No abstract

This study was to determine the extent of bacteria contamination and resistance to various antibiotics used commonly in microalgal culture. Seven different dose levels of chloramphenicol, dihydrostreptomycin sulphate, neomycin, penicillin G, streptomycin sulphate, penicillin G + streptomycin sulphate, and penicillin G + streptomycin sulphate + chloramphenicol were added to each culture of microalgae. The lethal effects on microalgae and bacteria were the highest in chloramphenicol and the lowest in penicillin G. The axenic culture of bacillariophyceae and dinophyceae was more difficult than that of chlorophyceae and haptophyceae because of their complicate external morphology. The efficient antibiotics and their concentrations for axenic cultures varied with microalgal species. The optimum quantity for antibiotic treatments were 2,000 ppm of dihydrostreptomycin for Chlorella ellipsoidea, neomycin 500 ppm of Isochrysis galbana and Heterosigma ahashiwo, chloramphenicol 500 ppm of Cyclotella didymus, and dihydrostreptomycin sulphate and neomycin 6,000 ppm of Thalassiosira allenii.

Rotifers of the genus Brachionus are commonly used as a live food for larval fish, and rotifers of different sizes are preferred according the mouth size of the fish. Rotifer species vary in size, and individual size can depend on the temperature and salinity of the rearing environment. We investigated the effects of temperature and salinity for two species, B. plicatilis (250-300 µm) and B. rotundiformis (100-220 µm). Two strains of B. plicatilis (CCUMP 36 and 48) and two strains of B. rotundiformis (CCUMP 51 and 56) were received from the Culture Collection of Useful Marine Plankton (CCUMP) at Pukyong National University and cultured with the green alga, Nannochloris oculata (KMMCC 16) from the Korea Marine Microalgal Culture Center (KMMCC). The growth and size of rotifers were examined at three water temperatures (16ºC, 24ºC, 32ºC) and four salinities (20 psu, 25 psu, 30 psu, 35 psu) under continuous light (40 molm -2 s -1 ). The maximum density and growth rate of B. rotundiformis were greater than those of B. plicatilis. The lorica length of B. plicatilis ranged from 215.4 to 269.7 µm and from 154.9 to 206.6 µm for B. rotundiformis, depending on strain, temperature and salinity. Rotifers were smaller when cultured at high temperatures, regardless of salinity. B. rotundiformis preferred higher salinity than B. plicatilis. The results demonstrated that the size of rotifers could be controlled to some extent by temperature and salinity.

The rotifer Brachionus plicatilis is one of the most important food organisms in aquaculture. The resting eggs produced by mictic female rotifers are easily stored and hatched, making them useful as the starter for the mass culture of rotifers in marine larval culture. This study examined the optimum preservation method for resting eggs to ensure a high hatching rate. To produce resting eggs, the marine rotifer B. plicatilis was cultured with Nannochloris oculata (KMMCC 16). The resting eggs were harvested and cryopreserved using 5% and 10% methanol (MeOH), dimethylsulfoxide (DMSO), and glycerol as cryoprotectant agents (CPAs). The cryopreservation comprised slow or rapid freezing and the resting eggs were stored for one month in liquid nitrogen (-196˚C). The resting eggs were also dried at different temperatures (30, 40, and 50˚C) and for different times (1, 2, and 3 h). In general, the hatching rates of the resting eggs preserved with CPA were higher than those without CPA and the slow freezing method was better than the rapid freezing method. However, the optimum CPA concentration for the hatching rate of the resting eggs varied with the freezing method and kind of CPA, and the CPA also affected the viability of the resting eggs. Dried resting eggs had a high, rapid hatching rate over 80%. The moisture content of the resting eggs cryopreserved in liquid nitrogen affected the hatching rate. Drying at 30˚C for 1 hour resulted in a high hatching rate of the resting eggs. In conclusion, drying at 30˚C for 1 hour and preservation in liquid nitrogen with the slow freezing method, without CPA, is recommended for a high hatching rate (ca. 95%) of rotifer resting eggs.