| Summary: | Many marine animals accumulate large amounts of lipid in their livers, muscles and other body tissues. Alkyldiacylglycerols and wax esters are two examples of unusual lipid classes which are stored as a large percentage of the total body lipids in some marine species, these lipids being unusual only in the sense that they do not occur in such high proportions in the body lipids of terrestrial species. 2. Several roles have been proposed for the marine lipid reserves generally and the neutral lipid classes alkyldiacylglycerols and wax esters in particular. It is considered that special biochemical mechanisms operate in species which store these lipid classes in quantity. The biosynthesis of their mutual precursor, fatty alcohol, is considered to be fundamental to these mechanisms. 3. In order to study the biosynthesis of alkyldiacylglycerols and wax esters in more detail, two animals typical of marine groups which accumulate these lipids were chosen as a source of experimental material. The dogfish, Squalus aoanthias, contains alkyldiacylglycerols that comprise about 40% of the total liver lipid which itself accounts for about 60% of the wet weight of the liver. The calanoid copepod, Euchaeta norvegiaa, contains in its body an oil sac where wax esters comprise 60% of the total lipid. The body lipid of this animal accounts for about 20% of the dry weight. 4. In vivo and in vitro incorporations of radioactively labelled precursors were used to study routes of neutral lipid biosyntheses in dogfish, tissues, especially liver. It was found that liver rapidly oxidised fatty alcohol to fatty acid which, was then exported to other tissues, especially muscle. Triacylglycerols were readily formed in liver and intestinal tissue from fatty acid precursors. 5. Wax esters were also readily biosynthesised in preparations of dogfish liver tissue, such biosynthesis requiring the presence of ATP and CoA. The formation of wax esters was enhanced by the addition of fatty alcohol to incubations of liver tissue. 6. A feature of the in in vitro work with dogfish tissues was the lack of a significant rate of biosynthesis of alkyldiacylglycerols compared to the other neutral lipids. There was also a very low rate of formation of fatty alcohol from fatty acid precursor and it is postulated that biosynthesis of fatty alcohol with consequent accumulation of alkyldiacylglycerols may be dependent on suitable reducing conditions existing in the cells,not realised in these experiments. Conditions must exist in the natural environment of the animal to facilitate the biosynthesis ofalkyldiacylglycerols for storage in the liver. Although this lipid class may be used as an energy reserve by dogfish, it is thought that its accumulation in the liver of this animal may be significant in buoyancy regulation. 7. Cell-free studies with preparations of whole Euohaeta showed that precursor fatty acid and fatty alcohol were readily incorporated into wax esters. Biosynthesis of wax esters was again found to be dependent on the presence of ATP and CoA. The presence of fatty alcohol but not fatty acid greatly enhanced production of wax esters. 8. Similar experiments with, cell-free preparations from Calanus finmarchicus and Acartia sp. confirmed, the Euchaeta results and showed additionally that Biosynthesis of fatty alcohol in calanoid copepods is dependent on the presence of reduced pyridine nucleotides. Thus, in common with the dogfish, the accumulation of a major lipid class was thought to require the presence of biosynthesised fatty alcohol. 9. A mechanism is presented which allows for the efficient conversion of glucose and fatty acids into wax esters in response to an intermittent high quantity of phytoplankton food for temperate species of copepod. It is argued that the limitations, normally present in lipid biosynthesis are eliminated by this mechanism so that a very extensive conversion of phytoplankton food to lipid may take place in a relatively short time. Wax esters are looked upon as long term energy reserves in calanoid copepods in place of the more usual triacylglycerols.
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