Respiratory quotients of particle-associated microbes track carbon flux attenuation in the mesopelagic Southern Ocean

Mesopelagic microbes and zooplankton, degrade and attenuate >90% of the 10 billion tonnes of Particulate Organic Carbon (POC) that sinks into the oceans’ interior annually. Approaches such as particle interceptors/incubators (called C-RESPIRE) can isolate the microbial assemblage attached to particles from that of zooplankton, enabling quantification of microbially-mediated POC flux attenuation. This metric yields patterns of POC degradation by microbes through the upper mesopelagic (200-500 m depth). Here, we investigate the temporal sequence of POC degradation in two steps. First, we intercept sinking particle assemblages from different depths (180-300 m) and hence with varying degrees of exposure to microbial activity. Second, we incubate these intercepted particles shipboard for 12h (short-term) and track degradation using Apparent Respiratory Quotients (ARQ, dDIC/dDO2). We also incubate shipboard (12h) a particle assemblage previously incubated (36h) in situ using C-RESPIRE (long-term). At a subantarctic and two polar sites, ARQs from short-term incubations exhibited a significant decrease with depth, consistent with particles deeper in the upper mesopelagic being exposed to a longer period of degradation and flux attenuation (as they settle). ARQs from all long-term incubations had significantly lower ARQs, and smaller depth-dependent gradients, than the short-term incubations. We interpret these trends as being driven in part by sequential changes in the stoichiometry of the microbially-altered POC substrates. Several ARQs of

海洋中层带的微生物与浮游动物每年可降解并削弱超过90%、总量达100亿吨的沉降至海洋内部的颗粒有机碳（Particulate Organic Carbon, POC）。诸如颗粒捕获器-培养箱（命名为C-RESPIRE）这类装置，可将附着于颗粒的微生物群落与浮游动物群落分离开来，从而实现对微生物介导的POC通量削弱过程的定量分析。该指标可揭示上层中层带（水深200~500米）内微生物介导的POC降解模式。本研究通过两个步骤探究POC降解的时间序列：其一，从不同水深（180~300米）捕获沉降颗粒群落，这些颗粒因所处水深不同，其暴露于微生物活动的时长存在差异；其二，将捕获的颗粒在船载环境下培养12小时（短期培养），并通过表观呼吸商（Apparent Respiratory Quotients, ARQ，即dDIC/dDO2）追踪降解进程。此外，我们还对此前利用C-RESPIRE完成36小时原位培养的颗粒群落开展12小时船载培养，即长期培养组。在亚南极海域与两处极地观测站点中，短期培养组的ARQ值随水深显著降低，这与上层中层带较深处的颗粒随沉降过程经历更长时间的降解与通量削弱的结论一致。所有长期培养组的ARQ值均显著低于短期培养组，且其随水深变化的梯度也更小。我们认为，这些趋势在一定程度上由微生物修饰后的POC底物的化学计量学序列变化所驱动。部分ARQ的