LAB 6: GROWTH RESPONSE OF Euglena gracilis & Selenastrum capricornutum IN RESPONSE TO ph

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1 LAB 6: GROWTH RESPONSE OF Euglena gracilis & Selenastrum capricornutum IN RESPONSE TO ph By: Billy Bob Joe Student No Lab Section: 0000 T.A: Brunette Blow Date: March 22,

2 Objectives: The main focus of this experiment was to study the effects of an environmental stress (ph) on population growth of two algal species: Euglena gracilis and Selenastrum capricornutum and to determine the optimal ph. These species are studied in isolation and in competition with each other to determine their effects on growth rate under various ph environments. Results: From Figure 1, it was noted that cell density of Euglena in isolation was higher when ph of the environment was low. On the other hand, Figure 2 showed that cells density increased as ph of the environment increased for the Selenastrum in isolation. Using the cell densities from the figures, the growth rates were calculated along with the doubling time and growth constants, Table 1. From the table, it is noted that the doubling time (the time it take the cell population to double) is a lot longer for Selenastrum then the Euglena at low ph. The reverse is true for Selenastrum at higher ph. Table 1. Euglena gracilis & Selenastrum capricornutum growth rates in isolation Selenastrum Euglena Growth Doubling Doubling Constant Time (t d ) Growth Rate (μ ) Growth Time (t d ) Growth Rate (μ ) ph (k) (days) (divisions / day) Constant (k) (days) (divisions / day) For the separate population that were grown in competition, the population density was calculated and recorded in Table 2 below. The table below illustrates that 2

3 Euglena population decreased and Selenastrum population increased as ph increased. It was also noted that there more Selenastrum initially then Euglena. Table 2. Euglena gracilis & Selenastrum capricornutum population density in competition ph 4 Cell density ( 10 cells / ml) Euglena Selenastrum Day 0 Day 7 Day 0 Day These population densities were then used to calculate to growth rates, which are illustrated below in Table 3. From this table, it can be seen that the growth rate is negative for Selenastrum at lower ph. The opposite is true for Euglena. Table 3. Euglena gracilis & Selenastrum capricornutum growth rates in competition Selenastrum Euglena ph Growth Constant (k) Doubling Time (t d ) (days) Growth Rate (μ ) (divisions / day) Growth Constant (k) Doubling Time (t d ) (days) Growth Rate (μ ) (divisions / day)

4 Figure 1. Growth Response of Euglena gracilis to ph 4

5 Figure 2. Growth Response of Selenastrum capricornutum to ph 5

6 Discussion: According to experimental data (Table 1), the Euglena s growth rate increases as the ph of the environment decreases. This is evident in the log phase (linear portion) or the exponential phase of the ph 3 curve of Figure 1, which grows the fastest (steeper line). Although the growth rate is higher at a ph of 3, Figure 1 indicates that there is a large decrease in cell density during the stationary phase of ph 3. This could mean that as the population becomes crowded (greater growth rate) there are limited resources which would decrease cell density. The cell density would decrease because all resources would be used up more rapidly and newly divided cells would not have any (Mason & Manne, 2003). Therefore an optimal environment whose ph is 4 would be better for Euglena as Figure 1 shows. This is because there is a stable stationary phase for ph of 4 in contrast to ph of 3, which allows for a longer life span. At higher ph, the growth rate of Euglena is low (Table 1.), and therefore the population density is also low at any one time compared to lower ph. The lag phase for Euglena is longer for higher ph then for lower ph. This indicates that, the organism takes longer to adapt or adjust to the environment when the ph stress is high (Mason & Manne, 2003). Thus, if the environment was strongly basic, the micro-organism would not survive or adapt well. In contrast, the Selenastrum prefers a higher ph environment as noted by Figure 1, which indicates that log phase (linear portion) of the ph 7 has greatest the magnitude (steeper line). Thus, a greater growth rate (as well as high cell density) is attained in a neutral environment (ph of 7) for the Selenastrum. This environment is also optimal according to Figure 1, since the stationary phase is stable. However, it is not concluded this because an environment whose ph is higher was not studied in this experiment. 6

7 Nevertheless, this does explain the fact that in competition, the Selenastrum population increased and the population of Euglena decreased as ph increased. However, the Euglena population does not decline significantly over the wide range of ph tested in this experiment. In facts, as ph is increased, the Euglena cells density decreases gradually as illustrated by Table 1. Thus, the Euglena would be predicted to survive better in competition with Selenastrum. So if both species were living in a lake which was becoming more acidic, it would be predicted that the Euglena would survive better. In competition, the Selenastrum population density as well as the growth rate (Table 3) was considerably low (in fact negative, perhaps they were dying) when compared to the population density that was grown in isolation. This apparent effect is probably interspecific competition of Euglena and Selenastrum for resources. Interspecific competition is competition between different species for scarce resources (Purves, 2000). That is, the macronutrients and micronutrients added to algal medium become limited more quickly and population of species decrease. However, the Euglena population was higher in competition then in isolation. This was possibly due to the fact that the Euglena can feed on dead organic material (perhaps that of Selenastrum), although it is also photosynthetic (Purves, 2000). On the other hand, this result can also be due to an experimental error, one of which may have been poor experimental design or following incorrect procedures. This error (greater number of Euglena in competition then in isolation) was perhaps due to greater number species present initially in competition then isolation (Table 2). Having the exact number of species to start out with is important may have prejudiced the results. Another source of error that prejudices the results was the unavoidable error of counting 7

8 the cells with the haemocytometer as noted by Table 2. The inaccurate counting is responsible for certain for the negative growth constant as well as negative time in Table 3. Conclusion: Despite the errors encountered in this experiment, it can be concluded that the Euglena is an acid tolerant micro-organism since its growth rate increases with decrease in ph. The greatest growth rate for the Euglena occurs in an environment whose ph is 3. In contrast to the Euglena, Selenastrum prefers nearly a neutral or somewhat a basic environment since its greatest growth rate occurs at a ph of 7. In competition, the Selenastrum population was low compared to the Selenastrum population in isolation. This was most likely due to interspecific competition. Although the same was not true for the Euglena population, this can be reasoned with the error or the alternative possibility mentioned in the discussion. References Citied: 1. Dr. Andrew Mason and Lisa Manne (2003) Introductory Biology Part I Laboratory Manual, University of Toronto at Scarborough Department of Life Sciences. Lab 3: pp Purves WK, Sadava D, Orians GH and Heller HC (2000) Life the Science of Biology, 6 th Edition, Sinauer Assoc. (Publishers). pp 976, 483 8