Is the microbial priming effect a potential threat to seagrass blue carbon storage?

Blue carbon habitats are systems that naturally sequester organic carbon into their sediments at faster rates than many terrestrial systems. By locking away this sediment organic carbon, blue carbon habitats act CO2 sinks.... [ view full abstract ]

Blue carbon habitats are systems that naturally sequester organic carbon into their sediments at faster rates than many terrestrial systems. By locking away this sediment organic carbon, blue carbon habitats act CO2 sinks. However, there are some processes that can potentially threaten to turn these sinks into sources such as enhanced microbial remineralisation of organic carbon via the microbial priming effect (MPE). MPE is a phenomenon in which access to fresh, labile organic carbon (LOC) ‘kick-starts’ sediment microbes to co-metabolise refractory organic carbon (ROC) that could not have been otherwise utilised. MPE has been studied in terrestrial ecosystems, however, little is known about how or if this mechanism occurs in coastal habitats. LOC in coastal environment may include autochthonous sources like algae and plant detritus as well as allochthonous sources from nearby streams or run-off from land. Seagrass meadows, a blue carbon habitat, have the potential to store ROC on millennial timescales but may frequently be the site of LOC input due to algae blooms under nutrient-loading conditions. Furthermore, physical disturbances like dredging or anchoring or propeller scars may expose deeply buried ROC to the surface and in contact with LOC. As a consequence, these LOC additions could potentially trigger MPE and threaten the ability of seagrass meadows to act as hotspots for capturing and sequestering ROC. This study aims to better understand the MPE mechanism in seagrass sediments using two LOC sources: microalgae to simulate an algae bloom and seagrass detritus to simulate a common LOC input. Both LOC sources were enriched with natural 13C isotopes before adding to sediments from 3 depths, 0-1 cm, 4-5 cm and 29-30 cm, which represent recent to 1,800-year-old ROC deposits. MPE was assessed by quantifying the captured CO2 produced as a byproduct of microbial metabolism. Mixing models applied to δ-13C stable isotope analysis were used to assess the proportion of LOC vs. ROC remineralised by the microbial population. Based on these results, we will discuss the implications of MPE in blue carbon habitats.