Forensic sciences: Effects of stress and perturbations on soil communities/Conclusion

Main results

In this study, effects of stress and perturbations on testate amoebae soil communities due to a cadaver and its body fluids were observed during four weeks. We were interested to discover if the effect of the cadaver is due to its body fluids only, whose effects are still unknown, or to its entirety. We used three different treatements, pig blood, cow manure and dead pigs heads. Control pots were set up as well.

The first purpose of the study was to establish a link between the ratio of survival testate amoebae and dead ones according to the different treatments throughout the time. The second purpose of the study was to look for a chronological pattern in the species succession through time, via changes in communities structure, in function of the treatments. For this purpose, the Shannon index of the samples was analysed in fonction of time and treatments. The pH and the released CO₂ flow changes induced by the treatments were also measured. All this was done because previous studies have shown an influence of corpse on testate amoebae survival rate and species succession through time. Therefore, these organisms could be used by forensic experts as a tool to establish long PMIs.

Our study did not work as well as expected. No concluant result has been obtained for the evolution of the living/dead ratio and the variation of the community structure through time. However, pH and respirometry have nonetheless given the expected results, with increases in the pH and the respiration rate for the blood and pig treatments.

However, our negativ results are useful to show some facts. We have seen that it is really difficult and time-consuming to work with and recognise testate amoeba with an optical microscope. Our results show us that, in a so small timespan, the communities changes are not enough relevant to see a variation. Furthermore, it was for us a good way to apply our knowledge and see the gap separating theory and practice.

Problems occurred

The bad results are due to many reasons. One of the main problem was the staining of living cells, which didn't work as expected at T0 and T1 for an unknown reason, and totally biased the living/dead ratio. The identification of testate amoebae gender was difficult to do with accuracy. This was due to our lack of experience in amoebae determination. This complicated a lot the communities analysis. Furthermore, it is fundamental to observe a larger number of amoebae per sample to obtain conclusive results. Moreover, it is possible that environmental changes induce encystment instead of death, which was not possible to observe. To finish, it is a bad idea to store the treatment pots in a greenhouse because the temperature is too high and it dries the soil and the treatments, especially pigs heads and manure. Thus, it alters the decomposition process and the amoebae development, which both need moisture.

Further studies

One way to improve this study could be the use of molecular tools to establish gender and species with higher precision. It could be also interesting to test in which way the bacterial communities changes, induced by the presence of corpse, has an influence on predator communities as testate amoebae. Furthermore, long-time studies should be made to establish when the amoebae presence decreases and when it recolonises the soil. The longer timespan can also reduce the influence of the daily weather, but it could show a greater seasonal influence. To finish, it could be useful to start a study in order to check if the community structure and living/dead ratio changes observed in previous studies are due to variation of pH, moisture rate, nutrient content of soil or variation in preys availability.