Photoprotection mechanisms of Nannochloropsis oceanica in response to light stress

Light is a pivotal environmental factor that affects the growth and productivity of photosynthetic organisms. Microalgae have evolved complex photoprotective mechanisms to circumvent damage from high light. Nannochloropsis produce approximately 4-5% eicosapentaenoic acid (EPA) of dry cell weight (dwt) under favorable growth conditions, and accumulate triacylglycerol (TAG) at up to 60% of dwt under nutrient-depletion and high light intensities. Nannochloropsis is becoming a strong research model for oleaginous microalgae and an excellent candidate for biofuel production. To study the photoprotective mechanisms of Nannochloropsis, proteomic and physiological dynamics were tested under different light intensities. When exposed to high light stress N. oceanica IMET1 cells showed a 45% and 38% decrease in total protein and sugar content, respectively, and a 44% increase in total lipid content; the growth of cells was not affected. Cell pigment content varied substantially: chlorophyll a and violaxanthin concentrations decreased by > 50%, while beta-carotene, zeaxanthin, astaxanthin, and canthaxanthin increased transiently by > 2-fold. Isobaric tags for relative and absolute quantification (iTRAQ) was used to label samples, and 452 proteins were identified from tandem mass spectrometry data. In response to high light stress, 150 of these proteins were significantly up-regulated and 58 were significantly down-regulated. The proteins were involved in pathways that include photosynthesis, pyruvate metabolism, and fatty acid biosynthesis. Results also indicate that photochemical adjustments in carbon metabolism, in particular, glycolysis, the Calvin cycle, and the citric acid (TCA) cycle, were activated to divert more acetyl-CoA towards carotenoid biosynthesis. This could be to scavenge reactive oxygen species to alleviate photodamage. A regulatory photoprotective model for N. oceanica IMET1 was proposed, which includes strategies of down-regulating light-harvesting antenna proteins, up-regulating oxidases, enhancing de-epoxidation within the xanthophyll cycle, accumulation of antioxidants, modulation of photosynthetic apparatus, and repartitioning of photosynthetic carbon into storage carbon.