This study was conducted to evaluate the patterns of prosperity and decay of genetically engineered microorganisms (GEMs) in microcosm systems and the interaction between GEMs and micro animals such as indigenous bacteria, protozoa, metazoa and algae. GEMs such as Escherichia coli and Pseudomonas putida were injected into (1) an aquatic flask-size microcosm which consisted of bacteria, Cyclidium glaucoma, Lepadella sp., Philodina sp., Aeolosoma hemprichi, Chlorella sp. and Tolypothrix sp. and (2) a natural lake model ecosystem, Lake Kasumigaura water. Six patterns of prosperity and decay of GEMs were recognized in the flask-size microcosm, (1) rapidly decreased, (2) rapidly decreased and survived keeping the low numbers, (3) slowly decreased, (4) slowly decreased and survived keeping the low numbers, (5) survived keeping their initial numbers, (6) survived during a long period and increased. The same patterns were also recognized in the natural lake model ecosystem. Furthermore, it was made clear that the prey-predator interaction between GEMs and micro animals was quite important to the prosperity and decay of GEMs. In other words, the prosperity and decay of GEMs was greatly affected by the predation of micro animals which inhabited the microcosm system.

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