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There is a growing consensus that photodegradation accelerates litter decomposition in drylands, but the mechanisms are not well understood. In a previous field study examining how exposure to solar radiation affects decomposition of 12 leaf litter types over 34 months in the Sonoran Desert, litter exposed to UV/blue wavebands of solar radiation decayed faster. The concentration of water-soluble compounds was higher in decayed litter than in new (recently senesced) litter, and higher in decayed litter exposed to solar radiation than other decayed litter. Microbial respiration of litter incubated in high relative humidity for 1 day was greater in decayed litter than new litter and greatest in decayed litter exposed to solar radiation. Respiration rates were strongly correlated with decay rates and water-soluble concentrations of litter. The objective of the current study was to determine why respiration rates were higher in decayed litter and why this effect was magnified in litter exposed to solar radiation. First, I evaluated whether photodegradation enhanced the quantity of dissolved organic carbon (DOC) in litter by comparing DOC concentrations of photodegraded litter to new litter. Second, I evaluated whether photodegradation increased the quality of DOC for microbial utilization by measuring respiration of leachates with equal DOC concentrations after applying them to a soil inoculum. I hypothesized that water vapor sorption may explain differences in respiration among litter age or sunlight exposure treatments. Therefore, I assessed water vapor sorption of litter over an 8-day incubation in high relative humidity. Water vapor sorption rates over 1 and 8 days were slower in decayed than new litter and not faster in photodegraded than other decayed litter. However, I found that 49-78% of the variation in respiration could be explained by the relative amount of water litter absorbed over 1 day compared to 8 days, a measure referred to as relative water content. Decayed and photodegraded litter had higher relative water content after 1 day because it had a lower water-holding capacity. Higher respiration rates of decayed and photodegraded litter were attributed to faster microbial activation due to greater relative water content of that litter.
ContributorsBliss, Michael Scott (Author) / Day, Thomas A. (Thesis advisor) / Garcia-Pichel, Ferran (Committee member) / Throop, Heather L. (Committee member) / Arizona State University (Publisher)
Created2019
Description
Predatory bacteria are a guild of heterotrophs that feed directly on other living bacteria. They belong to several bacterial lineages that evolved this mode of life independently and occur in many microbiomes and environments. Current knowledge of predatory bacteria is based on culture studies and simple detection in natural systems. The ecological consequences of their activity, unlike those of other populational loss factors like viral infection or grazing by protists, are yet to be assessed. During large-scale cultivation of biological soil crusts intended for arid soil rehabilitation, episodes of catastrophic failure were observed in cyanobacterial growth that could be ascribed to the action of an unknown predatory bacterium using bioassays. This predatory bacterium was also present in natural biocrust communities, where it formed clearings (plaques) up to 9 cm in diameter that were visible to the naked eye. Enrichment cultivation and purification by cell-sorting were used to obtain co-cultures of the predator with its cyanobacterial prey, as well as to identify and characterize it genomically, physiologically and ultrastructurally. A Bacteroidetes bacterium, unrelated to any known isolate at the family level, it is endobiotic, non-motile, obligately predatory, displays a complex life cycle and very unusual ultrastructure. Extracellular propagules are small (0.8-1.0 µm) Gram-negative cocci with internal two-membrane-bound compartmentalization. These gain entry to the prey likely using a suite of hydrolytic enzymes, localizing to the cyanobacterial cytoplasm, where growth begins into non-compartmentalized pseudofilaments that undergo secretion of vesicles and simultaneous multiple division to yield new propagules. I formally describe it as Candidatus Cyanoraptor togatus, hereafter Cyanoraptor. Its prey range is restricted to biocrust-forming, filamentous, non-heterocystous, gliding, bundle-making cyanobacteria. Molecular meta-analyses showed its worldwide distribution in biocrusts. Biogeochemical analyses of Cyanoraptor plaques revealed that it causes a complete loss of primary productivity, and significant decreases in other biocrusts properties such as water-retention and dust-trapping capacity. Extensive field surveys in the US Southwest revealed its ubiquity and its dispersal-limited, aggregated spatial distribution and incidence. Overall, its activity reduces biocrust productivity by 10% at the ecosystem scale. My research points to predatory bacteria as a significant, but overlooked, ecological force in shaping soil microbiomes.
ContributorsBethany Rakes, Julie Ann (Author) / Garcia-Pichel, Ferran (Thesis advisor) / Gile, Gillian (Committee member) / Cao, Huansheng (Committee member) / Jacobs, Bertram (Committee member) / Arizona State University (Publisher)
Created2022
Description
Decay of plant litter represents an enormous pathway for carbon (C) into the atmosphere but our understanding of the mechanisms driving this process is particularly limited in drylands. While microbes are a dominant driver of litter decay in most ecosystems, their significance in drylands is not well understood and abiotic drivers such as photodegradation are commonly perceived to be more important. I assessed the significance of microbes to the decay of plant litter in the Sonoran Desert. I found that the variation in decay among 16 leaf litter types was correlated with microbial respiration rates (i.e. CO2 emission) from litter, and rates were strongly correlated with water-vapor sorption rates of litter. Water-vapor sorption during high-humidity periods activates microbes and subsequent respiration appears to be a significant decay mechanism. I also found that exposure to sunlight accelerated litter decay (i.e. photodegradation) and enhanced subsequent respiration rates of litter. The abundance of bacteria (but not fungi) on the surface of litter exposed to sunlight was strongly correlated with respiration rates, as well as litter decay, implying that exposure to sunlight facilitated activity of surface bacteria which were responsible for faster decay. I also assessed the response of respiration to temperature and moisture content (MC) of litter, as well as the relationship between relative humidity and MC. There was a peak in respiration rates between 35-40oC, and, unexpectedly, rates increased from 55 to 70oC with the highest peak at 70oC, suggesting the presence of thermophilic microbes or heat-tolerant enzymes. Respiration rates increased exponentially with MC, and MC was strongly correlated with relative humidity. I used these relationships, along with litter microclimate and C loss data to estimate the contribution of this pathway to litter C loss over 34 months. Respiration was responsible for 24% of the total C lost from litter – this represents a substantial pathway for C loss, over twice as large as the combination of thermal and photochemical abiotic emission. My findings elucidate two mechanisms that explain why microbial drivers were more significant than commonly assumed: activation of microbes via water-vapor sorption and high respiration rates at high temperatures.
ContributorsTomes, Alexander (Author) / Day, Thomas (Thesis advisor) / Garcia-Pichel, Ferran (Committee member) / Ball, Becky (Committee member) / Hall, Sharon (Committee member) / Roberson, Robert (Committee member) / Arizona State University (Publisher)
Created2020