Amino Acids Are an Ineffective Fertilizer for Dunaliella spp. Growth

Abstract

Autotrophic microalgae are a promising bioproducts platform. However, the fundamental requirements these organisms have for nitrogen fertilizer severely limit the impact and scale of their cultivation. As an alternative to inorganic fertilizers, we investigated the possibility of using amino acids from deconstructed biomass as a nitrogen source in the genus Dunaliella. We found that only four amino acids (glutamine, histidine, cysteine, and tryptophan) rescue Dunaliella spp. growth in nitrogen depleted media, and that supplementation of these amino acids altered the metabolic profile of Dunaliella cells. Our investigations revealed that histidine is transported across the cell membrane, and that glutamine and cysteine are not transported. Rather, glutamine, cysteine, and tryptophan are degraded in solution by a set of oxidative chemical reactions, releasing ammonium that in turn supports growth. Utilization of biomass-derived amino acids is therefore not a suitable option unless additional amino acid nitrogen uptake is enabled through genetic modifications of these algae.

Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: influence of grazing-mediated total ammonia-nitrogen

“…However, the decreasing NPQ trend was not observed in urea-based grazing cultures. Urea dissociates into ammonia and carbon dioxide with the help of two intracellular enzymes, namely urease and urea amidolyase (Bekheet & Syrett, 1977); however, D. tertiolecta lacks the former enzyme (Murphree et al, 2017). The latter enzyme, urea amidolyase, is ATP-dependent and operates under metabolic control of the substrate, urea (induction) and product, ammonia (repression).…”

Metabolic and Transcriptional Profiles of Dunaliella viridis Supplemented With Ammonium Derived From Glutamine

“…While some algae species like Cyclotella cryptica and Chlorella spp. can take up some free amino acids and grow heterotrophically (Hellebust and Liu, 1974;Sauer et al, 1983), other oil producing algae are more selective in their ability to grow on amino acids as a nitrogen source (Kirk and Kirk, 1978;Ietswaart et al, 1994;Hellio and Le Gal, 1998;Murphree et al, 2017). Dunaliella viridis was previously shown to be unable to take up free amino acids from the medium with exception of histidine as a direct nitrogen source.…”

“…However, D. viridis can utilize NH 4 + released from abiotic degradation of glutamine (GLN), tryptophan, and cysteine. Histidine-supplemented growth medium supports algae growth at low rates, while GLN supplementation of algae cultures enables sufficient growth with a relatively high lipid content in the cells (Murphree et al, 2017). The ability to provide NH 4 + derived from deaminated amino acids like GLN, tryptophan, and cysteine can contribute to the recycling of organic nitrogen from the culture and reduce the input of synthetic nitrogen fertilizer.…”

“…The ability to provide NH 4 + derived from deaminated amino acids like GLN, tryptophan, and cysteine can contribute to the recycling of organic nitrogen from the culture and reduce the input of synthetic nitrogen fertilizer. D. viridis like other Dunaliella species is sensitive to ammonia toxicity (Gutierrez et al, 2016;Murphree et al, 2017) and shows growth inhibition and cell death at concentrations above 1 mM. Therefore, using amino acids as a slow-release source of ammonium would limit ammonia toxicity.…”

“…+ (Shin et al, 2015;Tan et al, 2016). GLN is converted over time into pyroglutamate and NH 4 + in medium used for Dunaliella cultivation and can support favorable growth and neutral lipid production in D. viridis (Murphree et al, 2017). However, the transcriptional regulation that differentiates NO 3 − and glutamine-derived ammonia (NH 4 + ) supported growth are unknown.…”