Enhanced microbial consumption of carbon-rich marine organic matter at experimentally elevated temperatures dictated by substrate C:N:P stoichiometry

Marine microbes are vital to oceanic ecosystems and influence the global climate through their paramount role in Earth's biogeochemical cycles. With this intricate role in ecosystems, it is important to understand the effect of increasing ocean temperatures on the cycling of organic matter (OM), which is hypothesized to contribute a positive feedback to future warming via an acceleration in microbial respiration of OM to CO₂. We experimentally investigated the temperature sensitivity of microbial consumption of marine particulate OM (POM) focused in the rapidly warming Gulf of Maine during the 2019 and 2020 fall phytoplankton bloom. The overall rate and quantity of microbial POM (C, N, and P pools) consumption at in situ versus elevated temperatures were quantified within bottle incubations over the course of two weeks. POM incubated at warmer temperatures (+5 to 6 °C) was generally consumed at a faster rate with an overall larger quantity consumed compared to cooler temperatures (12 to 14 °C). Significant interannual variability in consumption rates and temperature sensitivity (Q₁₀ parameter) across elemental pools was found and linked to the initial POM C:N:P stoichiometry. More nitrogen-rich POM was preferentially consumed at in situ temperatures, whereas carbon-rich POM, likely containing a terrigenous component, was preferentially consumed at warmer experimental temperatures. The empirically estimated temperature sensitivity (Q₁₀) ranged from 2.7 to 3.4 in 2019 versus 1.0–1.2 in 2020, variable between and amongst POM elemental pools, suggesting both temperature and organic matter substrate stoichiometry (composition) play an important role in dictating the microbial POM remineralization response to warming ocean temperatures.