Consequences of Mixotrophy on Cell Energetic Metabolism in Microchloropsis gaditana Revealed by Genetic Engineering and Metabolic Approaches

Abstract

Algae belonging to the Microchloropsis genus are promising organisms for biotech purposes, being able to accumulate large amounts of lipid reserves. These organisms adapt to different trophic conditions, thriving in strict photoautotrophic conditions, as well as in the concomitant presence of light plus reduced external carbon as energy sources (mixotrophy). In this work, we investigated the mixotrophic responses of Microchloropsis gaditana (formerly Nannochloropsis gaditana). Using the Biolog growth test, in which cells are loaded into multiwell plates coated with different organic compounds, we could not find a suitable substrate for Microchloropsis mixotrophy. By contrast, addition of the Lysogeny broth (LB) to the inorganic growth medium had a benefit on growth, enhancing respiratory activity at the expense of photosynthetic performances. To further dissect the role of respiration in Microchloropsis mixotrophy, we focused on the mitochondrial alternative oxidase (AOX), a protein involved in energy management in other algae prospering in mixotrophy. Knocking-out the AOX1 gene by transcription activator-like effector nuclease (TALE-N) led to the loss of capacity to implement growth upon addition of LB supporting the hypothesis that the effect of this medium was related to a provision of reduced carbon. We conclude that mixotrophic growth in Microchloropsis is dominated by respiratory rather than by photosynthetic energetic metabolism and discuss the possible reasons for this behavior in relationship with fatty acid breakdown via β-oxidation in this oleaginous alga.

Mixotrophy in a Local Strain of Nannochloropsis granulata for Renewable High-Value Biomass Production on the West Coast of Sweden

“…From day 6, a decrease in photosynthetic performance was detected in all tested samples, but this decrease was significantly more pronounced in mixotrophy from day 10. The decrease in Fv/Fm during mixotrophic growth has been previously reported for Nannochloropsis gaditana and the diatom Phaeodactylum tricornutum [11,19]. The enhanced growth together with the decrease in photosynthesis from day 10 in mixotrophic samples indicate a switch from the phototrophic to the mixotrophic regime.…”

“…Previous studies have investigated the mixotrophic growth of the Nannochloropsis genus using various external organic carbon sources [ 11 , 12 , 13 , 14 ], but only a few have explored the industrial potential of growing Nannochloropsis under this growth mode [ 12 , 14 ]. Ng has been previously proven as an industrially relevant strain due to its high biomass and its lipid productivity that is compatible with outdoor production during the summer season in Sweden [ 17 ].…”

“…Mixotrophic growth of the Nannochloropsis genus (i.e., Nannochloropsis gaditana and Nannochloropsis salina ) been investigated using glucose, glycerol, and acetate as external organic carbon sources or by adding the bacteria-rich medium Lysogeny broth (LB) [ 11 , 12 , 13 , 14 ]. However, only a handful of research projects have focused on understanding the industrial potential of growing Nannochloropsis under this growth mode [ 12 , 14 ].…”

Astaxanthin and eicosapentaenoic acid production by S4, a new mutant strain of Nannochloropsis gaditana

“…As reported in Table 3, the highest biomass productivity was observed at 500 µmol m −2 s −1 . The addition of glucose did not significantly influence cell growth, consistently with recent findings in this species [33]. The fraction of ketocarotenoids (Table 3) and astaxanthin per dry weight were similar in all conditions tested, with the exception of the highest irradiance used (2000 µmol m −2 s −1 ), where these values were reduced.…”

Metabolism of Scenedesmus obliquus cultivated with raw plant substrates

“…We explored differential expression within mitochondrial oxidative phosphorylation, photosynthesis, and carbon fixation pathways, particularly during late-log phase growth in ambient CO 2 and mid-log in 1% CO 2, to determine if there were any discernable patterns in the overarching energy metabolism of S. obliquus when switchgrass was present. It has been reported under mixotrophic conditions, the metabolism of some algae, like N. gaditana and C. reinhardtii, is dominated by respiration with an associated decrease in photosynthetic activity (Chapman et al, 2015;Bo et al, 2021). Differently, photosynthesis in C. sorokiniana is largely unaffected by the addition of acetate (Cecchin et al, 2018).…”