Nutrient loading accelerates breakdown of refractory dissolved organic carbon in seagrass ecosystem waters

Xia Zhang; Songlin Liu; Yunchao Wu; Hongxue Luo; Yuzheng Ren; Jiening Liang; Xiaoping Huang; Peter I Macreadie

doi:10.1016/j.watres.2024.123017

Nutrient loading accelerates breakdown of refractory dissolved organic carbon in seagrass ecosystem waters

Water Res. 2024 Dec 20:273:123017. doi: 10.1016/j.watres.2024.123017. Online ahead of print.

Authors

Xia Zhang¹, Songlin Liu², Yunchao Wu¹, Hongxue Luo², Yuzheng Ren², Jiening Liang², Xiaoping Huang³, Peter I Macreadie⁴

Affiliations

¹ Key Laboratory of Tropical Marine Bio-resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China.
² Key Laboratory of Tropical Marine Bio-resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; University of Chinese Academy of Sciences, Beijing 100049, China.
³ Key Laboratory of Tropical Marine Bio-resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; University of Chinese Academy of Sciences, Beijing 100049, China. Electronic address: xphuang@scsio.ac.cn.
⁴ Centre for Nature Positive Solutions, Biosciences and Food Technology Discipline, School of Science, RMIT University, Melbourne, VIC 3000, Australia.

PMID: 39721499
DOI: 10.1016/j.watres.2024.123017

Abstract

Nutrient loading is a major driver of seagrass ecosystem decline and also threatens the capacity for seagrass ecosystems to act as 'blue carbon' sinks. Dissolved organic carbon (DOC) represents a crucial component of carbon storage in seagrass ecosystems, with refractory DOC (RDOC) playing a key role in long-term (millennial time scale) carbon stocks. The processes governing RDOC are heavily influenced by microbial activity. While it is known that nutrient loading can weaken DOC sequestration potential by changing the DOC composition and transformation, the impact of nutrients on microbial communities that regulate the RDOC pool in seagrass ecosystems remains poorly understood. To address this gap, we conducted a 300-d laboratory incubation experiment to examine the effects of nutrient enrichment on DOC processing and microbial community dynamics. As expected, nutrient addition significantly accelerated the decline in DOC concentration, resulting in the residual DOC exhibiting a higher degree of humification and more depleted δ¹³C constituents. Concurrent with DOC degradation, microbial community composition shifted from a mix of r- and K-strategists in the early stages to a dominance of K-strategists and fungi in the later stages. Specific bacterial taxa, such as unidentified Rhodospirillales and Oceanococcus, were more prevalent in eutrophicated seagrass waters, while Magnetospira and Nocardioide were more abundant in less eutrophicated waters by the end of the incubation. We speculated that these microbial groups likely adapted to utilise more RDOC, contributing to its decline. The decline in RDOC was approximately 2-times greater in less eutrophicated seagrass waters compared to more eutrophicated waters (26.9 % and 14.5 % decline respectively), which suggests that less eutrophicated seagrass ecosystems are more vulnerable. This study provides evidence that high nutrient loading can enhance RDOC remineralization, ultimately weakening the long-term carbon sequestration potential of seagrass ecosystems.

Keywords: Carbon decomposition; Long-term incubation; Microbial community; Nutrient enrichment; Refractory DOC; Seagrass ecosystems.