Influence of Macrofaunal Burrows on Extracellular Enzyme Activity and Microbial Abundance in Subtropical Mangrove Sediment.

Bioturbation and bioirrigation induced by burrowing macrofauna are recognized as important processes in aquatic sediment since macrofaunal activities lead to the alteration of sediment characteristics. However, there is a lack of information on how macrofauna influence microbial abundance and extracellular enzyme activity in mangrove sediment. In this study, the environmental parameters, extracellular enzyme activities, and microbial abundance were determined and their relationships were explored. Sediment samples were taken from the surface (S) and lower layer (L) without burrow, as well as crab burrow wall (W) and bottom of crab burrow (B) located at the Mai Po Nature Reserve, Hong Kong. The results showed that the burrowing crabs could enhance the activities of oxidase and hydrolases. The highest activities of phenol oxidase and acid phosphatase were generally observed in B sediment, while the highest activity of N-acetyl-glucosaminidase was found in W sediment. The enzymatic stoichiometry indicated that the crab-affected sediment had similar microbial nitrogen (N) and phosphorous (P) availability relative to carbon (C), lower than S but higher than L sediment. Furthermore, it was found that the highest abundance of both bacteria and fungi was shown in S sediment, and B sediment presented the lowest abundance. Moreover, the concentrations of phosphorus and soluble phenolics in crab-affected sediment were almost higher than the non-affected sediment. The alterations of phenolics, C/P and N/P ratios as well as undetermined environmental factors by the activities of crabs might be the main reasons for the changes of enzyme activity and microbial abundance. Finally, due to the important role of phenol oxidase and hydrolases in sediment organic matter (SOM) decomposition, it is necessary to take macrofaunal activities into consideration when estimating the C budget in mangrove ecosystem in the future.