[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-P03] Decomposition characteristics of semi-persistent organic matter in the presence of cellulase possessing bivalves

*Misaki Kado¹, Yu Umezawa¹, Toru Miyata¹, Wen Liu², Yoshiki Horikawa¹, Mitunori Tarao¹, Ryota Kose¹, Tomohiro Kuwae³ (1.Tokyo University of Agriculture and Technology, 2.Kyoto University, 3.Port and Airport Research Institute)

Keywords:Corbicula japonica, endogenous cellulase, semi-persistent organic matter

In recent years, the decomposition characteristics of organic matter in aquatic ecosystems have attracted much attention from the viewpoints of carbon sequestration (i.e., blue carbon), and the natural decomposition characteristics of woody materials as plastic alternatives. Most of the decomposition experiments of semi-persistent organic matter in water have been focused only on the microbial decomposition. On the other hand, it has been reported that many benthic invertebrates possess cellulase genes derived from their own genome, namely endogenous cellulase genes, but not from symbiotic microorganisms in the gut, etc. Therefore, it is considered that endogenous cellulase potentially play an important role in the decomposition of organic matter such as cellulose. In this study, we cultured the endogenous cellulase secreting bivalve, Corbicula japonica, to investigate the decomposition characteristics of organic matter in the presence of the bivalves.
As biological samples, we prepared C.japonica, which secretes endogenous cellulase, and Ruditapes philippinarum, which does not. As organic materials to investigate the decomposition characteristics, we used Zostera marina, which has been often focused on as blue carbon, and pulp samples, which can be alternatives to plastic. In the experiment, we prepared three types of aquaria: a tank with C.japonica, a tank with R.philippinarum, and a tank without bivalves (control), and put organic materials in each tank to investigate the decomposition characteristics. The experiments were conducted for about two months in divided periods. After the culture experiments, calculation of dry weight difference, IR spectroscopy for structural analysis, and determination of reducing sugar production for cellulase activity were performed on cultured samples. We also performed microbial community composition analysis and microbial counting by DAPI staining on the cultured seawater. To confirm whether the bivalves used in the experiments secreted cellulase, cellulase detection was carried out using CMC plate assay.
After incubation experiments, the decrease in pulp samples was greater in the tank with C.japonica than in the control tank. Furthermore, cellulase was detected in C.japonica-feces. These results suggest that cellulose decomposition was accelerated by the presence of C.japonica, and the cellulase emitted by C.japonica is likely to work on pulp samples. On the other hand, the decrease in pulp samples was greater in R.philippinarum presence than in C.japonica. It is known that R.philippinarum have cellulase activity by symbiotic microorganisms, but do not secrete endogenous cellulase, and their cellulase activity is inferior to that of C.japonica ¹⁾. The reasons for the decrease in the amount of pulp samples in R.philippnarum tank were considered: some clams died immediately after the start of the experiment and cultured seawater became enriched, and the microbial composition of seawater changed. In contrast to the pulp samples, there was no significant difference (p<0.05) in Z.marina among the three tanks. It is considered that the presence of bivalves accelerated the decomposition of pulp samples, but did not affect the decomposition of Z.marina. This is probably due to the facts that the surface of Z.marina-leaf is covered with a cuticular layer and little cellulose is exposed, and there are repellent substances in Z.marina.

[Reference]1) Sakamoto et al., 2009. The Journal of Experimental Biology, 212, 2812-2818