Superior triacylglycerol (TAG) accumulation in starchless mutants of Scenedesmus obliquus: (I) mutant generation and characterization

Abstract

BackgroundMicroalgae are a promising platform for producing neutral lipids, to be used in the application for biofuels or commodities in the feed and food industry. A very promising candidate is the oleaginous green microalga Scenedesmus obliquus, because it accumulates up to 45% w/w triacylglycerol (TAG) under nitrogen starvation. Under these conditions, starch is accumulated as well. Starch can amount up to 38% w/w under nitrogen starvation, which is a substantial part of the total carbon captured. When aiming for optimized TAG production, blocking the formation of starch could potentially increase carbon allocation towards TAG. In an attempt to increase TAG content, productivity and yield, starchless mutants of this high potential strain were generated using UV mutagenesis. Previous studies in Chlamydomonas reinhardtii have shown that blocking the starch synthesis yields higher TAG contents, although these TAG contents do not surpass those of oleaginous microalgae yet. So far no starchless mutants in oleaginous green microalgae have been isolated that result in higher TAG productivities.ResultsFive starchless mutants have been isolated successfully from over 3,500 mutants. The effect of the mutation on biomass and total fatty acid (TFA) and TAG productivity under nitrogen-replete and nitrogen-depleted conditions was studied. All five starchless mutants showed a decreased or completely absent starch content. In parallel, an increased TAG accumulation rate was observed for the starchless mutants and no substantial decrease in biomass productivity was perceived. The most promising mutant showed an increase in TFA productivity of 41% at 4 days after nitrogen depletion, reached a TAG content of 49.4% (% of dry weight) and had no substantial change in biomass productivity compared to the wild type.ConclusionsThe improved S. obliquus TAG production strains are the first starchless mutants in an oleaginous green microalga that show enhanced TAG content under photoautotrophic conditions. These results can pave the way towards a more feasible microalgae-driven TAG production platform.

Photosynthetic efficiency and carbon partitioning in nitrogen-starved Scenedesmus obliquus

“…This is supported by the fact that fatty acid synthesis and starch synthesis are simultaneously active, and share the products of the Calvin cycle as a common carbon pre-cursor. Furthermore, fatty acid synthesis can readily take over the carbon flux through the starch synthesis pathway when this pathway is eliminated in starchless mutants of S. obliquus [6,37]. Finally, it is unlikely that starch or fatty acid synthesis is limited by the capacity of their synthesis machinery during nitrogen starvation.…”

Increased lipids production of Nannochloropsis oculata and Chlorella vulgaris for biodiesel synthesis through the optimization of growth medium composition arrangement by using bicarbonate addition

“…When [HCO3 -] is added to the cultivation medium, the carbon source will increase to bind to the Rubisco enzyme (Ribulose-1,5-biphosphate), resulting in G3P synthesis (glyceraldehyde-3-phosphate or triose-3-phospate) at The Calvin cycle will increase. Increased of G3P will increase the formation of glucose and lipids, because 28 G3P molecules will produce 14 molecules of glucose and 1 molecule TAG C18 [13]. According to Agustin, glucose is used as a source of energy for cell growth, so that biomass increases [11].…”

The fatter the better : selecting microalgae cells for outdoor lipid production

“…Microalgae technology has been focusing on growth and product formation via process optimization with a limited number of strains 121 . Behind this lies the idea that a marriage between process engineering and biology would bring microalgae for commodities into reality 12 . Such marriage never took place 2 , but countless knowledge has been accumulated on reactor design 116,122 , reactor operation and on microalgae cell biology 74,[123][124][125][126] .…”

“…Developing strains with superior performance can be achieved with multiple approaches: selection of better strains 10 , adaptive laboratory evolution 11 , random mutagenesis 12 and genetic manipulation 13 , or combination(s) of these 9,14 .…”

“…Process optimization is used to maximize the production of a certain compound using already available strains 31 . Strain improvement is used to select cells that display an increased baseline production of a certain compound, which can be achieved via breeding and artificial selection 13,16,32 and/or mutagenesis 12,14,21 . A combination of process optimization and strain improvement could significantly increase productivity and reduce production costs.…”