RISING WATER TEMPERATURES ALTER LIPID DYNAMICS AND REDUCE N‐3 ESSENTIAL FATTY ACID CONCENTRATIONS IN SCENEDESMUS OBLIQUUS (CHLOROPHYTA)1

The biosynthesis of nutritionally important polyunsaturated fatty acids (PUFAs) in phytoplankton is influenced by environmental temperature. We investigated the potential of climate warming to alter lipid dynamics of Scenedesmus obliquus (Turpin) Kütz. by comparing lipid and fatty acid (FA) profiles as well as FA metabolism (using 1‐¹⁴C] acetate) at 20°C and 28°C. We documented an overall decline (53%–37%) in the proportion of n‐3 PUFA (in particular, of α‐linolenic acid ALA; 18:3n‐3]), and a concomitant increase in saturated fatty acids (SAFAs) in total lipids (TLs) at 28°C, consistent with enhanced incorporation of radioactivity from 1‐¹⁴C] acetate into total 16:0, 18:1, and decreased incorporation into 18:2 and 18:3 FA (from 36% to 22% of the total) at 28°C. Glycerophospholipids were also affected by warming; ALA and stearidonic acids (SDAs; 18:4n‐3) both decreased (by 13% and 15%, respectively) in phosphatidylcholine (PC) and (by 24% and 20%, respectively) in phosphatidylethanolamine (PE). The characteristic FA in phosphatidylglycerol (PG; 16:1n‐13t) increased (by 22%) at 28°C. The activities of desaturases, which add double bonds to FA moieties, comprised the major suite of reactions affected by the temperature increase in TL and polar lipid (PL) classes. Climate modelers predict an increase in the number of extreme heat days in summer at temperate latitudes, with parallel projected increases in water temperatures of shallow water bodies. Our results suggest that the overall decrease in the essential n‐3 FA ALA in S. obliquus at higher water temperatures may lower food quality for higher tropic levels, adding another climate‐warming stress.