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Abstract Investigations of hydrostatic pressure effects on microbial enzymatic processing are challenging but critical for understanding element cycling in the world's oceans. In this study, we developed and tested a stainless‐steel pressure cell capable of withstanding up to 110 MPa of hydrostatic pressure (equivalent to ~ 11,000 m depth), featuring three sapphire windows that enable real‐time measurements of spectral absorption and fluorescence. The temperature‐controlled system (5–50°C) can be incorporated in standard laboratory spectrofluorophotometers. The hydrostatic pressure of the 2.6 mL sample is regulated using a manually controlled spindle pump. The system was tested with fluorometric enzyme assays and used to evaluate the activity of chitinase enzymes produced by piezophilic and piezosensitive marine bacteria at a constant temperature of 10°C. Experiments on enzymes from three bacterial strains revealed distinct and reproducible differences in pressure responses. This compact system enables precise monitoring of pressure‐sensitive activity of microbial communities and extracellular enzymes and thus provides an avenue for detailed investigations on the importance of hydrostatic pressure in regulating deep‐sea biogeochemical processing. 

Aquatic eddy covariance (AEC) is increasingly being used to study benthic oxygen (O 2 ) flux dynamics, organic carbon cycling, and ecosystem health in marine and freshwater environments. Because it is a noninvasive technique, has a high temporal resolution (∼15 min), and integrates over a large area of the seafloor (typically 10–100 m 2 ), it has provided new insights on the functioning of aquatic ecosystems under naturally varying in situ conditions and has given us more accurate assessments of their metabolism. In this review, we summarize biogeochemical, ecological, and biological insightsgained from AEC studies of marine ecosystems. A general finding for all substrates is that benthic O 2 exchange is far more dynamic than earlier recognized, and thus accurate mean values can only be obtained from measurements that integrate over all timescales that affect the local O 2 exchange. Finally, we highlight new developments of the technique, including measurements of air–water gas exchange and long-term deployments.