Multiple resources determine soil bacterial diversity and activity in an Antarctic polar desert

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Seasonal fluctuations in environmental variables elicit pulses (a.k.a. “hot moments’”) of ecosystem activity. Many of these fluctuations are predictable, creating similar yet transient conditions selecting for specific bacterial taxa. But shifts in resources often co-occur, complicating the ability to discern which environmental queues trigger bacterial responses. In this study, we used four forms of DNA stable isotope probing (SIP) to track the bacteria responding to four resource additions (C—mannitol, N— NH₄⁺ and NO₃^-, W—water, CN—mannitol, and NH₄⁺ and NO₃^-) and generating pulses of soil respiration in cold desert soils from the McMurdo Dry Valleys Long-Term Ecological Research site in Taylor Valley, Antarctica. We successfully separated isotopically labeled DNA of metabolically active or responding bacteria from unlabeled DNA of total communities for each SIP type (¹³C-DNA SIP with ¹³C-labeled mannitol, a sugar alcohol simulating inputs from algae and cyanobacteria; ¹⁵N-DNA with ¹⁵N- NH₄NO₃; ¹⁸O-DNA SIP with H₂¹⁸O; and a combination of ¹³C- and ¹⁵N-DNA SIP). Of the four distinct resource additions, CN additions facilitated a dramatic bloom of one bacterial species, Arthrobacter sp. (Micrococcaceae), constituting 70% of the relative recovery in the responding community and reducing in alpha diversity and richness. However, the combination of C and N also allowed 60% of the community to turnover, and the highest number of rare species to resuscitate, become metabolically active, and grow. The addition of inorganic N lead to a generally unique but stable responding community with only a 31% difference in composition and the highest number of abundant (>1%) and intermediate (0.1-1%) taxa shared between the responding and total communities. Water encouraged a high level of diversity and richness, promoting the persistence of rare bacteria, but not the appearance of new taxa. Regardless of differences in the responding communities, all additions generated pulses of CO₂, but soil respiration was at least 3.8-times higher in the CN than all other additions. Our findings suggest that individual and combinations of resources induce nutrient-specific changes in the assembly of complex bacterial communities by promoting dominance, sustaining diversity, and selecting for taxa within the core microbiome and rare biosphere.