Growth of high-elevation Cryptococcus sp. during extreme freeze–thaw cycles

Abstract

Soils above 6000 m.a.s.l. are among the most extreme environments on Earth, especially on high, dry volcanoes where soil temperatures cycle between -10 and 30 °C on a typical summer day. Previous studies have shown that such sites are dominated by yeast in the cryophilic Cryptococcus group, but it is unclear if they can actually grow (or are just surviving) under extreme freeze-thaw conditions. We carried out a series of experiments to determine if Cryptococcus could grow during freeze-thaw cycles similar to those measured under field conditions. We found that Cryptococcus phylotypes increased in relative abundance in soils subjected to 48 days of freeze-thaw cycles, becoming the dominant organisms in the soil. In addition, pure cultures of Cryptococcus isolated from these same soils were able to grow in liquid cultures subjected to daily freeze-thaw cycles, despite the fact that the culture medium froze solid every night. Furthermore, we showed that this organism is metabolically versatile and phylogenetically almost identical to strains from Antarctic Dry Valley soils. Taken together these results indicate that this organism has unique metabolic and temperature adaptations that make it able to thrive in one of the harshest and climatically volatile places on Earth.

ANaganishiain high places: functioning populations or dormant cells from the atmosphere?

“…This grouping has been confirmed by numerous other studies including the recent work of Liu et al (2015a, 2015b) who showed that N. friedmannii, N. antarcticus, N. vishniacii and N. bhutanensis are in the albidus clade of the Filobasidiales based on a seven-gene (RPB1, RPB2, TEF1, CYTB, and 3 rDNA genes) phylogeny. Also relevant to the present study, Vimercati et al (2016) showed that the isolate from Volcán Llullaillaco (discussed below) forms a monophyletic clade with N. friedmannii based on long-read 18S environmental sequences (1739 bp, JX099190) from Llullaillaco (Lynch et al 2012). Likewise, the Naganishia isolate from Volcán Saírecabur studied by Pulschen et al (2015) showed 100% identity with N. friedmannii over a 599 bp region of the LSU rRNA gene.…”

“…Concentrations of carbon and nitrogen are at or below detection limits (Table 1), even though deep sequencing of DNA in soils from 6030 m.a.s.l. indicates that there is foreign (Aeolian) plant DNA from lower elevations being deposited there (Vimercati et al 2016). In addition, these sites are all acidic, with pH values of 4.1–5.4 (Table 1), adding yet another dimension of stress to organisms living in these soils.…”

“…From south to north, microbial communities on Llullaillaco (6723 m) and Socompa (6051 m) have been studied extensively via culture-independent and culturing approaches (Costello et al 2009; Lynch et al 2012, Vimercati et al 2016; Solon et al 2017), whereas Naganishia and other yeasts have been cultured (Pulschen et al 2015) from moderately high slopes of Saírecabur (5971 m). Map is redrawn from Costello et al (2009).…”

“…The presence of foreign plant DNA in the plant-free areas on these volcanoes (Vimercati et al 2016) suggests that even the microbes in these soils may just be dormant propagules from far off locations. Many microbes can be transported over global distances (Kellogg and Griffin 2006; Schlesinger et al 2006), and biogeographic patterns of microbial community structure suggest widespread long-distance dispersal at global scales (Barberán et al 2007, Darcy et al 2011; Itani and Smith 2016).…”

“…In addition, these same yeasts are fairly easy to isolate from soils at elevations from 5000 (Pulschen et al 2015) to over 6000 m.a.s.l. (Vimercati et al 2016) in the Atacama region. These findings are somewhat surprising given the extreme dryness of these sites and the fact that they contain some of the lowest levels of total carbon (<0.03%) of any soil environment yet studied on Earth (Schmidt et al 2012).…”