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The UC Research Repository collects, stores and makes available original research from postgraduate students, researchers and academics based at the University of Canterbury.
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Recent Submissions
The effects of environmental pollution on bacterial conjugation.
(2024) McCabe, Andrew W.
Antibiotic resistant bacteria are an anthropogenically caused threat to medicine and social
cohesion. Whilst some of the drivers are known, such as the use and overuse of antibiotics,
others are just being discovered. Evidence that the vast number of environmental
contaminants is also an important driver is gaining acceptance. The lack of thorough testing
of the unintended antimicrobial effects of most of these contaminants leads to a severe lack
of knowledge on the impact of pollution on antibiotic resistance.
Bacteria can acquire antibiotic resistance via multiple mechanisms, including horizontal gene
transfer. One vector of transmission is conjugation. Conjugation (or āmatingā) is the process
by which plasmids move between cells. It is an important pathway through which many
bacteria acquire resistance.
Compounds known to be commonly found as contaminants/pollutants in waterways can also
have properties that could affect the frequency of conjugation. Some were chosen to test
whether exposure to them altered either or both inter- or intra- species plasmid transmission
frequencies.
Interesting patterns were observed indicating that the effect of exposure cannot necessarily
be predicted by the known chemistry of the contaminant or biochemistry of plasmid transfer.
Future research will determine if the hypothesised effect of each compound explains
observed effects on plasmid transmission frequencies.
I also examined if the plasmid transfer complex, which in this case is a type IV secretion
system, could confer resistance to heavy metal compounds. Whilst some plasmids have been
shown to carry heavy metal resistance genes, recent evidence suggests an independent effect
of the type IV secretion system on resistance.
Heavy metals are common environmental contaminants, due to leaching from plastics
amongst other sources, making exposures also common. If plasmid transfer genes have a
pleiotropic effect on heavy metal resistance, then heavy metals could substitute for
antibiotics to select for plasmids. Heavy metals would enrich for antibiotic resistance by
linkage.
The hypothesis was not supported in this research. However, a surprising observation was
made. One conjugative plasmid, RP4, increased susceptibility to zinc sulphate.
I have identified a zinc permease gene carried by RP4 that might increase zinc sulphate
uptake. Future research could determine if the permease gene is necessary and sufficient for
increased susceptibility to zinc sulphate.
The effect of thermomechanical processing parameters on the texture of Ti-6Al-4V forgings as a precursor to coarse grain growth.
(2023) Wiley, Richard
This thesis explores the relationship between forging parameters - total strain, strain rate, and
forging temperature - and the emergence of abnormal grains during subsequent heat treatment
of Ti-6Al-4V. Abnormal grain growth can compromise material properties, making its
understanding crucial for optimizing manufacturing processes. Through systematic experiments,
varying the forging temperature between 875 and 975Ā°C and the strain rate between 0.1sā»Ā¹ and
10sā»Ā¹ , the study reveals how different forging conditions influence the as-forged microstructure
through the alteration of the kinetics of flow softening and how this affects abnormal grain
formation. Microstructural analysis through optical and scanning electron microscopy methods
demonstrate correlations between these forging parameters and the occurrence of non-uniform
grain distributions. The insights gained offer strategies to mitigate abnormal grain growth during
heat treatment, advancing materials science and manufacturing practices.
The results show that forging temperature and strain rates impact prior beta grain size differently.
Higher forging temperatures at 975Ā°C lead to larger grain sizes due to increased boundary
mobility favouring grain growth. Lower forging temperatures at 875Ā°C produce more consistent
grain sizes due to uniform boundary energies resulting from a weaker texture. For samples forged
at 925Ā°C, varying strain rates cause significant differences in grain size due to the shift from
dynamic recovery to dynamic recrystallisation as the primary strain relief mechanism. This shift in
mechanisms is also reflected in stress-strain responses, with dynamically recrystallised samples
showing higher peak stress and more transience. The dynamic recrystallisation process leads to
the nucleation of strain-free grains with distinct orientations, contributing to a weaker textured
material.
The study validates that abnormal grain growth is influenced by strain rate and forging
temperature, with increased strain-strengthening precursor texture. Lower forging temperatures
lead to weaker texture and more normal grain growth. From the findings, it is recommended to
forge above critical rates for recrystallization, at the required forging temperature. The research
suggests forging above 5sā»Ā¹ up to 925Ā°C or 0.95 TĪ², although this is adjustable to 0.1sā»Ā¹ at
875Ā°C/0.9 TĪ². Billet size indirectly affects abnormal grain occurrence by affecting the strain rate,
with smaller billets displaying uniform grain size after annealing. Thus, changes in the thickness
of Ti-6Al-4V parts could cause abnormal grain growth in thicker sections during annealing and
should be considered prior to forging.
Regional flood frequency analysis in hydrologically diverse regions.
(2024) Fitriana, Farhana Sweeta
To reduce flood risk, it is crucial to accurately estimate design floods, which is a flood discharge associated with a specific annual exceedance probability. The main aim of this study is to devise an approach to carry out a Regional Flood Frequency Analysis within hydrologically similar sub-regions to estimate design floods for gauged and ungauged catchments with diverse climate and catchment characteristics. In such catchments, traditional regionalisation approaches usually fail due to a high level of inhomogeneity. The developed RFFA approach was evaluated using 363 catchments in New Zealand, a country with hydrologically diverse regions. It was found that using climate zones and catchment characteristics led to a higher degree of homogeneity than traditional sub-divisions. Cluster analysis based on catchment attributes was applied to further delineate homogenous regions, which resulted in 21 sub-regions. The two-parameter Log-Normal and Pearson 3 distributions were identified as the dominant regional probability distributions for these sub-regions. Next, the Generalised Additive Model coupled with the Index Flood L-moment approach was employed to estimate regionalised design floods of various return periods. Model performance was assessed using a Jackknife Resampling procedure. The results indicated significantly smaller error estimates for all estimated design floods than prior RFFA studies in New Zealand. The approach provides region-specific design values to inform flood risk management. It allows for robust design flood estimation in both gauged and ungauged catchments and can be implemented in other regions.
The rehabilitation of flood affected soils using biowastes.
(2024) Liu, Mingyuan
Climate change is causing increasingly frequent flooding events that result in the deposition of sediment and wood debris on valuable lands. Deposited sediments, sometimes >1 m deep, contain low concentrations of organic matter and plant nutrients. They typically become compacted resulting in dense, low-oxygen conditions that are unfavourable for plant growth. Recovering this land presents an urgent and unresolved problem. The application of mineral fertilisers on such sediments is ineffective because the nutrients are poorly retained, and the fertilisation does not ameliorate the physical condition of the sediment. Several studies have shown that composts and biosolids can restore fertility to disturbed soils that are devoid of organic matter and plant nutrients. I hypothesised that fertility could be restored to these sediment-covered lands through the application of biowastes (organic amendments), possibly in combination with mineral fertilisers. I aimed to determine the effect of sawdust (which could be derived from the pine slash), compost, and biosolids, on the growth of oats (Avena sativa L.) in a pot trial. Furthermore, I sought to determine the effect of sawdust combined with fertiliser (190 N kg ha -1 application) on the root morphology of oats. Treatments, replicated five times, included pine sawdust applied at ca. 75 t ha -1 equiv. and 150 t ha -1 equiv., compost applied at ca. 100 and 200 t ha -1 equiv., and biosolids applied at ca. 75 and 150 t ha -1 equiv. There were three configurations of the pine sawdust in the rhizoboxes (1) homogeneously mixed in sediment down to 38 cm, (2) surface application down to 23 cm, and (3) in patches with dimensions of 27 and 53 cm.
The addition of biowastes significantly increased the concentrations of C and plant nutrients of the sediment. The results showed that growth in the unfertilised controls was negligible, with germination rates <15%. Most of the organic treatments increased germination rate to >50%. Root development in the rhizoboxes was visually distinct between treatments. The addition of sawdust increased branching and root density. Mixing the sawdust with the sediment gave better results than surface application or application in patches. Addition of the biowastes significantly increased the N concentration of the plants.
This study showed that that fertility could be restored to sediments using amendments derived from pine slash, combined with fertiliser. The fertility of flood-deposited sediment was enhanced by all three amendments. There are several advantages to using pine slash instead of composts or biosolids, namely (1) The material is co-located with the sediment, so there needs to be minimal transport, (2) The removal of pine slash is anyway required from agricultural land, waterways and beaches, (3) Preparing chipped pine waste can be done in situ in a shorter time than the manufacture of compost, (4) pine slash does not introduce contaminants into the soil, such as heavy metals that may be associated with both composts and biosolids, (5) there are no social-cultural concerns with using pine slash on land, unlike biosolids, which are derived from human waste.
Future work should quantify the effects of these amendments on the chemical composition of a suite of agricultural plants and NZ-native vegetation. Field trials are required to determine the large-scale efficacy of this technique.
The frequency and mass-ratio distribution of binaries in young star clusters.
(2024) Alexander, Jason S.
Binary stars play a key role in the evolution of star clusters, therefore to develop our
understanding it is of great importance to determine the binary fractions of these clusters. In
this thesis, we focus on determining the frequency and mass-ratio distributions of binaries in
six young star clusters, specifically Collinder 69, š¼ Persei, Pleiades, NGC 6405, Trumpler
10, and UPK 640. Using data from Gaia DR3, the memberships of stars within these
clusters were determined through a selection and filtration procedure that resulted in our
final datasets.
We implemented an adapted version of the probabilistic generative model from Albrow
& Ulusele (2022) for colour-magnitude diagrams (CMDs) to ascertain the binary fractions
and mass-ratio distributions of each of the clusters. To account for overlapping populations
this method applies a Gaussian mixture model to represent stars as probabilities of being
single stars, binaries, or outliers. We used two different basis function sets to model the
mass-ratio distribution: Legendre polynomials and histograms. We found the results to be
consistent with each other.
We tested the sensitivity of our models to colour shifts of the isochrone. We ran our
model with incremental shifts in the colour up to a shift of Ā±0.01 mag. We found that the
effect of applying these colour shifts had a nominal effect on our results, and lie well within
quoted uncertainties. This shows that our model is extremely robust to the position of the
isochrone and that our results are well founded. We noticed that the small changes to the
results followed trends and we explain the cause of these effects.
The mass-ratio distributions of all our clusters exhibit a distinct preference to high-mass ratio binaries. Furthermore, we define š¹š75 as the ratio of the number of binaries which
have 0.75 ā¤ š ā¤ 1.0 to the number which have 0.5 ā¤ š ā¤ 1.0, and find all of our clusters
have š¹š75 > 0.5 and ranging between 0.598 and 0.852. The binary fraction for š ā„ šššš
is determined by integrating the mass-ratio distribution from 1 to šššš. We find that the
binary fraction of these clusters ranges between 0.170 and 0.274.
We find a strong positive correlation between the binary fraction for š ā„ 0.5 and š¹š75,
a moderate negative correlation between the age of a cluster and the binary fraction for
š ā„ 0.5, and a moderate negative correlation between the age of a cluster and š¹š75. To
account for the decreasing binary fraction with age, we propose a period of binary destruction
early in the lifetime of a star cluster (<ā¼ 100 Mya), where binaries are destroyed by stellar
disruptions at a higher rate than the escape of low-mass single stars. The proportion of equal mass
binaries may decrease with age because higher-order systems; which are unstable and
more likely to be disrupted over time, are blending into the binary region of the CMD,
or that high-mass-ratio binaries are more likely to form in a soft binary system and be
disrupted. Each of our clusters exhibited a high incidence of equal-mass binaries, and our
interpretations of the other trends directly imply that the frequency of equal-mass binaries
is more variable than at other mass-ratios, therefore it has the greatest effect on the binary
fraction.