AWA: Academic Writing at Auckland

Abstract

Amylase is an enzyme that is required to catalyse the biochemical digestion of starch polysaccharides. It is therefore required by organisms, specifically those who obtain a plant-based diet, to assist in this breakdown in order to gain nutritional value from their diet. This study was conducted in order to determine whether there are any differences in the enzyme activity of herbivorous and carnivorous gastropod species. The study involved two of New Zealand’s intertidal gastropods, T. smaragdus, the cat’s eye snail, and C. adspersa, the speckled whelk. Data was obtained by laboratory experiments which tested the amylase activity in the digestive gland of each species. The results of this study conclude that the herbivorous gastropod species had a much higher amylase activity level than the carnivorous species, which is due to the species’ high starch diet of micro and macroalgae. Overall, the results of this study give evidence to believe that there is a significant difference in the enzyme activity levels between herbivorous and carnivorous gastropods.

Introduction

Starch is a plant storage polysaccharide found in higher plants such as angiosperms. It is also found in green algal species, as well as in the form of floridean starch in red algal species (Barsby, Donald & Frazier, 2001). Starch is made up of two polysaccharides; a-amylose, a linear polymer, and amylopectin, a branched polymer. Enzyme a-amylase is responsible for the hydrolysis of the a-1,4-linkage glycosidic bonds of starch, and it hydrolyses starch into oligosaccharide molecules such as maltose (Munegumi, Inutsuka, & Hayafuji, 2016).

Turbo smaragdus, also known as the cat’s eye snail, is a native gastropod species which commonly resides in the intertidal/subtidal regions of New Zealand. This species is a herbivorous grazer that feeds on a range of microalgae and macroalgae species that are present in its rocky shore habitat (Alfaro, Dewas, & Thomas, 2007). To contrast, New Zealand speckled whelk Cominella adspersa is a carnivorous species, which occupies sublittoral mud flats and low intertidal areas (Stewart & Creese, 2004). According to Stewart and Creese (2004), it is not distinctive as to whether species from the genus Cominella are scavengers or predators, although mortality of clams in areas where species C. adspersa resides has shown to be higher, indicating a likelihood of common predatory occurrences. It is expected that species T. smaragdus will exhibit higher levels of a-amylase than carnivorous C. adspersa, caused by a higher intake starch due to a plant-based diet of micro/macroalgae. The aim of this study was to compare the amylase activity between herbivorous and carnivorous gastropods species to determine any differences in enzyme activity.

Results

The data for this study was collected in a laboratory environment; and was then compared with averages of the data sets collected by the class of University of Auckland ecological physiology students. The experiment involved the dissection of the digestive gland and subsequently used to measure amylase activity (see laboratory guide for full explanation of methods involved).  

Figure 1 shows the standard curve has an R² value of 0.97, which shows that 97% of the variation in amylase activity can be explained by the absorbance (nm). This indicates that the linear regression trend-line fits the data very well and therefore can be accurately used to make predictions of amylase activity. The positive increasing trend shows increased amylase activity (via absorbance readings) as the enzyme unit’s increases.

Figure 1: Scatter plot showing the standard curve of amylase activity at an absorbance of 590nm.

Figure 2 shows that there is a significant difference in mean amylase activity between the two species studied. T. smaragdus organisms had notably higher mean amylase activities than the carnivorous snails from species C. adspersa. However, it is interesting to note the difference between the two members of the herbivorous species Turbo smaragdus, as the second snail studied has a much higher mean amylase activity than the first.

Figure 2: Bar graph showing mean amylase activity levels for two herbivorous organisms and two carnivorous organisms.

The data for figure 3 was generated using an average slope from the class standard curves; therefore results with more error may have been generated. However, a clear difference is displayed for the overall amylase activity different between the two species even when considering the standard error values shown by the error bars. Herbivorous species T. smaragdus shows a significantly higher mean amylase activity across the class results when compared with carnivorous species C. adspersa.

 Figure 3: Bar graph showing the mean amylase activity of the two gastropod species, using averages from the class data with satisfactory results.

Discussion

According to Foster, Hodgson, & Boyd (1999), an important mechanism of algal digestion in mollusc species is enzymatic breakdown. Molluscs have a range of carbohydrases that have a greater specificity to the substrate, as well as a higher efficiency to catalyse reactions in comparison to the abilities of phyla such as Crustacea, Echinodermata and Annelida. In herbivores such as T. smaragdus, these glands capable of enzymatic secretion have the ability to act on starch molecules. When comparing the results of this study with the averages from the class data, the results from both clearly display a significant difference between the amylase activity levels found in the digestive glands of the species (see fig. 2 & 3). The herbivorous species of gastropod, T. smaragdus, exhibits a much higher mean amylase activity level than the carnivorous species, C. adspersa. The main cause for this difference is likely to be caused by the differences in diet between the two species. Herbivorous species T. smaragdus has a diet which consists primarily of algae, therefore explaining why there is more amylase activity occurring within the digestive gland. A study carried out by Foster, Hodgson, & Boyd (1999) on T. samarticus showed that this species potentially has carbohydrases that have the ability to digest storage and structural polysaccharides found in red, green and brown algal species. This was displayed by high levels of enzyme activity on the storage polysaccharides found in red and green algae, and low levels found on the brown algae, along with low levels for the structural polysaccharides. There is a possibility that the results they concluded were influenced by the availability of algal species within the habitat rather than true biological enzyme activities, and this is something which could be further researched. This may also apply to this study, as it was not taken into account what diet the organisms had been exposed to in the days before they died.

Foster, Hodgson, & Boyd (1999) have stated that there are three possible mechanisms of polysaccharide breakdown in molluscs; mechanical breakdown, biochemical breakdown (enzymatic), and via microbial fermentation. These mechanisms may be used in conjunction with one another. However, the study they reported discussed only the biochemical breakdown in rocky shore gastropod species T. samarticus, as was the case with this study (Foster, Hodgson, & Boyd, 1999).  As these digestive mechanisms may work simultaneously, further research into whether there is any degree of microbial fermentation occurring amongst either of these species could be undertaken, as this may further influence carbohydrase activity. Another factor to consider is any difference in enzyme activity between the esophageal gland and the digestive gland. A sea snail’s digestive enzymes are largely produced by the esophageal and digestive glands (Fretter & Graham, 1994). It would be interesting to further research the esophageal gland for T. smaragdus and C. adspersa, and make comparisons with the digestive gland enzyme activity levels to determine any differences in the location of this biochemical breakdown.

It may be of interest to determine what factors influence the diet choice made by herbivorous grazers. In a study carried out by Zemke-White & Clements (1999) on diet choice in marine herbivorous fishes, it was concluded that it is both the algal nutritional content along with the digestibility of the algae which influences the fish’s choice of diet. Their research found that for algal species that were a food source for the herbivorous fishes, the starch content along with the rate that the starch could be digested was higher than it was for algal species not classified as a food source for these fish. Zemke-White & Clements (1999) also highlighted the fact that in order to estimate bioavailability of an algal species, both the nutritional contents and the digestive ability/capacity of the herbivore consuming it need to be considered. Similar research could be undertaken in relation to marine intertidal gastropod species in order to determine the potential reasoning behind their dietary choices. This could involve the consideration of the algal starch content and the digestibility of these algal species by the herbivorous gastropod consuming it. Researching the effects of diet choice may be beneficial for the understanding of the high enzyme activity displayed by herbivorous species when compared to carnivorous species.

There is a chance that, due to the nature of the lab experiment, human error may have affected the results of the study. When considering figure 2 of the results section, although a clear difference is visible between the two species, there is also a notable difference between the values for each of the T. smaragdus organisms studied. It is a possibility that this interspecies difference may have been caused by error during the dissection of the organism; for example some other tissue from the organism may have been accidentally included in the digestive gland sample. It is also necessary to note that the results of the graph showing the class averages (see fig. 3) were calculated using the enzymatic activity from the absorbance of each group, but by using the average of the good standard curves that displayed a high R² value. Each group involved used a different spectrophotometer. Therefore, these averaged results are potentially not as accurate as they would have been if comparing the standard curve for each group separately.

In conclusion, the aim of this study was to compare the amylase activity between herbivorous and carnivorous gastropods species to determine any differences in enzyme activity. The results of this study conclude that the herbivorous cat’s eye snail T. smaragdus had a significantly higher amylase enzyme activity level in the digestive gland in comparison to carnivorous speckled whelk species C. adspersa. This is most likely due to the plant based diet consumed by the herbivorous snails, meaning this species requires higher amylase activity to break down a high starch diet. Further research may involve studying the algal starch content and the digestibility of these algal species by herbivorous gastropods to better understand the reason for a herbivorous species’ high enzyme activity. Further studies may also include researching into the esophageal gland for these species and making comparisons with the digestive gland enzyme activity levels to determine any differences, and also the consideration of whether there is any degree of microbial fermentation occurring amongst either of these. Factors they may have influenced the results of this study may have included a degree of human error, as well as using an average of many standard curves generated by different spectrophotometers, which may have altered results slightly. Overall, due to the high starch ingestion of herbivorous gastropod species compared to carnivorous species, a greater amylase activity is required to assist in this polysaccharide breakdown.

Reference list

Alfaro, A.C., Dewas, S.E., & Thomas, F. (2007) Food and habitat partitioning in grazing snails (Turbo smaragdus), northern New Zealand. Estuaries and Coasts, 30: 431–440.

Barsby, T. L., Donald, A. M., & Frazier, P. J. (2001). Starch: Advances in structure and function (Vol. 271). Royal Society of Chemistry.

Foster, G. G., Hodgson, A. N., & Boyd, C. S. (1999). Polysaccharolytic activity of the digestive enzymes of the macroalgal herbivore, Turbo sarmaticus (Mollusca: Vetigastropoda: Turbinidae). Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology,122(1), 47-52.

Fretter, V., & Graham, A. (1994). British prosobranch molluscs: their functional anatomy and ecology. Great Britain: The Dorset Press

Munegumi, T., Inutsuka, M., & Hayafuji, Y. (2016). Investigating the Hydrolysis of Starch Using α-Amylase Contained in Dishwashing Detergent and Human Saliva. Journal of Chemical Education. 93 (8): 1401–1405. doi: 10.1021/acs.jchemed.5b00545

Stewart, M. J., & Creese, R. G. (2004). Feeding ecology of whelks on an intertidal sand flat in north‐eastern New Zealand. New Zealand Journal of Marine and Freshwater Research, 38(5), 819-831.

Zemke-White, W. L., & Clements, K. D. (1999). Chlorophyte and rhodophyte starches as factors in diet choice by marine herbivorous fish. Journal of Experimental Marine Biology and Ecology, 240(1), 137-149.