Aquatic animal research in space station and its issues--focus on support technology on nitrate toxicity.

We studied the effects of accumulated nitrate in water on the spawning, hatching and development of medaka using a simple nitrifying filter and a combined filter having both nitrifying and denitrifying capabilities. A nitrate concentration of 100 mg NO3(-)-N/L was clearly of lethal toxicity to fish when they were exposed to nitrate in both adult and the growing phases. A nitrate concentration of 75 mg NO3(-)-N/L reduced the fertilization rate, delayed the hatching time and reduced the hatching rate of the eggs laid by adults and decreased the growth rate of juveniles. In addition, nitrate accumulations as low as 50 mg NO3(-)-N/L remarkably retarded spawning and lowered the number of eggs laid by fish exposed in the juvenile phase. The effects on the reproduction system may be initiated by a low concentration, approximately 30 mg NO3(-)-N/L.

Population dynamics of nitrifying bacteria in an aquatic ecosystem.

In a space environment such as Space Shuttle or Space Station, animal experiments with aquatic species in a closed system pose a crucial problem in maintaining their water quality for a long term. In nature, ammonia as an animal wastes is converted by nitrifying bacteria to nitrite or nitrate compounds, which usually become nitrogen sources for plants. Thus an application of the biological reactor with such bacteria attached on some filters has been suggested and experimentally studied for efficient waste managements of ammonia. Although some successful results were reported (Kozu et al. 1995, Nagaoka et al. 1998, Nakamura et al. 1997, 1998) in the space applications, purely empirical approaches have so far been taken to develop a biological filter having a stable nitrifying activity. In this study, we constructed a mathematical model to deal with the dynamics of the ammonia nitrifying processes in a biological reactor. The model describes population dynamics of the ammonia-oxidizing bacteria and the nitrite-oxidizing bacteria cultivated on the same filter. We estimated parameters involved in the model using the experimental data. The result shows that these estimated parameters could be applied to general cases and that the two bacteria are in a symbiotic relationship; they can better perform when both coexist, as has been empirically recognized. Based on the model analysis, we discuss how to prepare a high performance biological filter.

Application of nitrifying and denitrifying processes to waste management of aquatic life support in space

Since a biological filter with nitrifying bacteria was firstly applied to aquatic animal experiments in IML-2 mission, the reactor system has been further studied to combine both nitrifying and denitrifying reactions under aerobic environment allowing an efficient removal of inorganic nitrogen from animal wastes. The isolated denitrifying bacteria had an activity under aerobic condition with rice straw providing a metabolic carbon source for the reaction. The advantage of the aerobic biological filter having both nitrifying and denitrifying activities may allow to reduce the size of the life support system and also for its manageability. The paper reports characteristics of the biological filter systems used for the IML-2 mission and the improved combined filter system having both nitrifying and denitrifying activities, and discuss its application to space experiments.

Telescience testbed experiments of intracellular recordings in the Xenopus oocyte.

The feasibility of intracellular recordings under constraints for experimental conditions in outer space were tested at a telescience testbed of the National Space Development Agency of Japan. We attempted to study the dose-response relationship of adenosine-induced potential changes in the Xenopus oocyte. The testbed simulated the distance from a ground control room to oocytes in an orbital laboratory; signals were delayed and compressed from one "station" to other. A microelectrode was inserted into the oocyte using remote control on the manipulator and on the xyz-planes of the platform with stereoscopic pictures observed through a head mounted display. When the transfer rate of the visual signals was decreased from 1.5 Mbps to 128 Kbps, insertion of the electrode became almost impossible because of reduced picture quality. Once the electrode was inserted, however, dose-dependent adenosine responses could be observed with little trouble.

Isolation and properties of the Capnocytophaga ochracea bacteriocin.

A bacteriocin from a subgingival plaque isolate of Capnocytophaga ochracea was purified and characterized. It was isolated from cell extract and had a molecular weight of 100,000. Streptococcus sanguis, Streptococcus mitis, Streptococcus mutans and Propionibacterium acnes were susceptible to the bacteriocin.