Metabolism in other small organisms, such as bacteria, are measured by taking the metabolism of a very large population of organisms, and then dividing that by the count of organisms in order to obtain the metabolic rate of a given individual.
In this paper on the metabolism of E. coli, they looked at the various input and output chemicals. Glucose, biomass, oxygen, and carbon dioxide were all observed. Observation of multiple variables also helps in the reduction of errors by using the stoichiometric balance to make sure that no errors have occurred in the measurements.
The methodology section of the paper had the following techniques:
Continuous cultivations were performed at 28 °C, at a stirring speed
of 800 r.p.m., and the pH was controlled at pH 6·6 with 2·5 mol NaOH
l−1. The 2-l bioreactor (model SGI 7F-Set2; Setric) was equipped with
standard measuring and control units (temperature, pH, dissolved
oxygen, stirrer speed). A constant aeration rate of 1·33 v.v.m. was
achieved by a mass flow controller (Brooks). Continuous cultivation
was performed for at least eight residence times at a given dilution
rate; steady-state conditions were verified by constant optical
density and oxygen and carbon dioxide transfer rates. Except for
glucose, all other substrates were in excess. The concentration of
dissolved oxygen was always maintained above 20 % of air saturation.
Culture liquid was withdrawn from the bioreactor into an ice-cooled
beaker. Cell dry mass was determined gravimetrically after
centrifugation of a known sample volume. The pellet was washed twice
with 0·9 % w/v NaCl and dried at 40 °C under vacuum for 48 h.
Elemental biomass composition was analysed with a PE 2400 II elemental
analyser (Perkin Elmer). Glucose was measured enzymically by a YSI
glucose analyser (model 2700; Yellow Springs Instrument Co.). Organic
acids, i.e. acetic acid, formic acid, lactic acid and pyruvic acid,
were analysed by HPLC using an HPX-87H Aminex ion-exclusion column
(Bio-Rad). The column, connected to a UV detector (λ=210 nm) and RI
detector, was eluted at 50 °C with 5 mmol H2SO4 l−1 at a flow rate of
0·5 ml min−1.
If the tardigrades are grown in solution, an identical methodology can be used to measure respiration rate.