A New Highly Sensitive Method to Assess Respiration Rates and Kinetics of Natural Planktonic Communities by Use of the Switchable Trace Oxygen Sensor and Reduced Oxygen Concentrations

Oxygen respiration rates in pelagic environments are often difficult to quantify as the resolutions of our methods for O₂ concentration determination are marginal for observing significant decreases during bottle incubations of less than 24 hours. Here we present the assessment of a new highly sensitive method, that combine Switchable Trace Oxygen (STOX) sensors and all-glass bottle incubations, where the O₂ concentration was artificially lowered. The detection limit of respiration rate by this method is inversely proportional to the O₂ concentration, down to <2 nmol L⁻¹ h⁻¹ for water with an initial O₂ concentration of 500 nmol L⁻¹. The method was tested in Danish coastal waters and in oceanic hypoxic waters. It proved to give precise measurements also with low oxygen consumption rates (∼7 nmol L⁻¹ h⁻¹), and to significantly decrease the time required for incubations (≤14 hours) compared to traditional methods. This method provides continuous real time measurements, allowing for a number of diverse possibilities, such as modeling the rate of oxygen decrease to obtain kinetic parameters. Our data revealed apparent half-saturation concentrations (K_m values) one order of magnitude lower than previously reported for marine bacteria, varying between 66 and 234 nmol L⁻¹ O₂. K_m values vary between different microbial planktonic communities, but our data show that it is possible to measure reliable respiration rates at concentrations ∼0.5–1 µmol L⁻¹ O₂ that are comparable to the ones measured at full air saturation.