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Isoprene Emission in Darkness by a Facultative Heterotrophic Green Alga - PubMed

  • ️Wed Jan 01 2020

Isoprene Emission in Darkness by a Facultative Heterotrophic Green Alga

K G Srikanta Dani et al. Front Plant Sci. 2020.

Abstract

Isoprene is a highly reactive biogenic volatile hydrocarbon that strongly influences atmospheric oxidation chemistry and secondary organic aerosol budget. Many phytoplanktons emit isoprene like terrestrial pants. Planktonic isoprene emission is stimulated by light and heat and is seemingly dependent on photosynthesis, as in higher plants. However, prominent isoprene-emitting phytoplanktons are known to survive also as mixotrophs and heterotrophs. Chlorella vulgaris strain G-120, a unicellular green alga capable of both photoautotrophic and heterotrophic growth, was examined for isoprene emission using GC-MS and real-time PTR-MS in light (+CO2) and in darkness (+glucose). Chlorella emitted isoprene at the same rate both as a photoautotroph under light, and as an exclusive heterotroph while feeding on exogenous glucose in complete darkness. By implication, isoprene synthesis in eukaryotic phytoplankton can be fully supported by glycolytic pathways in absence of photosynthesis, which is not the case in higher plants. Isoprene emission by chlorophyll-depleted mixotrophs and heterotrophs in darkness serves unknown functions and may contribute to anomalies in oceanic isoprene estimates.

Keywords: Chlorella vulgaris; chlorophyll; glycolysis; heterotrophy; marine isoprene; photosynthesis in unicellular eukaryotes; volatile hydrocarbons.

Copyright © 2020 Dani, Torzillo, Michelozzi, Baraldi and Loreto.

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Figures

Figure 1
Figure 1

Light and dark isoprene emission by Chlorella vulgaris (A) on chlorophyll (Chl) basis, (B) on culture dry weight (DW) basis. Emission in the light was measured from phototrophic cultures (minimal mineral medium). Emission in the dark was measured from heterotrophic cultures (glucose medium; N ≥ 6 technical replicates, t test, α = 0.05).

Figure 2
Figure 2

Illustration of proposed interactions among glycolytic pathways, photosynthesis, and isoprene biosynthesis in (A) photoautotrophic eukaryotes in continous light (B) heterotrophic eukaryotes in continuous darkness. Metabolic cross-talk among CBB cycle (active only in light), the three glycolytic pathways namely the OPP pathway, the ED pathway, and cytosolic glysolysis, is indicated by prominent arrows. The thickness of arrows indicate the potential flux strength. The CBB cycle and glycolytic pathways co-contribute GAP, pyruvate to the MEP pathway and isoprene synthesis in light. The three glycolytic pathways sustain carbon and energy supply for isoprene synthesis in darkness. CBB, Calvin-Benson-Bassham; OPP, oxidative pentose phosphate; G-6-P, glucose-6-phosphate; ED, Entner-Doudoroff; 6-P-gluconate, 6-phosphogluconate; KDPG, 2-keto-3-deoxy-6-phosphogluconate; Ribulose-5-P, ribulose-5-phosphate; 3-PGA, 3-phosphoglycerate; 2-PGA, 2-phosphoglycerate; GAP, glyceraldehyde-3-phosphate; PEP, 2-phosphoenol pyruvate; MEP, 2-C-methyl-D-erythritol 4-phosphate; and DMADP, dimethylallyl diphosphate.

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