Carbon conversion in mixotrophic cultivation by a thermophilic cyanobaterium producing PHB

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In the present research, the utilization properties of carbon and energy sources were characterized in the photomixotrophic cultures of a thermophilic cyanobacterium (Synechococcus sp.MA19) and liverwort cells (Marchantia polymorpha HYA-2F), and the following results were obtained.(1) Sucrose was selected as an organic carbon source which was utilized by strain MA19 under a photomixotrophic condition. It was found that phosphate was a key factor governing the productivity of poly-β-hydroxybutyrate (PHB) by the strain.(2) In the cultures of stratn MA19 with sucrose medium in the range of incident light intensity of 0-67 W/m^2, the cellular carbon recovery of 1.1 and CO_2 evolution ratio of -0.18 were obtained when the total light energy absorbed by the cells was 5.8×10^6 J/g-cells. This result inducated that CO_2 released via respiratory actions was trapped and reused for the cell formation by photosynthetic activity of the strain.(3) A bioreactor system was developed for the photomixotrophic cultures of M.polymorpha HYA-2F, which consisted of a glass vessel with external light irradiation and a light moderator installed in a process controller. Batch culture in the bioreactor was conducted in glucose medium while keeping the value of absorbed light energy at 3.5×10^2 W/kg by regulating incident light intensity ranging from 19 to 220 W/m^2. During this culture under light control, CO_2 evolution was suppressed and the high value of cell yield was achieved compared with that in the culture conducted at constant incident light intensity of 50 W/m^2.(4) The characteristics of energy conversion through respiratory and photosynthetic actions in the mixotrophic cells were examined based on the metabolic flux analysis. In the culture with light regulation where the efficiency of overall energy conversion to ATP was considered, the enhanced cell yield was obtained on the basis of energy provided to the system in comparison with those in the cultures without light control.