Abstract
Arthrobacter spp. are coryneform Gram-positive aerobic bacteria, belonging to the class Actinobacteria. Representatives of this genus have mainly been isolated from soil, mud, sludge or sewage, and are usually mesophiles. In recent years, the presence of Arthrobacter spp. was also confirmed in various extreme, including permanently cold, environments. In this study, 36 psychrotolerant and metalotolerant Arthrobacter strains isolated from petroleum-contaminated soil from the King George Island (Antarctica), were screened for the presence of plasmids. The identified replicons were thoroughly characterized in order to assess their diversity and role in the adaptation of Arthrobacter spp. to harsh Antarctic conditions. The screening process identified 11 different plasmids, ranging in size from 8.4 to 90.6 kb. A thorough genomic analysis of these replicons detected the presence of numerous genes encoding proteins that potentially perform roles in adaptive processes such as (i) protection against ultraviolet (UV) radiation, (ii) resistance to heavy metals, (iii) transport and metabolism of organic compounds, (iv) sulfur metabolism, and (v) protection against exogenous DNA. Moreover, 10 of the plasmids carry genetic modules enabling conjugal transfer, which may facilitate their spread among bacteria in Antarctic soil. In addition, transposable elements were identified within the analyzed plasmids. Some of these elements carry passenger genes, which suggests that these replicons may be actively changing, and novel genetic modules of adaptive value could be acquired by transposition events. A comparative genomic analysis of plasmids identified in this study and other available Arthrobacter plasmids was performed. This showed only limited similarities between plasmids of Antarctic Arthrobacter strains and replicons of other, mostly mesophilic, isolates. This indicates that the plasmids identified in this study are novel and unique replicons. In addition, a thorough meta-analysis of 247 plasmids of psychrotolerant bacteria was performed, revealing the important role of these replicons in the adaptation of their hosts to extreme environments.
Highlights
Almost 80% of the Earth’s biosphere is permanently cold, i.e., the temperature in these regions stays below 5◦C throughout the whole year
The results showed that the main group of bacteria (36 isolates, named the ANT strains in this work) belong to the Arthrobacter genus (Romaniuk et al, 2018)
This observation corroborates previous reports that Arthrobacter spp. are among the most frequently isolated cultivable soil bacteria, even in polar regions, e.g., they comprised 14.7% of cultivable bacteria isolated from soil samples collected at the Browning Peninsula, Eastern Antarctica and 22% of isolates originating from postglacial soils of Ecology Glacier, King George Island, Antarctica (Zdanowski et al, 2013; Pudasaini et al, 2017)
Summary
Almost 80% of the Earth’s biosphere is permanently cold, i.e., the temperature in these regions stays below 5◦C throughout the whole year. Antarctica is known to be one of the coldest regions on Earth, with an average annual temperature ranging between −48◦C at the South Pole and −2◦C on island areas. It is an extreme habitat for indigenous organisms, because of the low temperatures, and due to the occurrence of other harsh environmental conditions, including increased UV radiation, strong and drying winds, and the low amount of accessible nutrients (Martianov and Rakusa-Suszczewski, 1990; D’amico et al, 2006; Kejna et al, 2013; Cowan et al, 2014; Grzesiak et al, 2015; Kosek et al, 2017). Intercontinental atmospheric long-range transport of pollutants to Antarctica seems to be highly limited, mostly due to the strong westerlies at mid-latitudes (40–60◦ S), which isolate the continent in winter (Kukucka et al, 2010; Stohl and Sodemann, 2010)
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