食品工場排水における土着細菌を活用したウキクサ植物 Lemna gibba の生産収率向上の試み

The Duckweed, Lemnoideae plants, are recently highlighted as one of the useful biomass with multiple uses. The development of effective production of the duckweed biomass using wastewater as cost-free fertilizer is the challenge of this study. I conducted a trial to improve the growth of duckweed Lemna gibba in the treated food factory wastewater (WW), namely A-wastewater (A-WW) and K-wastewater (K-WW). These WW contained significantly higher sodium (Na) and phosphate (PO4), while lower nitrogen (N) by 100-folds compared to a popular duckweed medium, Hoagland. Thus, the duckweed did not grow well in A-WW and K-WW. Plant growth-promoting bacteria, PGPB, are currently expected to become a new nature-based technology tool for increasing biomass production including the duckweed. In chapter I, I call attention to the usage of appropriate PGPB, especially in A-WW and K-WW with uneven nutritional conditions. Active PGPB, Acinetobacter calcoaceticus P23 and Pseudomonas fulva Ps6, previously obtained from the environmental water in which duckweed naturally grows, did not promote but instead inhibited the growth of L. gibba in the WW conditions. Thus, I explored indigenous bacteria from the WW that exhibited growth promotion activity towards L. gibba. I demonstrated for the first time that among the indigenous bacteria that naturally grow in the WW unrelated to duckweed habitat, there exist a specific bacterium that can promote the growth of L. gibba in both WW conditions, which was Chryseobacterium sp. 27AL. Moreover, I determined the key factors that caused the growth inhibition activity of the above-mentioned non-indigenous PGPB strain P23 in the K-WW. It was found that P23 shifted its growth promotion activity on duckweed to growth inhibition in the low N condition. This result suggested that P23 showed nutrient competition with duckweed on the N uptake in the WW condition. Nevertheless, there is still a challenge remained of the PGPB application in WW, where the biomass yield of L.gibba could not fully recover. The WW contained the unbalanced minerals that significantly reduced L. gibba growth and also affected PGPB activities. Thus, in chapter II, I further identified factors for growth inhibition of L. gibba in the WW and demonstrated that modification of WW improved L. gibba growth and PGPB growth promotion activities. The result showed that low N and excess PO4 conditions caused the growth inhibition of L. gibba. Supplementation of KNO3 or NH4OH and the application of CaCO3 to the WW resulted in the recovery of L. gibba growth. Moreover, all the PGPB, including non-indigenous P23, showed higher duckweed growth promotion activities in the mineral-modified WW. Overall, the mineral supplementation within the acceptable level combined with the domestic PGPB application offers a promising way to significantly improve duckweed biomass production in the WW. On the other hand, during the trial of wild duckweed biomass production in an open-air A-WW tank, Thailand, I observed the overgrowth of microalgae, which dramatically reduced the duckweed growth probably because of their nutrient competition. Therefore, in chapter III, I asked if indigenous PGPB for L. gibba also have antagonistic activity towards the microalgae. Some eukaryotic microalgae were isolated from wastewater samples, including genera Chlorella, Coelastrella, Desmedesmus, and Parachlorella. In addition, Microcystis aeruginosa, a prokaryotic microalga well-known to form harmful algae blooms(HABs) by producing microcystin toxin, was also used for examination. I found that some PGPB including 27AL have the ability to inhibit the growth of microalgae, especially M. aeruginosa. The effect of PGPB was further explored in the co-culture of duckweed L. gibba and M. aeruginosa. The result strongly suggested that Chryseobacterium 27AL could specifically suppress M. aeruginosa and still promoted the growth of duckweed, L. gibba, significantly compared to control without bacterial inoculation. This result successfully demonstrated that wonderful PGPB, 27AL, has a dual function of enhancing the duckweed growth while inhibiting duckweed's competitor, microalgae. Overall, this study uncovered the hidden potentials of indigenous PGPB that contribute to the improvement of the duckweed biomass's production efficiency using factory wastewater. To put the strategies developed here into practical use, it is needed to evaluate the feasibility of the scale-up duckweed cultivation system in the actual WW treatment sites for the future