Harvey, Harry J.
(2021)
Impacts of environmental structure on resilience of yeasts to stress.
PhD thesis, University of Nottingham.
Abstract
Ubiquitously, the environments of microorganisms have three-dimensional structure that create heterogeneous distributions of the space in which microorganisms reside. In particular, the soil environment is a complex porous medium inhabited by a vast array of microorganisms, essential for Earth processes like biogeochemical cycling. However, these microorganisms are subject to environmental perturbation. The physiological impacts of environmental stress on microorganisms are well studied, but whether (and to what extent) soil structure impacts exposure and response of microorganisms to stressors remains poorly understood. In this thesis, it was hypothesised that environmental structures could influence the stressor exposure of cells (and hence stressor survival) within them, and that the extent of this protective effect would depend on the type and scale of environmental structure. To examine this overall hypothesis, the influence of environmental structures on microbial response (or survival) to stress were assessed: first in relation to soil aggregation, then within macroscale pores ranging from 0.5 – 2 mm in diameter and last within micrometre scale structures using microfluidic approaches.
A method was developed to manufacture soil aggregates from natural soils with defined quantities of soil yeast in the aggregate exterior or interior. This was used to examine the impact of soil aggregation on microbial survival of a small panel of stressors (anoxia, lead nitrate, and heat stress). Results indicated that yeast cells inside aggregates were protected from acute heat stress relative to cells at the aggregate exterior, whereas effects of aqueous lead nitrate or anoxia were similar on cells at either location. The protective effect against heat stress was compromised after prolonged heat exposure but was accentuated within compacted versions of soil aggregates, providing evidence that soil compaction, a common consequence of agricultural activity, can influence microbial stress resilience.
In further experiments, structured environments with millimetre-scale pores were developed by setting up vessels containing glass beads of different sizes. Yeast ii inocula and stressors were introduced to these to explore the relationship between the environmental pore size and stress survival. Here, it was demonstrated that survival of yeast in response to lead nitrate within these structures increased with decreasing average pore size. This trend was reproduced using additively manufactured (3D-printed) lattice structures, containing pores of similar size ranges to the less-uniform glass bead structures.
Finally, microfluidic devices were used to determine whether structure at the microscale impacted microbial survival of stress. These devices contained either fabricated soil-like structures, or small microspheres to create simplified structures within otherwise homogeneous environments. At this scale, an impact of environmental structure was less clear. However, in the simplified microsphere environments, results suggested that cells within more confined spaces (I.e., more surrounded by protective structures) were less exposed to stressor (copper sulfate), which was introduced as a flowing solution within the microfluidic devices.
Taken together, results from this thesis suggest that environmental structure can determine microbial (exposure to and) survival of stress, at scales of structure ranging from micrometres to millimetres. The new methodologies and results developed within this thesis provide a foundation upon which the relationship between microbial perturbation and environmental structure can be further explored.
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