent gas vesicle in the corner. The scalebar is 1µm. (Burns et al. 2007 IJSEM) Right: Electron tomography of a single H.walsbyi cell. Gas vesicles (GV) line the cell periphery, while electron-dense blobs of acidic polymers fill the inside of the cell. (Bolhuis et al. 2006 BMC Genomics, OA)

Unlike protistologists, bacteriologists are blessed with small genomes, and can thus sequence whatever they like with little pain (relatively). This means they get to sequence all the cool things they want, which is a little unfair. Haloquadratum has been sequenced (Bolhuis et al. 2006 BMC Genomics); there is also unusually high amounts of polymorphisms in ribosomal DNA both within species and within the genomes themselves, thought to be an adaptation to their extreme environments (López-López et al. 2007 J Mol Evol), although the adaptation aspect is to be taken with a grain of salt as it is difficult to distinguish from consequence.

Haloquadratum belongs to a large group of extreme halophilic archae, the Halobacteriales. Obligatory latest tree (note the taxonomic mess, and the swaths of poorly-understood organisms in need of attention):

Phylogeny of salty extremophilic Halobacteriales; as with anything microbial, taxonomic chaos is inevitable. Unusually high rDNA polymorphism within single species doesn't help much either. (Modified from Minegishi et al. 2010 IJSEM (abbreviations deciphered in red))

The salty waters Haloquadratum inhabits are no regular salty waters – the salinity exceeds that of typical seawater by a factor of ten. In other words, really salty. Not only that, but Haloquadratum thrives in Mg-rich waters just below the lethal concentration (past which nothing lives). This extreme salinity means the cells are no longer hypertonic relative to the medium, and in fact may be hypotonic – recall classic experiments involving dipping onion cells in saltwater to show shrinkage. In fact, despite living in aqueous environments, these organisms have very little access to water, and the extremely saline muck is also highly anoxic (low in oxygen). In other words, not the ideal vacation spot for most life.

Presumably, there is considerable selective pressure to optimise for a very different surface-area-to-volume ratio, particularly to enhance gas exchange (otherwise severely hampered by salinity). Thus, not only does the non-hypotonic environment with respect to the cell enable it to easily deviate from its spherical tendencies due to lack of turgor (Walsby 2005), but in fact favours it to expand its surface area by being a flat sheet.

There are also triangular hypersaline Archaea, Haloarcula japonica:

Left: Triangular Haloarcula japonica cells under normal conditions. Right: H.japonica after lowering magnesium concentrations – the cells become rounded "spheroplasts". (Horikoshi et al. 1993 Cell Mol Life Sci, modified therein from Nakamura et al. 1992)

Quite fascinatingly, the decrease of magnesium concentrations in H.japonica cultures results in the cells becoming spherical! A specific glycoprotein appears to be responsible for maintaining H.japonica's triangular shape, and is released upon lowering magnesium levels, allowing the cells to spring back to their natural rounded selves (Horikoshi et al. 1993 Cell Mol Life Sci). This is also a cool case of membrane morphology being predominantly regulated by a single protein, which is not so common. There's a whole area of research based around the shaping of membranes with various proteins...really cool stuff too. Interestingly, the triangular H.japonica divides asymmetrically (Hamamoto et al. 1988 FEMS Microbiol Let), resembling the division of triangular stomatal lineage meristemoids in Arabidopsis...

Extreme environments can provide opportunity for some extreme geometric experimentation, as shown by a couple flat square and triangular archaeans here. While prokaryotes are notoriously dismissed for being morphologically 'plain', many are far from it, and even have elaborate cell structures within, but that's for another day. But zoologists and botanists take note: the little awesome that is left outside the protist kingdom is hoarded by prokaryotes, ha!

References:

Bolhuis, H., Poele, E., & Rodriguez-Valera, F. (2004). Isolation and cultivation of Walsby's square archaeon Environmental Microbiology, 6 (12), 1287-1291 DOI: 10.1111/j.1462-2920.2004.00692.x

Bolhuis, H., Palm, P., Wende, A., Falb, M., Rampp, M., Rodriguez-Valera, F., Pfeiffer, F., & Oesterhelt, D. (2006). The genome of the square archaeon Haloquadratum walsbyi : life at the limits of water activity

BMC Genomics, 7 (1) DOI: 10.1186/1471-2164-7-169

Burns, D., Janssen, P., Itoh, T., Kamekura, M., Li, Z., Jensen, G., Rodriguez-Valera, F., Bolhuis, H., & Dyall-Smith, M. (2007). Haloquadratum walsbyi gen. nov., sp. nov., the square haloarchaeon of Walsby, isolated from saltern crystallizers in Australia and Spain INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 57 (2), 387-392 DOI: 10.1099/ijs.0.64690-0

Hamamoto, T., Takashina, T., Grant, W., & Horikoshi, K. (1988). Asymmetric cell division of a triangular halophilic archaebacterium FEMS Microbiology Letters, 56 (2), 221-224 DOI: 10.1111/j.1574-6968.1988.tb03181.x

Horikoshi, K., Aono, R., & Nakamura, S. (1993). The triangular halophilic archaebacteriumHaloarcula japonica strain TR-1 Experientia, 49 (6-7), 497-502 DOI: 10.1007/BF01955151

López-López, A., Benlloch, S., Bonfá, M., Rodríguez-Valera, F., & Mira, A. (2007). Intragenomic 16S rDNA Divergence in Haloarcula marismortui Is an Adaptation to Different Temperatures Journal of Molecular Evolution, 65 (6), 687-696 DOI: 10.1007/s00239-007-9047-3

Minegishi, H., Kamekura, M., Itoh, T., Echigo, A., Usami, R., & Hashimoto, T. (2009). Further refinement of the phylogeny of the Halobacteriaceae based on the full-length RNA polymerase subunit B' (rpoB') gene INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 60 (10), 2398-2408 DOI: 10.1099/ijs.0.017160-0

Oren, A., Pri-El, N., Shapiro, O., & Siboni, N. (2006). Buoyancy studies in natural communities of square gas-vacuolate archaea in saltern crystallizer ponds Saline Systems, 2 (1) DOI: 10.1186/1746-1448-2-4

Walsby, A. (2005). Archaea with square cells Trends in Microbiology, 13 (5), 193-195 DOI: 10.1016/j.tim.2005.03.002

(h/t Opisthokont, who owes my boss few hours of my productive work time)