RESPIRE: an in situ particle interceptor to conduct particle remineralization and microbial dynamics studies in the oceans’ Twilight Zone

Sinking biogenic particles are centers of remineralization in the ocean's interior, a stratum which has large vertical biogeochemical gradients in the mesopelagic (i.e., 100–1000 m depth) realm. Due to difficulties in studying this region, our understanding of particle remineralization is based upon investigations employing biogeochemical algorithms, proxies, or shipboard incubations of deep-water samples. Here we circumvent many of these issues by fabricating and deploying a REspiration of Sinking Particles In the subsuRface ocEan (RESPIRE) particle interceptor in the open ocean to nonintrusively intercept settling particles at fixed depths (100–300 m). After a preprogrammed particle interception phase, the RESPIRE interceptor switches to a particle incubation mode, during which remineralization rate at in situ temperature and pressure is tracked using a dissolved-oxygen optode time series. A rotating (every 10 min) indented sphere transfers particles into a chamber that houses the dissolved-oxygen optode, and minimizes access to particles by mesozooplankton. Deployments of typically 72 h (36 h interception: 36 h incubation) yield rates of particle remineralization primarily by attached microbes, and concurrently provide samples for geochemical analyses and microbial assays to explore the relationship between bacterial community structure, enzymatic solubilisation, and trends in particle remineralization. The interceptor has also been used to follow temporal changes in microbial dynamics associated with remineralization. RESPIRE provides more realistic conditions within which remineralization rates can be measured in the ocean's interior, and will foster new linkages between the disparate disciplines of microbial physiology and particle biogeochemistry in the ocean's Twilight Zone.