Oxygen measurement
Oxygen is the most advantageous electron acceptor during the process of respiration. That is why all higher organisms use oxygen to breathe. The oxygen is also involved in the re-oxidation of most reduced compounds produced by other types of anaerobic respirations (e.g. H2S, Fe2+, Mn2+). In addition, during daytime, photosynthesis by benthic microalgae produces oxygen Therefore, by quantifying oxygen consumption rates, we can have a net measure of both aerobic and anaerobic respiration processes, and of photosynthesis during light, occurring in the sediment.
How do we measure it?
In our laboratory, we usually use two different type of techniques, one using microsensors to measure vertical profiles of oxygen across the sediment water interface and with depth in the sediment and another by measuring the changes in oxygen concentrations in closed core incubations over time.
These methods provide similar but complementary information.
Microsensors
Profiles made with microsensors provide important information as to the total flux of oxygen across the sediment water interface, but also to the deeper layers, the depth of maximum oxygen production in light corresponding to the layer of microalgae, and the depth of the oxic zone (when oxygen concentration goes to zero). However, given that this method is based on a microsensor, fluxes measured are solely diffusive (movements of oxygen at a microscale) and are very localized to a small area around the tip of the sensor (a few microns).
Whole core incubations
Whole core incubations and measurements of the oxygen changes, provides a more integrated estimation of the oxygen flux as it includes a larger area, the consumption due not only of the microorganisms but also of larger macroorganisms (worms, molluscs, crustaceans etc). It also allows to include the effect of another process, that of advection that moves the oxygen molecules at larger spatial scales (for example by waves, the ventilation of burrows etc). This is why, fluxes measured by whole core incubation are called sometimes total fluxes.
To do this, we close the core (cylinder) containing our sediment ecosystem. On the inside of the lid there is a gel that contains a special dye that reacts with oxygen. Using an optical fibre we can measure through the transparent lid the changes in the fluorescence of the compound as the oxygen concentration changes inside the core. Quite cool..
However, both of them allow us to study the sediment metabolism and depending on the specific aims of our experiment, we use one or the other, or both. The more tools, the better!
