Nitrate and Phosphate Regimes Induced Lipidomic and Biochemical Changes in the Intertidal Macroalga Ulva lactuca (Ulvophyceae, Chlorophyta)

This study was carried out in order to understand the lipid and biochemical alterations resulting from different nutritional regimes of nitrate and phosphate in Ulva lactuca. The algal thalli cultured in artificial seawater (ASW) showed higher levels of carbohydrates and non-polar lipids and increased phosphatase activities, accompanied by degradation of polar lipids, proteins and pigments. Further, higher levels of lipid hydroperoxides indicated reative oxygen species (ROS)-mediated non-enzymatic lipid peroxidation due to nutritional limitation-induced oxidative stress. Those thalli cultured in ASW supplemented with nitrate showed responses corresponding to nitrate addition, such as an increase in pigments, monogalactosyldiacylglycerols, polyunsaturated fatty acids and nitrate reductase. In addition, these thalli showed partial induction of phosphatases, low phospholipids, and high sulfolipid and 1,2-diacylglyceryl-3-O-4'-(N,N,N-trimethyl)-homoserine (DGTS) due to phosphate limitation. Similarly, algal thalli cultured in ASW supplemented with phosphate showed down-regulation of phosphatases, an increase in phospholipids due to availability of phosphate as well as a decrease in nitrate reductase, pigment, monogalactosyldiacylglycerols and polyunsaturated fatty acids due to nitrate limitation. On the other hand, algal thalli cultured in ASW supplemented with both nitrate and phosphate showed recovery of lost pigments and proteins, a high monogalactosyldiacylglycerol/digalactosyldiacylglycerol ratio, high unsaturation and high oxylipin levels (both C18 and C20). Further, the accumulation of indole-3-acetic acid in nutrient-limited thalli and of kinetin and kinetin riboside in nutrient-supplemented thalli indicated their antagonistic roles under nutrient stress. Thus, U. lactuca copes with nitrate and phosphate nutritional stress by altering the metabolic pathways involved in lipid biosynthesis including a shift in lipid classes, fatty acids, oxylipins and indole-3-acetic acid/kinetin cross-talk.