Environmental influences on the docosahexaenoate content of the triacylglycerols and phosphatidylcholine of a heterotrophic, marine dinoflagellate, Crypthecodinium cohnii

DINOFLAGELLATE HOST-PARASITE STEROL PROFILES DICTATE KARLOTOXIN SENSITIVITY(1)

We examined the sterol profile of Karlodinium veneficum (D. Ballant.) J. Larsen, Akashiwo sanguinea (Hiraska) Ge. Hansen et Moestrup, Alexandrium tamarense (M. Lebour) Balech, Alexandrium affine (H. Inoue et Fukuyo) Balech, Gonyaulax polygramma F. Stein, and Gymnodinium instriatum (Freud. et J. J. Lee) Coats, along with their Amoebophyra parasites. There were no consistent sterol profiles that characterized the genus Amoebophyra. Instead, in five out of six comparisons, the host and parasite sterol profiles where highly correlated. The one exception, Amoebophyra sp. ex Alex. tamarense, was least like its host in sterol profile and also possessed the widest host range for infection. There was little correlation between host and parasite in fatty acid profiles, with the parasite being deficient in fatty acids characteristic of the plastid [e.g., 18:5(n-3) associated with galactolipids of the thylakoids, as previously published by Adolf et al. (2007)]. Those hosts and parasites with sterol profiles dominated by desmethyl sterols were most sensitive to karlotoxin toxicity. In the host-parasite pairs most sensitive to karlotoxin addition, recovery of the intact karlotoxin molecule was poorest. Given the sensitivity to karlotoxin, some species of Amoebophyra may avoid infection of K. veneficum.

Effect of n-dodecane on Crypthecodinium cohnii fermentations and DHA production

The potential use of n-dodecane as an oxygen vector for enhancement of Crypthecodinium cohnii growth and docosahexaenoic acid (DHA) production was studied. The volumetric fraction of oxygen vector influenced the gas-liquid volumetric mass transfer coefficient k (L) a positively. The k (L) a increased almost linearly with the increase of volumetric fraction of n-dodecane up to 1%. The stirring rate showed a higher influence on the k (L) a than the aeration rate. The effects of this hydrocarbon on C. cohnii growth and DHA production were then investigated. A control batch fermentation without n-dodecane addition (CF) and a batch fermentation where n-dodecane 1% (v/v) was added (DF) were carried out simultaneously under the same experimental conditions. It was found that, before 86.7 h of fermentation, the biomass concentration, the specific growth rate, the DHA, and total fatty acids (TFA) production were higher in the CF. After this fermentation time, the biomass concentration, the DHA and TFA production were higher in the DF. The highest DHA content of biomass (6.14%), DHA percentage of TFA (51%), and DHA production volumetric rate r (DHA) (9.75 mg l(-1 )h(-1)) were obtained at the end of the fermentation with n-dodecane (135.2 h). The dissolved oxygen tension (DOT) was always higher in the DF, indicating a better oxygen transfer due to the oxygen vector presence. However, since the other C. cohnii unsaturated fatty acids percentages did not increase with the oxygen availability increase due to the n-dodecane presence, a desaturase oxygen-dependent mechanism involved in the C. cohnii DHA biosynthesis was not considered to explain the DHA production increase. A selective extraction through the n-dodecane was suggested.

Analysis of docosahexaenoic acid biosynthesis in Crypthecodinium cohnii by 13C labelling and desaturase inhibitor experiments

The lipids of the heterotrophic microalga Crypthecodinium cohnii contain the omega-3 polyunsaturated fatty acid (PUFA) and docosahexaenoic acid (22:6) to a level of over 30%. The pathway of 22:6 synthesis in C. cohnii is unknown. The ability of C. cohnii to use 13C-labelled externally supplied precursor molecules for 22:6 biosynthesis was tested by 13C NMR analysis. Furthermore, the presence of desaturases (typical for aerobic PUFA synthesis) was studied by the addition of specific desaturase inhibitors in the growth medium. The addition of 1-(13)C acetate or 1-(13)C butyrate in the growth medium resulted in 22:6 with only the odd carbon atoms enriched. Apparently, two-carbon units were used as building blocks for 22:6 synthesis and butyrate was first split into two-carbon units prior to incorporation in 22:6. When 1-(13)C oleic acid was added to the growth medium, 1-(13)C oleic acid was incorporated into the lipids of C. cohnii but was not used as a precursor for the synthesis of 22:6. Specific desaturase inhibitors (norflurazon and propyl gallate) inhibited lipid accumulation in C. cohnii. The fatty acid profile, however, was not altered. In contrast, in the arachidonic acid-producing fungus, Mortierella alpina, these inhibitors not only decreased the lipid content but also altered the fatty acid profile. Our results can be explained by the presence of three tightly regulated separate systems for the fatty acid production by C. cohnii, namely for (1). the biosynthesis of saturated fatty acids, (2). the conversion of saturated fatty acids to monounsaturated fatty acids and (3). the de novo synthesis of 22:6 with desaturases involved.

Purification and properties of the fatty acid synthetase complex from the marine dinoflagellate, Crypthecodinium cohnii

De novo biosynthesis of fatty acids in the heterotrophic marine dinoflagellate, Crypthecodinium cohnii, has been studied in vitro. Fatty acid synthetase was located in the cytosol and its activity was dependent on acetyl-CoA, malonyl-CoA, NADPH2 and NADH2. The enzyme was purified 100-fold using ion-exchange chromatography on DEAE-Sephadex A-25, adsorption to hydroxyapatite and gel filtration on Sepharose 4B columns. Very active endogenous proteases were separated from the fatty acid synthetase at the first step of purification. The purified enzyme had a molecular weight of about 400000, as judged from gel filtration, sucrose density gradient centrifugation and polyacrylamide gel electrophoresis under non-denaturing conditions. Polyacrylamide gel electrophoresis under denaturing conditions in the presence of SDS and urea revealed one major protein band of Mr 180000, suggesting that the enzyme is composed of two multifunctional subunits of apparently identical molecular weight. Reaction products of the C. cohnii fatty acid synthetase are free fatty acids due to the presence of a thioesterase activity in the purified enzyme complex. The main product is palmitate. Docosahexaenoic acid (C22:6, n-3), the major fatty acid component of C. cohnii lipids, is not directly synthesized by the enzyme.

Dinosterol in model membranes: fluorescence polarization studies

The interaction between the phospholipids of Crypthecodinium cohnii, a heterotropic marine dinoflagellate, and its major sterol dinosterol is less soluble in model membranes and as effective in increasing the microviscosity. These results indicate that the unique side chain of dinosterol does not play a special role in terms of complementary interaction with the phospholipids of this organism.