Perspectives on the microbial carbon pump with special reference to microbial respiration and ecosystem efficiency in large estuarine systems

[摘要] Although respiration-based oxidation of reduced carbon releases CO₂ intothe environment, it provides an ecosystem with the metabolic energy foressential biogeochemical processes, including the newly proposed microbialcarbon pump (MCP). The efficiency of MCP in heterotrophic microorganisms isrelated to the mechanisms of energy transduction employed and hence isrelated to the form of respiration utilized. Anaerobic organisms typicallyhave lower efficiencies of energy transduction and hence lower efficienciesof energy-dependent carbon transformation. This leads to a lower MCPefficiency on a per-cell basis. Substantial input of terrigenous nutrientsand organic matter into estuarine ecosystems typically results in elevatedheterotrophic respiration that rapidly consumes dissolved oxygen, potentiallyproducing hypoxic and anoxic zones in the water column. The loweredavailability of dissolved oxygen and the excessive supply of nutrients suchas nitrate from river discharge lead to enhanced anaerobic respirationprocesses such as denitrification and dissimilatory nitrate reduction toammonium. Thus, some nutrients may be consumed through anaerobicheterotrophs, instead of being utilized by phytoplankton for autotrophiccarbon fixation. In this manner, eutrophied estuarine ecosystems becomelargely fueled by anaerobic respiratory pathways and their efficiency is lessdue to lowered ecosystem productivity when compared to healthy and balancedestuarine ecosystems. This situation may have a negative impact on theecological function and efficiency of the MCP which depends on the supply ofboth organic carbon and metabolic energy. This review presents our currentunderstanding of the MCP mechanisms from the view point of ecosystem energytransduction efficiency, which has not been discussed in previous literature.

[学科分类] 地球化学与岩石