Team:Dundee/Project/DetectionComparison

Introduction

The current method for regulating toxic levels of microcystin does not involve directly detecting microcystin, but instead uses cyanobacteria cell counts. One direct method for detecting microcystin is to take water samples and carry out high performance liquid chromatography (HPLC). This process takes approximately 24 hours. Using our biological detector we hope to reduce this time. We examine the affect that this lengthy detection time can have on the change in numbers of cyanobacteria and microcystin found in the water body that is being tested. This then allows us to determine whether faster detection methods are necessary.

Theory

Making the following assumptions:

• t = 0 is the time water samples are taken
• b0 is the initial number of cyanobacteria at t=0
• t is the time in hours after the water samples are taken
• cyanobacteria undergo binary fission every hour
• cyanobacteria growth is uninhibited
• each cyanobacteria releases N microcystin molecules

We arrive at these equations:

where b(t) is the number of cyanobacteria at time t and MC(t) the number of microcystin molecules.
HPLC results are obtained 24 hours after the water samples are taken i.e. t=24. We compare this against our aim of a 1 hour detection time t=1 by evaluating equation (2).

Dividing MC(24) by MC(1) we recover an expression for MC(24) in terms of MC(1).

Results

At the detection times, t = 24 hours and t = 1 hour, the ratio for the number of microcystin molecules is 8.4 million : 1.

Correspondingly after 24 hours there can be up to 8.4 million times more microcystin molecules present than there is after 1 hour. Putting this ratio into perspective, this is the same as the height of the Empire State Building being compared with the combined height of 7 E.coli.

Therefore in the time period between collection of samples and obtaining results there can be vast increases in the concentration of microcystin present in the water body. This emphasises that HPLC is an unsuitable method for toxin detection and that a 1 hour detection period is much more efficient.

Generally we can conclude that faster detection methods are necessary and our biological detector is worthwhile pursuing if we can reduce this detection time.