Influence of nitrogen source on photochemistry and antenna size of the photosystems in marine green macroalgae, Ulva lactuca.

Ulva lactuca is regarded as a prospective energy crop for biorefinery owing to its affluent biochemical composition and high growth rate. In fast-growing macroalgae, biomass development strictly depends on external nitrogen pools. Additionally, nitrogen uptake rates and photosynthetic pigment content vary with type of nitrogen source and light conditions. However, the combined influence of nitrogen source and light intensity on photosynthesis is not widely studied. In present study, pale green phenotype of U. lactuca was obtained under high light (HL) condition when inorganic nitrogen (nitrate) in the media was substituted with organic nitrogen (urea). Further, pale green phenotype survived the saturating light intensities in contrast to the normal pigmented control which bleached in HL. Detailed analysis of biochemical composition and photosynthesis was performed to understand functional antenna size and photoprotection in pale green phenotype. Under HL, urea-grown cultures exhibited increased growth rate, carbohydrate and lipid content while substantial reduction in protein, chlorophyll content and PSII antenna size was observed. Further, in vivo slow and polyphasic chlorophyll a (Chl a) fluorescence studies revealed reduction in excitation pressure on PSII along with low non-photochemical quenching thus, transmitting most of the absorbed energy into photochemistry. The results obtained could be correlated to previous report on cultivation of U. lactuca through saturating summer intensities (1000 µmole photons m-2 s-1) in urea based: poultry litter extract (PLE). Having proved critical role of urea in conforming photoprotection, the application PLE was authenticated for futuristic, sustainable and year-round biomass cultivation.