The Hot Potato: Starch, Thermal Adaptation and Enzymes

By Karen Scott (@DrKarenScott)

When the word geography is mentioned, microbiology is not always the first thought, however this was probably the main focus throughout my PhD studies. Being able to understand the microbiology of the environment can greatly assist in putting together the bigger picture, especially where sustainable organic waste management practises are concerned, as they rely ultimately on understanding the mechanisms that control organic matter decay.

This study builds upon my earlier findings on gully pot waste degradation and enzyme activity (see my previous blog) assessing the effect of starch on degradation rate and enzyme activity (α- and β-glucosidase) under mesophilic (moderate temperature typically between 20 and 45 °C) and thermophilic (relatively high temperature between 45 and 122 °C) conditions. A key focus was to assess how the enzymes potentially adapt to different temperature regimes to assist in understanding whether the effect observed was an acclimatisation effect, or it is in fact a true temperature effect that is a result of the composting process. This reciprocal design measured the differences between increased or decreased assay temperatures compared to the temperatures the waste was composted at. The enzyme activity was measured to assess the degradation dynamics. The experiment took place over a 6 week period where mesocosms (a mix of water and sieved gully waste) were incubated under mesophilic (30ºC) and thermophilic (50ºC) conditions, a starch additive was added to half of these mesocosms.

The messy job - mixing the slurry ready for the experiment

This study confirmed previous findings showing that the waste was able to decompose under both sets of temperature conditions. Interestingly though, the losses observed were similar throughout, indicating the losses were temperature independent, which appears to contradict the general literature, where temperature has been shown to be a main controlling factor during composting. However, many other studies have indicated that temperature is not necessarily the driving force in every situation.

The results for the enzyme activity did not follow the same suit, showing greater activity in those that had the starch addition, confirming that a starch addition significantly increases the activity of both α- and β-glucosidase, albeit weakly for the β-glucosidase. Whilst β-glucosidase activity is on average higher with the higher assay temperatures, this was not statistically significant in this instance, indicating neither in situ temperature or assay temperature are significant explanatory variables. However, the effect of increased activity of β-glucosidase with increasing temperature is consistent with previous results. Interestingly, the interactive effect of assay and in situ temperature on α-glucosidase activity is weakly significant which would be indicative of an adaptive effect of temperature on the enzymes responsible for this activity. 

The mean activity for b- and a-glucosidase over the sampling period. the labels for each bar shows the in situ temperature followed by the assay temperature.

Similar results were obtained when α-glucosidase activity was investigated with the starch amended samples only. Therefore, the data for α-glucosidase assay provides evidence for thermal adaptation in this environmental system. The results presented here would be consistent with α-glucosidase being under selection (in an evolutionary context). Whilst α-glucosidase activity is on average twice as high when assayed at 50ºC than 30ºC for the 50ºC starch amended samples, it is only 20% higher when assayed at 30ºC as compared with 50ºC for the 30ºC starch amended samples. Therefore it appears that as enzymes become adaptive to increasingly high temperatures, they become increasingly less adaptive at lower temperatures.

This study demonstrates a positive association between enzyme activity and its putative substrate (starch) and also the effect of temperature appears to have an adaptive selective influence on enzyme activity. Examining these temperature variables provided a better understanding of the processes occurring and displaying the wastes ability to decompose, to a degree, under conditions similar to those that it would be subjected to if using ex situ management system. Whilst complete agreement between enzyme activity and in situ degradation was not observed, it does leave scope for more investigation into enzyme parameters which could potentially shed more light on these issues.

The paper which this blog was based on can be found here

Adams, J. D. W. and Scott, K. M. (2013). Enzymatic analyses demonstrate thermal adaptation of α-glucosidase activity in starch amended gully waste. Bioresource Technology 127(0): 231-235.