Isoprene Emission in Darkness by a Facultative Heterotrophic Green Alga.

K G Srikanta Dani,Francesco Loreto,Giuseppe Torzillo,Rita Baraldi,Marco Michelozzi

doi:10.3389/fpls.2020.598786

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.

Highlights

Isoprene, the most prominent of all biogenic volatile organic compounds (BVOCs) in the atmosphere, influences atmospheric oxidation status and secondary aerosol yield over terrestrial ecosystems globally, and over marine emission hotspots locally (Henze et al, 2008; Wang and Ruiz, 2017)
The heterotrophic cultures grown on glucose in darkness contained 0.42 ± 0.06% Chl DW−1 after up to 6 days of dark acclimation, and they were significantly chlorophylldepleted compared to light-grown photoautotrophic cultures (2.6 ± 0.3% Chl DW−1; N = 6 independent cultures each, p < 0.05; t test)
Glycolysis is the primary source of carbon and energy for all anabolic processes including isoprene synthesis in heterotrophic Chlorella growing in darkness

Summary

Introduction

The most prominent of all biogenic volatile organic compounds (BVOCs) in the atmosphere, influences atmospheric oxidation status and secondary aerosol yield over terrestrial ecosystems globally, and over marine emission hotspots locally (Henze et al, 2008; Wang and Ruiz, 2017). Light- and heat-responsive isoprene emissions have been the norm for known isoprene-emitting phytoplankton (Exton et al, 2013; Dani et al, 2017). Field-based estimations of isoprene from marine and freshwater sources routinely rely on correlations between chlorophyll abundance, photosynthesis, and species distribution (Hackenberg et al, 2017; Booge et al, 2018; Steinke et al, 2018; Rodríguez-Ros et al, 2020). Many prominent marine isoprene emitters, such as dinoflagellates and coccolithophores, are facultative heterotrophs and mixotrophs (Johnson, 2015; Dani and Loreto, 2017). Mixotrophic and heterotrophic growths often lead to chlorophyll depletion in such organisms (Terrado et al, 2017), and the presumed equations among chlorophyll abundance, photosynthesis, species distribution, and isoprene emission in mixed-ocean layers become tenuous. It was important to verify if a unicellular eukaryote could emit isoprene in absence of photosynthesis

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