Succession of the Fungal Community of a Spacecraft Assembly Clean Room When Enriched in Brines Relevant to Mars

Spacecraft can carry microbial contaminants from spacecraft assembly facilities (SAFs) to the cold arid surface of Mars that may confound life detection missions or disrupt native ecosystems. Dry hygroscopic sulfate and (per)chlorate salts on Mars may absorb atmospheric humidity and deliquesce at certain times to produce dense brines, potential sources of liquid water. Microbial growth is generally prohibited under the nonpermissive condition of extremely low water activity in the frigid potential brines on Mars. Here we challenged the microbial community from samples of the Jet Propulsion Laboratory SAF with the extreme chemical conditions of brines relevant to Mars. Enrichment cultures in SP medium supplemented with 50% MgSO4 or 20% NaClO3 were inoculated from washes of SAF floor wipes. Samples were taken for each of the first four weeks and then at six months after inoculation to follow changes in the SAF microbial community under high salinity for long periods. Metagenomic DNA extracts of community samples were examined by Illumina sequencing of 18S rRNA gene sequences using fungal primers. The fungal assemblage during the first month of enrichment was predominantly common Ascomycetes, primarily Saccharomyete yeasts. Basidiomycetes were detected, mainly in the Microbotryomycetes and Tremellomycetes. Fungi were much less abundant in enrichment cultures at 50% MgSO4 than at 20% NaClO3. After 6 mo of enrichment, few fungi remained. Microbes persisting from the JPL SAF microbial community in aged cultures enriched at extreme salinities might be the most capable of subsequently surviving and proliferating at the near surface of Mars. The SAF fungal assemblage did not survive and proliferate as well as the SAF bacterial community.