[A-CG41] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG41-10] Evaluation of cellulase activity for the degradation of organic matter in seagrass beds

*Misaki Kado¹, Yu Umezawa¹, Toru Miyata¹, Suzuki Kento¹, Haruko Kurihara², Makoto Watanabe¹, Yoshiki Horikawa¹, Toshihiro Miyajima³ (1.Tokyo University of Agriculture and Technology, 2.University of the Ryukyus, 3.Marine Biogeochemistry Group, Division of Ocean-Earth System Science, Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Cellulose, Seagrass beds, Enzyme activity

Seagrasses and mangroves fix and store carbon dioxide in the oceans. Carbon fixed in the ocean ecosystems is called “blue carbon” and is expected to mitigate global warming. Because carbon dioxide is released back into the atmosphere after the decomposition of seagrass-derived organic matter, understanding the decomposition characteristics of the seagrass leaves is useful in assessing blue carbon storage. The seagrass vegetation zone that extends on the landward side of the fringing coral reefs is expected to function as "buffer vegetation zones" that contribute to the conservation of coral reefs by absorbing and trapping nutrients and other dissolved and suspended solids. On the other hand, organic matter decomposition in the seagrass beds may also benefit coral communities by continuously releasing regenerated nutrients at an adequate level. From these perspectives, investigating on-site organic matter decomposition characteristics is useful for evaluating the function of the seagrass beds. Therefore, based on the hypothesis that organic matter degradation characteristics in seagrass beds are also affected by cellulase activity, we conducted field sampling in the Bisezaki reef, Okinawa, Japan, and several experiments in the laboratory.
Sediment samples were collected from the northern seagrass zone, southern seagrass zone, and sandy non-vegetated zone at the Bisezaki reef in September 2022. The southern seagrass beds have a higher nutrient load than the northern seagrass beds, probably due to wastewater discharge. The surface 3 cm sediment samples were collected with acrylic cores and analyzed the activity of cellulase decomposing cellulose, which is the main component of the seagrass leaves. Enzyme activity was determined by measuring the concentration of reducing sugars produced by incubating the sediment with cellulose (carboxymethyl cellulose) using the DNS method. A system without bacterial activity was also prepared using sodium azide, and the results were compared between both experiments.
Results showed that the cellulase enzyme activity was higher in the sediment from the seagrass bed than in the non-vegetated sandy sediment. Therefore, it is considered that the seagrass leaves are more likely to be decomposed in the sediments in the seagrass zone than those in the sand without seagrasses. There was no significant difference between the northern and southern seagrass beds, but the trend was that the concentration of reducing sugars was higher in the northern area. In our presentation, we will discuss the function of seagrass beds on the conservation of coral communities.