title: Marine Biodiscovery - An Exploration of Chemical Diversity, Antibiotic Discovery, and Invertebrate Natural Product Chemistry
Creators: name: Voser, Tanja M
Publisher: name: Griffith University
Year: 2022
Subjects: subject: marine natural products
subject: drug discovery
subject: trends
subject: marine microbial natural products
subject: chemistry
subject: Australian
subject: bacteria
subject: antimicrobial
subject: microorganisms

description: This thesis covers a wide range of research on marine natural products with a focus on their role in drug discovery. Chapter 1 starts with a general introduction to marine natural product chemistry, describing current trends and problems. It gives an insight into the importance of the discovery of new antibiotics in this pressing time where multidrug-resistant bacteria are on the rise. Chapter 2 is a meta-analysis of current trends in marine microbial natural product research and an analysis of how much of their chemistry overlaps with the chemistry of terrestrial microbial natural products. Over the past decade, research has greatly shifted to focus on marine microbial natural products, at the expense of marine macro-organism studies. An assessment of the structural overlap between 55,817 published marine and terrestrial microbe and marine macroorganism natural products, using structural fingerprints and scaffold clustering, was enlightening. The results showed that currently 76.7% of the chemistry found in marine microbes is vastly similar to the chemistry isolated from terrestrial microbes. This overlap is mainly due to the use of terrestrial isolation and culturing methods that select for the growth of terrestrial-like bacteria instead of unique marine bacteria. As a result, the unique chemistry associated with truly marine microbe species that are in close symbiotic relationships with marine macro-organisms is being missed. Chapter 3 describes the antimicrobial assay development and screening of a large collection of Australian marine invertebrate specimens for activity against four different strains of bacteria. It resulted in 12.5% of all the specimens tested, showing activity against the drugresistant strain of Staphylococcus aureus (MRSA) but lower bioactivity rates against Pseudomonas aeruginosa (0.76%) and Escherichia coli (0.76%). This illustrates how much harder it is to find activity against Gram-negative bacteria such as P. aeruginosa and E. coli. To investigate this challenge further, drug combination screening was undertaken to assess the potential of resurrecting antibiotics, for the use against bacterial strains that have developed resistance, by combining the antibiotic with marine invertebrate extracts. The combinatorial assays were unsuccessful, thus for further investigations I concentrated on the specimens that exhibited activity in the main assay. Analysis of the sponge specimen Aaptos aaptos that showed activity against S. aureus resulted in the isolation of demethylaaptamine as the bioactive component. Small quantities of a series of peptides with molecular weights in the range of 3,000- 4,000 Da were also isolated from the sponge. Unfortunately, a detailed structure determination could not be undertaken due to COVID-19 state border restrictions, which prevented recollection of the sponge. The subsequent two chapters report on the chemistry of antimicrobial bioactive marine invertebrate extracts. Chapter 4 describes the structures of two new betaine molecules isolated from the bryozoan Amathia lamourouxi. Their structures were determined through analysis of 1D and 2D NMR and mass spectrometric data. Crude extracts of the specimen showed antimicrobial activity at 2.5 mg/mL. Although the yield of active compounds was too small to be completely isolated and identified they were associated with brominated alkaloids. Unfortunately, recollection of this species was also hindered by state border closure due to the COVID-19 pandemic. Chapter 5 describes the isolation and identification of two new amphiphilic polyamines that together with a mixture of relatives were extracted from the marine sponge Aaptos lobata. The two pure compounds and the mixture of amphiphilic compounds showed moderate bioactivity against both drug sensitive and resistant S. aureus, and P. aeruginosa. Chapter 6 describes a multiplatform investigation of the chemistry of the ascidian B. leachii. This species was targeted because it had a similar chemical profile to Aaptos aaptos, small alkaloid and large (&gt;3500 Da) peptide and was more accessible after the COVID-19 state border closures. The investigation used different analytical tools like LC-MS and advanced NMR techniques, including DOSY to characterise the diversity of compounds found in the mixture, while MALDITOF imaging was used to identify the specific locations of these metabolites within the ascidian tissue. This study demonstrated the power of MALDI imaging to provide an insight into the chemical ecology of marine species and helped to establish relationships between marine invertebrates and their associated microorganisms. This finding will aid future specific targeting of tissue regions within marine invertebrates for symbiotic microbe isolation and identification of natural products. Finally, Chapter 7 combines and discusses all the findings of this thesis and examines the future of marine drug discovery. The research reported here has explored the chemical diversity of microbial natural products, discovered new compounds, some with antimicrobial activity, and investigated the chemistry of marine invertebrates and their interesting symbiotic relationship with microorganisms.