— Written by Ean Eberhard

When looking at the cell abundance across three light intensities, Low (60µmol(photon)/m²/s), Medium (400µmol(photon)/m²/s), and High (800µmol(photon)/m²/s) and four different temperatures, I found that the light intensity does have an effect on the cell abundance. At the lowest light intensity, the highest temperature of 30°C showed to have very little growth while all other temperatures steadily grew. However, when the light intensity increased at the highest temperature the cell abundance began to increase. This alone suggest that there is a complex response of phytoplankton to multiple stressors. If one was to do an experiment where all temperatures were only tested across the same low light intensity, then he would conclude that when temperatures are high there is very little growth in cell abundance and thus high temperatures are detrimental to phytoplankton. This statement is not true however because as stated previously, as the intensity of light increased, the high temperature cultures were growing well. The common trend across all three light intensities was that the 25°C temperature had the highest cell abundance. Now what about the growth rate?

When looking at the growth rates across the four different temperatures I found a common trend that the lowest light intensity had the slowest growth rate. This low growth rate can be compared to the cell abundance and show a very steady increase in number. The other common trend is that the growth rate of both medium and high light intensities seems to be pretty similar across all four temperatures. Both these higher light intensities grow fast quick then decline in growth rate not soon after especially at 25°C. At 20°C the higher light intensity cultures do not decline in growth rate nearly as fast but also have smaller cell abundance. Again, this proves that there is an interaction between both temperature and light intensity in relation to both cell abundance and cell growth. So, we know the relationship between cell abundance and growth rate but what about the cell size? When the abundance increases are the cells increasing in size or decreasing in size?

Taking a look at the cell size compared to the cell abundance across the three light intensities and four temperatures I found a general trend of a lower cell size when there is a higher abundance. However, there is more to look at. In the acclimation phase at the lowest light intensity there is reason to note that at the highest temperature of 30°C the cells abundance increases the least, as stated before. Although the cell abundance is not increasing the cell size is significantly increasing. The exact reason for this is unknown and further analysis is needed however, I can assume that the cells are allocating its resources towards cell growth and not cell division. This increase in the individual cell size may mean a greater number of chlorophyll, increasing the cells chance to obtain more light (as we know they want at the low light intensity and high temperature treatment) and grow in abundance.  The cell abundance in medium light shows that 30°C and 25°C have similar abundances at experimental day two but the 25°C has a much larger cell size. This means there is higher biomass at 25c. This higher biomass means that at a higher temperature there is less biomass. However, at the high light, the highest temperature of 30°C has the highest biomass as they have the largest cells while similar to the 25°C abundance. At medium light intensity, the 25°C has a higher biomass until we increase the light intensity and find that the 30°C has higher biomass. This again shows a more complex interaction between stressors; the higher light intensity counteracts the original assumption that the higher the temperature the lower the biomass. It seems that with a high light intensity and high temperature the cells seem to be doing fairly well in regard to biomass. The question is, is this biomass good biomass or not (quality food or bad food, this could affect the trophic cascade). In some cases, such as in medium light, the high temperature means less carbon will go into the system which means less carbon for trophic levels and less absorbed from atmosphere. In high light we can see that the 15 and 20 have larger cells than the 25 who has the highest abundance. Yet the highest temperature at highest light intensity is doing best, this means that with different combinations of stressors we have different complex responses. Again, we can ask are those larger cells better and more efficient than a bunch of smaller cells?

Does that mean that its producing more of lower quality cells? A further chemical analysis of the cells material composition is needed to understand the quality in regard to trophic cascade. Finally, I can take a look at the QY Max values of each culture to get an idea of the cells efficiency within its environment.

A look at the QY Max values across the four temperatures at the three light intensities shows a general trend of higher light intensities across all temperatures had the lowest efficiency while the low and medium intensities would trade places for most efficient dependent on the temperature. When I compare the 25°C high light intensity treatments efficiency and cell size I find that it has a very small cell size with a low efficiency however when I compare this to the cell abundance I find that this treatment has a very high cell abundance. If one was to simply use cell abundance as proxy for the health of the phytoplankton than he would not have the full picture and say that these cells are doing very well, however, they are not. Another finding is that in the high light intensity at 20°C I found that as the cell size increases the cell efficiency also increases. This means that an overall finding is that the lower the cell size the lower the efficiency and vice versa.

In conclusion, it is clear that one cannot simply look at one factor of a phytoplankton’s cell response to stress when determining its health. There are multiple physiological changes to consider and that must be compared to one another to effectively interoperate what is happening to the cells under stress. It is also clear that there is a complex response induced by interactions between multiple stressors that are not simply addictive.  We know this because the combination of High light and high temperature was actually positive for cell abundance and because if one was to do this experiment at one light intensity they would find that the high temp is detrimental however that is not always the case, high temperature at high light intensities means high growth.

There is still plenty to be considered and analyzed in this experiment. A few things include POC (particulate organic carbon), where we can ask about C/N ratios and how much carbon is being taken up by the cells or is the biomass of the same quality? Secondly, we should consider biogenic silica and ask about the difference in cell structure. One other consideration is looking at the light curves where we can ask about photosynthesis rates and whether CO₂ is being processed well by the cells. There are plenty of other questions to be answered and are underway. This was only the beginning.