15 OCTOBER 1999
USE OF CHLORELLA VULGARIS FOR THE CULTURE OF MOINA MICRURA
Chlorella vulgaris is currently commercially produced in Japan and is recognized as a nutritional supplement for humans. The alga has been certified to be high in vitamins, minerals and dietary fiber. Recently, Japanese researchers have begun investigating its use as a food for domesticated animals and for the cultivation of fish. Its use in fish hatcheries is beginning to revolutionize how fish hatcheries operate in Japan. In Hawaii, researchers and extension personnel have begun investigating the use of this freshwater phytoplankton for use in five feeds culture of both marine and freshwater fish hatcheries.
Because it is a live cell there are several restrictions for its importation into Hawaii and SGES aquaculture extension specialist have been working since last year to obtain the necessary permits for its importation. These were obtained in April of this year allowing for small-scale testing of the product in Hawaii. The first experiment focused on the culture of Moina micrura, a popular live food organism in the freshwater ornamental fish trade. The initial goal of the experiment was quite simple. Define what was the maximum density of Moina that was attainable using this microalgae. The density of 10 individuals/ml attained using the "green water" method is the highest density that has been reported in the literature. However, the results using the Chlorella as a food source clearly shows that there is now a new standard (approximately 30 individuals/ml) that needs to be recognized. Apparently what happens is that when fresh Chlorella is used to culture the Moina, the amount of food no longer is a limiting factor as the slurry is approximately 5 billion cells/ml and can be continuously added to the culture. The maximum cell density attained in the "green water" cultures ranges between 2-5 million cells/ml. In addition, since the Chlorella is a live cell suspension there are a lot fewer problems with water quality than when using dry mixes (e.g., yeast). Interestingly, at 30 individuals/ml the second and third parameters that appear to be limiting factors begin to surface. These are dissolved oxygen and pH which were observed at 4 ppm and 6.7, respectively, when culture density was at 30 individuals/ml. There appears to be enough individuals that the dissolved oxygen is being cleared faster than it can be replaced, even with vigorous aeration. The resulting carbon dioxide levels due to respiration are high enough that the culture medium is plunged into an acid pH.
While these initial results appear to be very encouraging there are some serious short falls and one is the limited shelf life (2-3 weeks) of the Chlorella suspension. Our initial attempts at using it after it has been frozen shows that it does not support the growth of the Moina. Lastly, there is an associated cost to the use of this product as it currently costs $20/liter ($8/liter for the algae, $12/liter for shipping). For these reasons we have also begun investigating growing Moina using dry mixes (e.g., Rotimac, Microfeast L-10, spirulina powder) to see how they perform.
(excerpts from article by Clyde S. Tamaru, Sea Grant Extension Service/Aquaculture Development Program, Vernon Sato, Anuenue Fisheries Research Center; Harry Ako, Department of Environmental Biochemistry; in Makai, University of Hawaii Sea Grant College Program, Vol. 21, No. 8, August 1999)