Flux balance analysis of Chlorella sp. FC2 IITG under photoautotrophic and heterotrophic growth conditions

Abstract

Quantification of carbon flux distribution in the metabolic network of microalgae remains important to understand the complex interplay between energy metabolism, carbon fixation, and assimilation pathways. This is even more relevant with respect to cyclic metabolism of microalgae under light-dark cycle. In the present study, flux balance analysis (FBA) was carried out for an indigenous isolate Chlorella sp. FC2 IITG under photoautotrophic and heterotrophic growth conditions. A shift in intracellular flux distribution was predicted during transition from nutrient sufficient phase to nutrient starvation phase of growth. Further, dynamic flux analysis (dFBA) was carried out to capture light-dark metabolism over discretized pseudo steady state time intervals. Our key findings include the following: (i) unlike heterotrophic condition, oxidative pentose phosphate (PP) pathway, and Krebs cycle were relatively inactive under photoautotrophic growth; (ii) in both growth conditions, while transhydrogenation reaction was highly active, glyoxalate shunt was found to be nonoperative; (iii) flux distribution during transition period was marked with up regulation of carbon flux toward nongrowth associated (NGA) maintenance energy, oxidative phosphorylation, and photophosphorylation; (iv) redirection of carbon flux from polysaccharide and neutral lipid resulted in up regulation of Krebs cycle flux in the dark phase; (v) elevated glycolytic and acetyl-CoA flux were coupled with induction of neutral lipid during light cycle of the growth; (vi) significantly active photophosphorylation in the light phase was able to satisfy cellular energy requirement without need of oxidative PP pathway; and (vi) unlike static FBA, dFBA predicted an unaltered NGA maintenance energy of 1.5 mmol g(-1) DCW h(-1).

Advances in metabolic modeling of oleaginous microalgae

“…[39], C. reinhardtii [23], and Chlorella sp. [79]. Mixotrophic growth, as a kind of hybrid condition, has shown biomass yields falling between ones observed for autotrophic and heterotrophic.…”

“…Overcoming the steady-state assumption of standard FBA is vital for the modeling of highly dynamic systems, which are characteristic of photosynthetic microorganisms [33, 37, 79]. These organisms have evolved under cyclic light/dark conditions, which require switching between different phenotypic states.…”

Genome-Scale Metabolic Model for the Green Alga Chlorella vulgaris UTEX 395 Accurately Predicts Phenotypes under Autotrophic, Heterotrophic, and Mixotrophic Growth Conditions

“…(Table II) assume that the fluxes differ between light and dark only by the layout of the core carbon network (tricarboxylic acid, glycolysis, and PPP), and the rest of the metabolism (synthesis of amino acids, DNA, and RNA) does not vary significantly in terms of relative fluxes (Yang et al, 2000;Muthuraj et al, 2013;Wu et al, 2015). Despite previous reports that assume the lack of change in the flux distribution for nucleotide metabolism (Yang et al, 2000;Muthuraj et al, 2013;Wu et al, 2015), we found that fluxes vary widely depending on the growth condition. Our results suggest that the anabolic part of the metabolism is not independent of growth conditions, and iCZ843 provides an accurate qualitative and quantitative analysis of the metabolism.…”

“…The relation of nucleotide composition (28 RNA/DNA) was taken from Muthuraj et al (2013). Literature and experimental data were normalized assuming an idealized size and weight for C. vulgaris (see below).…”

“…FC2 IITG, C. protothecoides, Chlorella variabilis, and Chlorella pyrenoidosa. These smaller networks are limited in scope and contain only core carbon and photosynthetic metabolism, with an average of 500 reactions and 390 metabolites (Yang et al, 2000;Muthuraj et al, 2013;Wu et al, 2015;Juneja et al, 2016). Here, we report the reconstruction of a genome-scale metabolic network for C. vulgaris UTEX 395 (iCZ843) based on the recently assembled genome sequence.…”