New and fast method to quantify respiration rates of bacterial and plankton communities in freshwater ecosystems by using optical oxygen sensor spots

Abstract

A new method of respiration rate measurement based on oxygen luminescence quenching in sensor spots was evaluated for the first time for aquatic bacterial communities. The commonly used Winkler and Clark electrode methods to quantify oxygen concentration both require long incubation times, and the latter additionally causes signal drift due to oxygen consumption at the cathode. The sensor spots proved to be advantageous over those methods in terms of precise and quick oxygen measurements in natural bacterial communities, guaranteeing a respiration rate estimate during a time interval short enough to neglect variations in organism composition, abundance, and activity. Furthermore, no signal drift occurs during measurements, and respiration rate measurements are reliable even at low temperatures and low oxygen consumption rates. Both a natural bacterioplankton sample and a bacterial isolate from a eutrophic river were evaluated in order to optimize the new method for aquatic microorganisms. A minimum abundance of 2.2 x 10(6) respiring cells ml(-1) of a bacterial isolate was sufficient to obtain a distinct oxygen depletion signal within 20 min at 20 degrees C with the new oxygen sensor spot method. Thus, a culture of a bacterial isolate from a eutrophic river (OW 144; 20 x 10(6) respiring bacteria ml(-1)) decreased the oxygen saturation about 8% within 20 min. The natural bacterioplankton sample respired 2.8% from initially 94% oxygen-saturated water in 30 min. During the growth season in 2005, the planktonic community of a eutrophic river consumed between 0.7 and 15.6 micromol O(2) liter(-1) h(-1). The contribution of bacterial respiration to the total plankton community oxygen consumption varied seasonally between 11 and 100%.

FIG. 1.

Technical setup of oxygen measurement method.

FIG. 2.

Oxygen concentration changes between 410 and 430 μmol liter⁻¹ at fluctuating temperatures (4.1 ± 0.2°C) measured in sterile distilled water for 60 min. An oxygen concentration of 420 μmol liter⁻¹ is equivalent to 103% oxygen saturation. Saturation ranged from 101 to 104%.

FIG. 3.

Changes in oxygen concentration (μmol liter⁻¹) in sterile minimal medium at 20.3°C as a function of time (min). The intersection point with the y axis was 278.5 μmol O₂ liter⁻¹, which is equivalent to 99% oxygen saturation at the given temperature.

FIG. 4.

Oxygen consumption (μmol liter⁻¹) of strain OW 144 (16 × 10⁶ cells ml⁻¹) in minimal medium (0.5% glucose) within 20 min at 20°C.

FIG. 5.

Respiration rate (μmol O₂ liter⁻¹ h⁻¹) of a geometrically diluted bacterial culture (OW 144) cultivated in minimal medium (0.5% glucose) at 20°C (P < 0.001; r² = 0.73). The insert shows the respiration rate of OW 144 at abundances below 100 × 10⁶ cells ml⁻¹ (P < 0.001; r² = 0.47).

FIG. 6.

Cell-specific respiration (fmol O₂ cell⁻¹ h⁻¹) of isolate OW 144 at different glucose concentrations (μmol liter⁻¹) at 20°C. Significant differences were observed between 0 μM glucose and 50 μM (*a) or 200 μM (*b) glucose (P < 0.01).

FIG. 7.

Oxygen concentration (μmol liter⁻¹) of a bacterioplankton sample (20.2 × 10⁶ ± 1.9 × 10⁶ total cells and 1.8 × 10⁶ ± 0.1 × 10⁶ respiring cells ml⁻¹) from the River Warnow at 21.6°C (P < 0.001; r² = 0.88). In biotope water, 100% oxygen saturation equals 272.8 μmol O₂ liter⁻¹ at 21.6°C.

FIG. 8.

Community respiration and BR (μmol O₂ liter⁻¹ h⁻¹) in the River Warnow during spring and early summer 2005, at temperatures ranging from 2.1 to 22.3°C. The second y axis shows the fraction of highly active (respiring) bacteria (10⁶ ml⁻¹).