Molecular insights into the role of particulate organic matter in biogeochemical cycling in freshwater ecosystems

Abstract

Organic matter is widely recognised as one of the main drivers of the structure and function of aquatic ecosystems. It is a source of energy and nutrients for aquatic microorganisms, and it plays an important role in regulating physical processes as well as in the transport and bioavailability of metals and pollutants. However, the current understanding of the mechanisms concerning the dynamic processing of organic matter in the water column is still mainly speculative. This work aims to investigate the role of suspended particles on the cycling of organic matter in freshwater at the molecular-level.

The chemical and molecular composition of suspended particles was first investigated. A selection of methods based on mass spectrometry was used to target several compound classes, including total hydrolysable amino acids, neutral carbohydrates, lipids, and macromolecular derived compounds. Other compounds of potentially anthropogenic origin were also identified. Information of individual biomarkers suggested that the suspended material is C and N rich, exhibiting a relatively labile nature. Changes in the concentration and composition were influenced by a sewage treatment works effluent.

Following the characterisation, a compound-specific stable isotope probing approach was applied to test assimilation of dissolved compounds by particle-attached microbial communities. This approach allowed the demonstration of two main processes: a) the preferential assimilation of an organic substrate vs an inorganic, and b) the direct assimilation of the organic molecule into newly synthesised proteinaceous biomass.

The same approach was used to assess differences in the mechanistic processing of organic substrates under contrasting conditions. Rates of assimilation of ¹³C and ¹⁵N into individual hydrolysable proteinaceous amino acids were significantly different across sites that resulted in contrasting processing of the organic substrate. However, rates were not correlated to environmental conditions, suggesting that the drivers of DOM metabolism are more complex and perhaps strongly influenced by changes in microbial community structure.

The results of this dissertation provide new molecular-level insights into the role of suspended particles in the cycling of organic and inorganic substrates in freshwater. A further integrated approach that incorporates the dissolved and particulate domains is central in the better understanding of biogeochemical cycling in aquatic ecosystems.

Date of Award	6 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Richard P Evershed (Supervisor) & Penny J Johnes (Supervisor)