Investigating the bioavailability of dissolved organic nitrogen using freshwater algae

Abstract

Nitrogen (N) inputs to freshwaters are increasing, largely due to increasing human impacts in the environment. The role of Dissolved Organic N (DON) as a nutrient resource in freshwaters is poorly understood, even though it is an important and, in oligotrophic waters, dominant N source. Its role as a bioavailable nutrient resource is examined here. Six algal isolates were obtained from two streams: a eutrophic farmland stream, and an oligotrophic peatland stream. Each site was well-characterised with detailed data available on stream N, P and C composition. The isolates were cultured in the laboratory, identified using DNA barcoding (via ITS2), including largely cosmopolitan species with differing associated microbiomes (via 16S), and tested experimentally to determine their ability to utilise different DON chemistries (urea, glutamate) alongside inorganic N (nitrate, ammonium) compounds. Species-specific differences were observed in the rates of N uptake, even between closely related species. Compound-specific differences in uptake rates were also observed for individual isolates. In most sites and species, there was preference for nitrate uptake over ammonium. Some species utilised DON compounds (urea or glutamate) in preference to inorganic N compounds. Algal species ability to use DON as a nutrient resource was not obviously linked to stream character. Finally, dual-labelled 13C, 15N stable isotopes of urea and glutamate were used to trace uptake into algal biomass using a recently isolated Chlorella vulgaris strain. This showed bulk uptake of N in both treatments, with C uptake in the glutamate treatment, suggesting different metabolic pathways for differing DON chemistries. This research demonstrates that algae will respond and grow on DON compounds, in some circumstances at faster rates than on inorganic N compounds, and that there is both species-specific preference and compound-specific difference in uptake rates.

Date of Award	2 Dec 2021
Original language	English
Awarding Institution	University of Bristol
Sponsors	Natural Environment Research Council
Supervisor	Penny J Johnes (Supervisor), Richard P Evershed (Supervisor), Patricia Sanchez-Baracaldo (Supervisor) & Stephen Maberly (Supervisor)