Extracellular enzymes, microbes and decomposition of organic matter in coastal mangrove sediments

Abstract

Extracellular enzymes, primarily produced by microorganisms, affect ecosystem processes because of their essential role in degradation, transformation and mineralization of organic matter. However, such data on sediments in mangrove ecosystems are especially scanty. Therefore, there is a need to investigate extracellular enzymes in mangrove ecosystems in order to estimate the C cycling dynamics on a global scale. The objectives of this research were to investigate the activities of enzymes involved in C and nutrient cycling, to assess microbial nutrient acquisition by applying enzymatic stoichiometry, to delineate the relationships among enzyme activities, microbial parameters as well as decomposition of sediment organic matter (SOM) in a coastal mangrove of Hong Kong.

Strong variations in activities of hydrolases (β-glucosidase (GLU), β-N-acetyl-glucosaminidase (NAG), and acid phosphatase (ACP) involved in C, N and P cycling, respectively), but not activity of phenol oxidase (PHO, degrading recalcitrant organic matter) have been observed. However, evident variations of PHO activity were found in rhizosphere and crab-affected sediments. Hydrolase activities are positively correlated to sediment C, N and P as well as bacterial abundance, suggesting a feedback and succession of microorganisms to limitation of natural resources. On the other hand, a significant correlation between ACP and PHO activity indicates SOM degradation is controlled by P availability. Coincidentally, the enzymatic stoichiometry also suggests that this ecosystem is extremely microbial P-limited. Moreover, at molecular level, the remarkable correlations between bacterial laccase-like communities (relating to SOM degradation) and sediment C:P as well as N:P have repeatedly suggested a P-limited condition for SOM-degrading microbial communities. Therefore, it can be known that the increase of P limitation will slow down SOM decomposition and thus protect C storage in this coastal mangrove ecosystem.

Furthermore, the effects of N and biochar additions on SOM decomposition have been investigated. Relative to the controls, N addition has dual role in C storage since both loss and accumulation of organic C was observed in mangrove forest and intertidal zone sediments, respectively. Sediment organic C was slightly increased after biochar addition, although the increase of PHO and GLU activity indicates a trend of C loss. This contradictory phenomenon is probably due to the entrapped or adsorbed enzymes catalyzing the oxidation or hydrolysis of SOM. Although the information is still limited on this subject, the results of this study may help to better understand the potential strategies on preventing C loss in mangrove ecosystems.

Overall, the thesis indicates that SOM in this coastal mangrove is microbial P-limitation, and SOM decomposition may be slowed down so as to sequester C into sediments if microbial P limitation is exacerbated. Therefore, in further study, the importance of P in SOM degradation and microbial communities should be considered for C cycling and climate change mitigation.