Ecophysiological Distinctions of Haloarchaea from a Hypersaline Antarctic Lake as Determined by Metaproteomics
Authors: Tschitschko Bernhard , Williams Timothy J. , Allen Michelle A. , Zhong Ling , Raftery Mark J. , Cavicchioli Ricardo 
Published: March 2016

ABSTRACT

Deep Lake in the Vestfold Hills is hypersaline and the coldest system in Antarctica known to support microbial growth (temperatures as low as −20°C). It represents a strong experimental model because the lake supports a low-complexity community of haloarchaea, with the three most abundant species totaling ∼72%. Moreover, the dominant haloarchaea are cultivatable, and their genomes are sequenced. Here we use metaproteomics linked to metagenome data and the genome sequences of the isolates to characterize the main pathways, trophic strategies, and interactions associated with resource utilization. The dominance of the most abundant member,
Halohasta litchfieldiae
, appears to be predicated on competitive utilization of substrates (e.g., starch, glycerol, and dihydroxyacetone) produced by
Dunaliella
, the lake's primary producer, while also possessing diverse mechanisms for acquiring nitrogen and phosphorus. The second most abundant member, strain DL31, is proficient in degrading complex proteinaceous matter.
Hht. litchfieldiae
and DL31 are inferred to release labile substrates that are utilized by
Halorubrum lacusprofundi
, the third most abundant haloarchaeon in Deep Lake. The study also linked genome variation to specific protein variants or distinct genetic capacities, thereby identifying strain-level variation indicative of specialization. Overall, metaproteomics revealed that rather than functional differences occurring at different lake depths or through size partitioning, the main lake genera possess major trophic distinctions, and phylotypes (e.g., strains of
Hht. litchfieldiae
) exhibit a more subtle level of specialization. This study highlights the extent to which the lake supports a relatively uniform distribution of taxa that collectively possess the genetic capacity to effectively exploit available nutrients throughout the lake.

IMPORTANCE
Life on Earth has evolved to colonize a broad range of temperatures, but most of the biosphere (∼85%) exists at low temperatures (≤5°C). By performing unique roles in biogeochemical cycles, environmental microorganisms perform functions that are critical for the rest of life on Earth to survive. Cold environments therefore make a particularly important contribution to maintaining healthy, stable ecosystems. Here we describe the main physiological traits of the dominant microorganisms that inhabit Deep Lake in Antarctica, the coldest aquatic environment known to support life. The hypersaline system enables the growth of halophilic members of the
Archaea
: haloarchaea. By analyzing proteins of samples collected from the water column, we determined the functions that the haloarchaea were likely to perform. This study showed that the dominant haloarchaea possessed distinct lifestyles yet formed a uniform community throughout the lake that was collectively adept at using available light energy and diverse organic substrates for growth.