Article: Macroalgal Blooms Trigger the Breakdown of Seagrass Blue Carbon by Liu and colleagues (2020)
Background: Seagrass meadows are important ecosystems as they serve as vital nurseries for many fish and other animal species, as well as play crucial roles in nutrient cycling. Because seagrasses are highly productive ecosystems and have high turnover rates, they are also important for their role in sequestering (i.e., storing) carbon and reducing the progress of climate change. As the grasses grow, they take in CO2 from their environment, and when they die, the carbon within their leaves becomes stored as detritus (i.e., partially decomposed organic matter). Seagrass environments are threatened by excess nutrient inputs (e.g., fertilizer runoff). Excess nutrients cause macroalgal blooms at the surface; this blocks sunlight from reaching seagrass--causing it to die. After an algal bloom, there is also a major algal dieback creating algal detritus which mixes with seagrass detritus. While the carbon in seagrass is recalcitrant (i.e., difficult for microbes to eat), carbon stored in algal detritus is labile (i.e., difficult for microbes to eat). In this study, Liu and colleagues explored “co-metabolism,” a mechanism by which microbes are able to breakdown recalcitrant detritus (e.g., seagrass detritus) and release CO2 and CH4 only when a more easily accessible energy source (i.e., algal detritus) is also available.
Methods: Living seagrass leaves, macroalgae, sediment, and seawater were collected near Port Phillip, Australia. Seagrass and macroalgae were left in a dark room for 7 days to cause death and detritus formation. With these detritus samples, five different experimental microcosms were established with four replicates of each type: seagrass only, low biomass algae only, high biomass algae only, seagrass and low biomass algae together, and seagrass and high biomass algae together. Throughout the 62-day experiment, greenhouse gas (e.g.,CO2 and CH4) production was monitored periodically. Before and after the experiment, the weights of detritus were recorded and the microbial communities were analyzed.
Findings: As the organic matter decayed through the incubation period, microcosms with seagrass and high biomass algal detritus together produced significantly more greenhouse gasses than all other experimental groups. High biomass algal detritus only was the next highest producer of greenhouse gases, which was followed by seagrass and low biomass algal detritus together and low biomass algal detritus only. Seagrass detritus grouped with high amounts of algal detritus had lower remaining biomasses after the incubation period, suggesting higher rates of decomposition than other experimental groups. No significant differences in microbial community structures were found between groups.
Conclusions: The increase in decomposition rate and greenhouse gas production when seagrass and algal detritus are paired suggest that co-metabolism is a likely mechanism causing reduced carbon sequestration in seagrass meadows. This is further supported by the seagrass and high algal detritus biomass grouping having higher greenhouse gas production than the combination of the seagrass detritus group alone and the high algal detritus group alone, suggesting a synergistic effect. Researchers found that co-metabolism did not result in a significant shift in the microbial community. Instead, the co-metabolism effect is due to the same microbial community being supplied with enough energy from the labile algal detritus to subsequently breakdown seagrass detritus. Nutrient loading caused by human activity is already threatening important seagrass habitats by causing algal blooms that shade and outcompete them. Here, Liu and colleagues have uncovered how algal blooms are reducing the role seagrass meadows have in sequestering carbon, providing yet another reason to curtail excess nutrient runoff.
Figure: Schematic of co-metabolism effect between macroalgal and seagrass detritus (i.e., dead organic matter). The increase in macroalgal blooms surrounding seagrass leads to an increase in mixed detritus (i.e., seagrass detritus + macroalgal detritus). Hypothesis studied: If labile (i.e., easy to breakdown) macroalgal detritus is mixed with recalcitrant (i.e., difficult to breakdown) seagrass detritus, then the microbial communities breaking down the labile organic matter will be primed to break down the recalcitrant organic matter leading to increased CO2 and CH4 emissions and decreased stored carbon.
Reference:
Liu, Songlin, et al. "Macroalgal Blooms Trigger the Breakdown of Seagrass Blue Carbon." Environmental Science & Technology 54.22 (2020): 14750-14760.
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