Planococcus halocryophilus; growth in subzero halophilic conditions

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Planococcus halocryophilus (OR1)

Color-enhanced scanning electron micrograph of the Planococcus halocryophilus bacterium. Credit: N.C.S. Mykytczuk et al., the ISME Journal (7 February 2013) © Nature Publishing Group [1].


By Ethan Hanson



Classification:
Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillaes
Family:Planococcaceae
Genus: Planococcus

Species:

Planococcus halocryophilus



Planococcus halocryophilus is an aerobic, gram-positive bacterium that is found in arctic permafrost. This extremophile is characterized as both halophilic and psychrophilic, thriving in an environment of high salinity as well as an extremely low temperature. This bacterium's reproduction capability is measured at the lowest recorded temperature, measured at -15ºC. Planococcus halocryophilus continues to preserve itself at temperatures as low as -25ºC. The bacterium is accountable for effects of global warming, bringing about sizable CO2 emissions concurrent to melting permafrost. Astrobiological research towards this extremophile is resonant owing to its similitude of potential target environments for life on Mars.

Section 1

Planococcus halocryophilus (OR1) cells are non-spore forming motile cocci that occur individually or in pairs, with a diameter of 0.8-1.2 micrometers. In colonies, P. halocryophilus appears an opaque, shiny orange color in clusters measured between 2.0-3.0 mm. As stated before, they are gram-positive, aerobic, and both psychrotolerant as well as halotolerant. They can grow at temperatures between -15 and 37ºC, however their optimal temperature of growth is approximately 25ºC. Figure 1 exhibits an Arrhenius plot of log transformed growth rate constants for OR1. Individual measurements come from viable cell count in relation to temperature. Here, the optimal temperature of growth is projected at 25ºC with a maximum 10^8 colony forming units per mL, and a significant drop in growth rate when temperature falls below 10ºC. At <10ºC, the R2 value (slope) increases from approximately 0.49 to approximately 0.94. Growth rate seems to negate below -10ºC.

P. halocryophilus are capable of growth in liquid water of high salinity, up to 19% NaCl, but maintain the ability to grow in conditions of 0% NaCl. When compared to other strains of Planococcus, P. halocryophilus was the only species with a growth temperature range below zero. OR1 also measured out to be the most tolerant to NaCl (ahead of P. maritimus at 17% NaCl). A major physiological feature that distinguishes P. halocryophilus is its transformation of the cellular envelope at temperatures below zero. OR1 cells grown at subzero temperature gain winding nodular sheet encrustations around each individual cell. Its hypothesized that the purpose of this cellular envelope addition is to provide extra protection to the interior of the cell during low-temperature replication. Figure 2 shows three electron microscopic images of P. halocryophilus, (a) and (b) under scanning EM and (c) under transmission EM. Images (a) and (c) show comparison of cell division planes, for cells replicating at 25ºC (a), versus dividing cells at -15ºC (c). The images show dramatic contrast by the nodular cell envelope, showing a 3-dimensional representation of the encrustation in image (c).

P. halocryophilus is an aerobic heterotroph that has the ability to utilize at least 25 different carbon sources (glucose, fructose, galactose, glycerol, rhamnose, mannose, glucosamine, glutamine, maltose, ribose, cellobiose, trehalose, mannitol, lactose, lactic acid, sucrose, gelatin, pectin, dextrin, serine, alanine, arginine, acetic acid, gluconic acid, and glutamic acid), when provided individually.

A cellular fatty acid profile of P. halocryophilus was used to determine which fatty acids were most abundant under specific conditions (control at 25ºC and 0% NaCl) of high salinity and of subzero temperatures separately. Under conditions of high salinity, the most abundant were branched fatty acids and unsaturated fatty acids. Under subzero temperature conditions, the most abundant were straight-chain fatty acids and saturated fatty acids. Very few outliers showed greatest abundance in control conditions. This data is important to membrane fluidity of P. halocryophilus because of the relative abundance of saturated and unsaturated fatty acids. Having more unsaturated fatty acids in conditions of high salinity provides evidence for higher fluidity in the cell membrane, allowing for greater osmotic regulation in ideal temperature conditions. In contrast, the high abundance of saturated fatty acids in subzero conditions provide evidence for lower cell fluidity, in order to protect the cell when metabolic energetics are low due to temperature.

In conditions of cold-specific stress, energy metabolism carries on through increased abundance of substrate shuttling genes and components of the electron transport chain. In subzero conditions (-15ºC), while most metabolic enzymes are repressed, the tricarboxylic cycle and ATP synthase keep the cell alive. In conditions of salt-specific stress, carbohydrate metabolism is extricated by glycogen synthesis, stores induced by specific genes that are activated only under nutrient limited conditions. P. halocryophilus proves its unique resourcefulness from specific response methods through fastidious induction and regulation of metabolic activity.
Bacterium Planococcus Halocryophilus Offers Clues about Microbial Life on Enceladus, Mars. www.sci-news.com/space/article01105-planococcus-halocryophilus-bacterium.html.

Color-enhanced scanning electron micrograph of the Planococcus halocryophilus bacterium. Credit: N.C.S. Mykytczuk et al., the ISME Journal (7 February 2013) © Nature Publishing Group [2].
Color-enhanced scanning electron micrograph of the Planococcus halocryophilus bacterium. Credit: N.C.S. Mykytczuk et al., the ISME Journal (7 February 2013) © Nature Publishing Group [3].

Section 2

Include some current research, with at least one figure showing data.

Color-enhanced scanning electron micrograph of the Planococcus halocryophilus bacterium. Credit: N.C.S. Mykytczuk et al., the ISME Journal (7 February 2013) © Nature Publishing Group [4].

Genome

Include some current research, with at least one figure showing data.

Section 4

Conclusion

References



Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2018, Kenyon College.