Aeolian dust deposition as a driver of cyanobacterial community structure in biological soil crusts

Biocrusts are comprised of soil-dwelling microbes well known for stabilizing desert soils. Unstable soil is typically colonized first by motile cyanobacteria that can burrow under the surface to avoid sun exposure when in a dry state. They produce long, sticky sheaths and large trichome bundles that bind soil particles. Biocrusts dominated by such cyanobacteria are rather inconspicuous and thus termed “light biocrusts.” Some non-motile cyanobacteria can produce the dark sunscreen pigment scytonemin. They are typically considered to be secondary colonizers of the soil surface and their development marks the formation of “dark biocrusts.” Contrasting with this general paradigm, we observed both light and dark biocrusts growing side by side in a natural desert area in Pinal County, Arizona. Because light biocrusts developed as a band along a nearby dirt road, we hypothesized that aeolian dust deposition from road traffic may have contributed to this spatial patterning. To test this, we used inoculum from the natural site to grow biocrust in the laboratory with and without inputs of dust deposition, characterizing resulting biocrusts by appearance, microscopy, community composition based on 16S RNA, as well as proxy pigment analyses. Light biocrusts developed on soils receiving regular dust inputs, while undusted soils developed dark biocrust, an outcome traceable primarily to a more rapid growth of motile, non scytonemin-producing cyanobacteria under dust deposition. However, similar experiments carried out with well-developed crusts resisted dust-driven community shifts, even after extended treatments. We conclude that dust can swiftly affect community assembly pathways, but that it is much less of a factor, if at all, in driving shifts in established communities, and can partly explain biocrust spatial patterning in our site, and likely elsewhere.