Chemoautotrophy in subzero environments and the potential for cold-adapted Rubisco.

Area of Science:

• Microbiology
• Astrobiology
• Biochemistry

Background:

• Autotrophy, the fixation of inorganic carbon, is crucial for life on Earth, with Rubisco being a key enzyme.
• While plants and cyanobacteria are well-studied, bacteria in extreme environments also utilize Rubisco for chemolithoautotrophy.
• Arctic cryopeg brines and sea ice represent unique, subzero, hypersaline environments harboring diverse microbial life.

Purpose of the Study:

• To characterize autotrophic pathways and Rubisco diversity in Arctic subzero, hypersaline environments.
• To investigate the abundance and forms of Rubisco in the genus Thiomicrorhabdus from these environments.
• To model Rubisco kinetics and assess potential cold adaptation in extreme environments.

Main Methods:

• Metagenome-assembled genome (MAG) reconstruction from Arctic samples.
• Genomic surveys of Thiomicrorhabdus across diverse environments.
• Kinetic modeling of Rubisco carboxylation rates under varying conditions (CO2, O2, temperature).

Main Results:

• The Calvin-Benson-Bassham (CBB) cycle was prevalent, with distinct Rubisco forms dominating each environment.
• Four MAGs with chemolithoautotrophic potential were identified, with Thiomicrorhabdus being the most abundant.
• Rubisco form II, constitutively present in subzero environments, showed potential for cold adaptation with a more exposed active site, outcompeting form I at low O2.

Conclusions:

• Subzero Rubisco form II from Thiomicrorhabdus warrants further investigation for unique kinetics and thermal stability.
• This research expands the understanding of autotrophic functionality limits in extreme Earth environments.
• The findings have implications for identifying potential autotrophy on other planetary bodies like Enceladus, Europa, and Mars.