Amino Acids Are an Ineffective Fertilizer for Dunaliella spp. Growth.

Chengsong Zhao,Jacob T Dums,Colin A Murphree,Heike Sederoff,Jeffrey Macdonald,Andrey Tikunov,Siddharth K Jain,Danielle Y Young,Guillaume Pilot,Nicole Khoshnoodi

doi:10.3389/fpls.2017.00847

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.

Highlights

Autotroph algae have gained attention in recent years because they are potentially a valuable bioproduct feedstock
D. viridis lacked any available sequence that might code for a urea transporter, and does not appear to possess a sufficient set of enzymes needed for urea conversion (Table 1)
It is unlikely to be the case that Dunaliella spp. can access or assimilate the carbon from Gln and Cys, as we found no evidence that these amino acids cross the cell membrane

Summary

Introduction

Autotroph algae have gained attention in recent years because they are potentially a valuable bioproduct feedstock These organisms could produce large quantities of high value proteins, chemicals, or combustible hydrocarbons on a relatively small acreage, and may operate with less environmental impact than conventional agriculture or fossil fuels (Wijffels and Barbosa, 2010; Georgianna and Mayfield, 2012; Rasala and Mayfield, 2015). The viability of growing algae at scale for any purpose is limited by the requirements these organisms have for nitrogen and phosphorous fertilizers The demand for these fertilizers is such that if only 10% of transportation fuels needed in the US for 2010 were replaced by algal biodiesel, it would require an amount of nitrogen fertilizer equivalent to 175% of total annual US production (Grobbelaar, 2004; Chisti, 2013). This is considerable, as the production of synthetic N-fertilizer via the Haber-Bosch process is reliant on

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Conclusion