PHOTOSYNTHESIS IN THE DEEP BLUE SEA: PHOTOPHYSIOLOGICAL RESPONSES OF PHAEODACTYLUM TRICORNUTUM TO DYNAMIC AND STATIC IRRADIANCES
Journal of Phycology
Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically with Storeatula major (Cryptophyceae) as prey. G. galatheanum , like many photosynthetic dinoflagellates, contains high amounts of n-3 long-chain-polyunsaturated fatty acids (LC-PUFA) such as docosahexaenoic acid (DHA, 22:6n-3) and the hemolytic toxic fatty acid 18:5n-3. We hypothesize that a benefit of phagotrophy in G. galatheanum is the acquisition of precursor linolenic acid (18:3n-3) that fuels
... -PUFA synthesis. Phototrophs grew at 0.37 d Ϫ 1 , while mixotrophs grew at 0.40 d Ϫ 1 with a feeding rate of 0.62 d Ϫ 1 . Photosynthesis was lower in mixotrophs (3.7 pg C cell Ϫ 1 h Ϫ 1 ) than phototrophs (4.9 pg C cell Ϫ 1 h Ϫ 1 ). DHA levels were higher in mixotrophs [3.7 ( ϩ / Ϫ 0.11) pg cell Ϫ 1 ] than phototrophs [3.0 ( ϩ / Ϫ 0.16) pg cell Ϫ 1 ] and prey [0.4 ( ϩ / Ϫ 0.01) pg cell Ϫ 1 ]. 18:5n-3 levels [1.7 ( ϩ / Ϫ 0.03) pg cell Ϫ 1 ] were similar in phototrophs and mixotrophs. An intermediate in n-3 LC-PUFA synthesis, 20:4n-3, accumulated in mixotrophs [0.6 ( ϩ / Ϫ 0.27) pg cell Ϫ 1 ] relative to phototrophs (not detected) and prey [0.03 ( ϩ / Ϫ 0.002) pg cell Ϫ 1 ]. Low ratios of linolenic acid to DHA in phototrophic G. galatheanum (0.14) relative to mixotrophic G. galatheanum (0.29) and prey (2.14) are consistent with substrate limitation of LC-PUFA synthesis in phototrophs. Accumulation of 20:4n-3 suggests incomplete conversion of linolenic acid to DHA, possibly due to conditions in batch culture. We conclude that precursors for n-3 LC-PUFA biosynthesis in G. galatheanum may be acquired through ingestion of S. major , and may partially control feeding/photosynthesis in mixotrophic populations.