MICROBIAL PIGMENTS AND MEMBRANE LIPIDS AS INDICATORS OF SOIL CRUST INPUTS TO ATMOSPHERIC DUST

Soil crusts are a significant source for atmospheric dust in the desert southwest, USA. Atmospheric fine dust particulate matter (PM_2.5) included up to 50% crustal material and coarse particles (PM_>2.5) included 50-90% crustal material for sampling sites in Pinal County, AZ. We analyzed microbial pigment and membrane lipid biomarkers in soil crusts and ambient dust to constrain the relative inputs on agricultural and native soils to dust samples at rural and urban sampling sites. We also conducted dust resuspension experiments for native microbiotic soil crusts to examine biomarker partitioning into PM_2.5 and PM₁₀ fractions. All of the biomarkers were more concentrated in PM_2.5, the fine fraction, which can be transported longer distances in the atmosphere.

Microbiotic soil crusts develop in arid landscapes where plant cover is sparse and the soil surface is exposed to sunlight. Well-developed crusts form on native soils, where soil cyanobacteria secrete polysaccharides that bind mineral grains together and diverse microbial populations grow. Cyanobacteria deposit the pigment scytonemin in the extracellular polysaccharide layer, serving a photoprotective role in screening UV radiation. The average scytonemin concentration in native soil crusts was 961 nmol/gOC compared with just 7 nmol/gOC in agricultural soil crusts. Other microbial biomolecules also had higher concentration in native soils, including Chlorophyll a (176 vs. 104 nmol/gOC), glycolipids (4.9 vs. 1.4 umol/gOC), and phospholipids (2.4 vs. 1.5 umol/gOC). Conversely, glycosphingolipids were more abundant in crusts from agricultural soils (148 vs. 110 nmol/gOC). The glycosphingolipids may derive from sphingomonads (Sphingomonadaceae), Gram-negative bacteria that have been identified previously in arid soils. Phospholipids and glycosphingolipids were most abundant in ambient urban dust, suggesting that agricultural soils were the primary crustal input.