Electrical properties of biological tissues have been studied for over a century [1]. A large variety of different biological tissues have been investigated with the help of impedance spectroscopy, a detailed review on various human and animal tissue and blood samples can be found, e.g., in the review by Gabriel et al. [2]. Fricke and Curtis [3] investigated the impedance of yeast cell suspensions in the early 1930´s. They concluded that the impedance at the surface of the yeast cell is derived from a poorly conducting membrane which acts as a static capacitor. The dielectric properties of biological cells and their different components (cell wall, membranes and cytoplasm) were summarized by Markx and Davey [4]. Over the last couple of decades a lot of research was done on the impedance of cell suspensions, in which a relationship between capacitance and viable cell number was reported [5, 6, 7, 8]. Fehrenbach et al. [5] used online capacitive measurements for biomass estimation of
In general, the use of impedance spectroscopy has two main advantages:
It is fast, and can therefore be applied for online-monitoring;
It is a non-destructive method and hence can be used for measuring living organisms.
The objectives of this work are to measure mass, size and diet-based distinguishability (differences, variations) of the aquatic worms
All measurements were performed using an Impedance/Gain Phase Analyzer HP 4194A (Hewlett-Packard, California, U.S.A) which was connected via four BNC cables to a BDS 1200 connection head containing a BDS 1309 measurement cell (NOVOCONTROL Technologies, Germany) thereby applying voltage and measuring current separately in a bipolar electrode configuration. The BDS 1309 consists of two gold plated electrodes with a Teflon® isolation ring in between, the diameter of the electrodes was 11 mm and the distance between the electrodes was 6.1 mm. This sample cell is especially designed for high permittivity liquids. The software WinDETA (NOVOCONTROL Technologies, Germany) was used to calibrate the system using the stray capacity of the cell (1.2 pF) and to perform the measurements. We measured the absolute impedance and phase of different worm biomass quantities in a frequency range from 100 Hz to 10 MHz.
All measurements were performed at room temperature. The analyzer settings were chosen to provide 37 data points on a logarithmic scale from 100 Hz to 10 MHz with threefold averaging. The obtained impedance and phase spectra were fitted with “EIS Spectrum Analyser” software [18] using the Powell algorithm [19] to fit the spectra. Fig 1(a) depicts an equivalent circuit for the system simulating two parallel ways for the current to pass through the cell: electrode – water – electrode (
The aquatic worms investigated in this work are called
The worms which we used for our experiments were cultured under the same experimental conditions but on different feeds, mainly TetraMin® fish feed and secondary potato sludge. TetraMin® is a commercial tropical fish feed which provides a complete diet for ornamental fish. Secondary potato sludge was collected from the process water of a potato starch factory.
In order to represent the environment of a real reactor (and thus the applicability of this method) as closely as possible, tap water was used as basis for the growth medium. Because living aquatic worms are constantly exchanging substances via their permeable skin (uptake of nutrients and exclusion of catabolic substances), this tap water was pH buffered. A controlled temperature and water flow system with 12 plastic flow-through beakers was utilized to test the different worm foods. Each beaker had a working volume of 770±8 mL and a surface area of 57 cm2. Water was continuously discharged with a drain pipe positioned approximately 2 cm above the artificial sediment (at the 150 mL mark). The beakers were submerged in a temperature controlled bath at 19±1 °C and retained in dark conditions until the food containing sediment was replaced and/or sampling occurred. Water was supplemented at a flow rate of 4.34 mL min-1, resulting in refreshment rates of 4 and 8 day-1, respectively. The water (pH 7.2, hardness 77 mg CaCO3) was a mixture of 70% tap water (Leeuwarden, the Netherlands) and 30% softened water. The water was disinfected with UV-C light to prevent inoculation of the beakers with microorganisms and was aerated to keep oxygen on a saturated level, pH was controlled by dosing HCl (37%) with the use of pH controlled dose pump (StepDos O8S). It was then supplied on the surface of each beaker by a needle dispenser, creating a constant flow of small droplets.
The total wet weight of the aquatic worms was determined with the help of a fine mesh on a laboratory balance. Prior to weighing, worms were kept in fresh buffered tap water without any food for 2 hours in order to give them time to purge their guts. Afterwards the worms were cleaned with fresh buffered tap water. The clean worms were collected on top of the mesh and paper towels were gently pressed against the back of the mesh for 10 seconds to drain the remaining water. Average individual weight was calculated by dividing worm biomass by worm number.
As a concluding statement, we would like to point out that
Fig. 3a shows the measured impedance spectra for 6 different amounts of biomass of living
At frequencies above 1 kHz the impedance decreases with worm biomass due to higher conductivity of the worm’s intra- and extracellular body fluids compared to the surrounding tap water. At frequencies below 1 kHz an increase of the impedance occurs due to additional capacitive effects due to polarisation of the worm membranes, represented by
The largest phase differences were found at 570 Hz, but the correlation between biomass and phase response is not linear at that frequency. Up until approximately 2 kHz, the phase response of the smallest amount of worms measured (30 mg) is slightly higher than that of tap water. In order to investigate this feature, the influence of the worms on the buffered tap water was investigated. The buffer was measured before and after 60 min (the typical time of a threefold measurement) exposure to living aquatic worms. The result is given in Fig. 4 showing the same features as the 0 mg and 30 mg curves in Fig. 3: A decrease of the impedance at all frequencies accompanied by a small increase of the phase in the low frequency range only. This effect can be explained by the fact that the skin of the worms may leak small amounts of proteins or salts, thereby reducing impedance and increasing electrode polarisation. If there is sufficient water left in the cell, its impedance is lower at low frequencies where ion movement plays a major role. This effect disappears as soon as the biomass reaches a certain volume. In order to avoid this effect we chose 90 kHz as measurement frequency, which allows good resolution and small deviations in between single measurements (small error bars), and the results (Fig. 5) are not influenced by small composition changes of the buffered tap water (see Fig. 4).
It is important to point out that the general contribution of electrode polarisation at this frequency is very small (see Fig 1c). Since it is a feature of the liquid, not of the worms, it can be assumed constant for all worm concentrations. Because the evaluation is based upon phase differences, electrode polarisation contributions cancel each other out. In Fig. 5 we present the aforementioned linear relation between worm biomass and phase response.
Apart from the worm mass investigation, as feasibility study two groups of worms feeding on different diets were compared in order to see whether such a differentiation is possible. Fig. 5 shows that this might indeed be the case, since the slopes for worms with different diets are different. A covariance analysis [28] was performed to determine whether this difference is significant or not. In detail, this covariance analysis was executed using an independent variable
Covariance analysis of the regression slopes of mass (m) against phase (φ) at 90 kHz of worms fed with either TetraMin® (T) or secondary potato sludge (S) as shown in Fig. 5. SD means standard deviation.
TetraMin® (T) | Sec. potato sludge (S) | |
---|---|---|
18.00 | 30.00 | |
140.00 | 142.00 | |
118.27 | 110.09 | |
-4.11 | -4.50 | |
3.31 | 3.63 | |
-1.00 | -1.00 | |
-46.18 | -65.32 | |
< 0.0001 | < 0.0001 | |
-0.0278 | -0.0328 | |
-0.2227 | 0.163 | |
-0.0308 | ||
-0.534 |
The covariance analysis shows that the difference between the two slopes is highly significant (P<0.0001); so the slope of the regression lines depends on the diet of the worms. As can be seen in Fig. 4, changes in the composition of the buffered tap water due to the metabolism of the worms are not visible at this frequency (90 kHz). Instead this effect is caused by small changes in the composition of the worms which can be associated with the composition of the feed. A steeper decline equals a stronger phase response dependence which in turn signifies a higher capacitance of the sample. Such an increased capacitance of the worms fed with potato sludge results either from more worms with a high permittivity, or fewer worms with a lower permittivity than the worms fed with TetraMin®. Whereas the exact composition of aquatic worms is complex and cannot be simplified easily, it can be expected that the substance with the lowest permittivity accumulated in a worm is fat, and the highest permittivity is found in ionic solutions within the worms. A possible, reasonable consequence from this train of thought would be that the worms fed with potato sludge either contain less fat or more ionic solutions than the ones fed with TetraMin®. A validation of this assumption which will include an actual fat measurement and the analysis of the admittance spectra is planned and will be subject of a work subsequent to this one.
Because individual worms always differ in size from each other, the ratio of worm surface to worm mass and thus the amount of membrane (skin) per worm is different at each measurement. This difference is visible in the region where membrane polarization is predominant, but does not influence measurements at higher frequencies, as illustrated in Figs 6 and 7: Fig. 6 shows the dependence of average worm size on the phase response at 440 Hz. At this frequency, the phase shift consists of electrode polarization (see also Fig. 1b) and α-dispersion. Although clearly larger than at 90 kHz the electrode polarization contributions can again be assumed to be constant for different worm concentrations, thereby cancelling each other out when phase shifts are compared. The resultant slope is thus a measure for α-dispersion. Whereas this evaluation worked well for the data presented, we would like to emphasize that it should be applied with cation since electrode polarization may vary with time or other factors that cannot be controlled. Fig. 7 shows the independence of the average worm size (no significant phase difference) in the β-region at 90 kHz for the same group of worms.
Measurement of impedance and phase shift in the frequency range from 100 Hz to 10 MHz provides information about the biomass of living aquatic worms. We have shown that the biomass of the aquatic worm