The Initial Research On Marine Yeasts With High-protein Content

In recent years the maricultural industry of China has been developing very rapidly with its cultured sea-food yield becoming the highest in the world. Therefore, the demand of feed for cultured marine animals is increasing steadily. However, there is still a lack of feed which is composed of single cell protein (SCP) from marine yeasts with high content of protein and other nutrients. Although fish powder is a very important protein source of marine animal, with the development of maricultural industry, the demand for fish powder and the price of it increase every year, which prevents the development of maricultural industry. In order to find a new protein source for the sea-food, the screening of marine yeast with high protein-conent was done in this study. After the nucleic acid content, the fatty acids composition and the amino acid composition were valued, we found that strain G7a, one of the marine yeasts with high protein-content, could produce high content of protein on a wide range of carbon sources, including inulin from Jerusalem artichoke.Total 327 marine yeast strains from seawater, sediments, mud of salterns, guts of the marine fish and marine algae were obtained. After crude protein of the yeasts was estimated by the method of Kjehldahl, eight strains of the marine yeasts with high-protein content were found. The biomass(g/l)and protein content (w/w) of the eight marine yeasts are as follows: YA03a: 2.1, 57.4; W6b: 3.5, 49.8; N3c: 7.1, 42.5; N11b: 8.1, 40.4; G7a: 5.3, 41.4; N17: 9.3, 39.2; G7b: 2.9, 33.5; 4#2: 7.9, 30.4. The results of routine identification and molecular methods show that YA03a is closely related to Hanseniaspora uvarum, whereas the strain N17 could be in close relationship to Candida tropicalis. The strain G7a is assigned to Cryptococcus aureus. The strain G7b and 4#2 are identical to those of Aureobasidium pullulans, respectively. The strains N3c and N11b are closely related to Yarrowia lipolytica, and the strainW6b is identical to Metschnikowa reukau.The value of nutrition shows that the nucleic acids content of the eight marine yeasts is between 2.3% and 7.8%. All the yeast strains tested have a large amount of C18:0 and C18:1 fatty acids except that YA03a has a large amount of C16:1. All the yeast strains are characterized by lacking of the two polyunsaturated fatty acids (C20:5 and C22:6) essential for marine animals. This means that the marine yeasts would need to be supplemented with live algae rich in these two polyunsaturated fatty acids when they are used as the components of feed for cultured marine animals. All the yeast strains tested have a large amount of essential amino acids, especially lysine and leucine which are very important nutritive components for marine animals. After the routine identification and molecular methods we found that the marine yeast strain G7a which is identified to be a strain of Cryptococcus aureus contains high level of protein and could grow on a wide range of carbon sources. The most important is that it has inulinase activity so that it can use inulin from Jerusalem artichoke as carbon and energy sources. The optimal medium for single cell protein production of G7a is seawater containing 6.0% (ww/v) of Jerusalem artichoke extract and 4.0% (w/v) of the hydrolysate of soybean meal while the optimal conditions for single cell protein production are pH 5.0 and 28.0°C. After fermentation for 56 h, 10.1 g/l of dry cell weight and 53.0% (w/w) of crude protein in the cells were achieved, leaving 0.051% (w/v) reducing sugar and 0.072% (w/v) total sugar in the fermented medium. The yeast strain G7a only contains 2.1% (w/w) of nucleic acid, but its cells contains a large amount of C16:0 (19.0%), C18:0 (46.3%) and C18:1 (33.3%) fatty acids and has a large amount of essential amino acids, especially lysine (12.6%) and leucine (9.1%) and strain G7a also has vitamin C (2.2 mg per 100 g of cell dry weight). These results show that the marine yeast strain G7a is suitable for single cell protein production. However, because yeasts have rigid cell wall which can not be digested by the marine animals, the protein of the marine yeast G7a should be released before it is used as single cell protein by marine animals. So the research of yeast mutants that could autolyse at non-permissive temperatures was carried out. An expected autolysis mutant G47 from strain G7a is obtained by mutageneis. 2.89mg/ml (about 108 cells/ml) from the mutant G47 was released within 30 min. We also found that the mutant has the same protein content and cell mass as its wild type G7a.In conclusion, the marine yeast strain G7a and its autolysis mutant G47 have highly poteintial applications in single cell protein production...