Apoptosis, cell death
There are three major organelles that play a role in cell death, otherwise known as apoptosis. Chromatin condensation and genomic DNA fragmentation working together with cell membrane blebbing are said to be morphological hallmarks of terminal stages of apoptosis. Chromatin will undergo a phase change from heterogeneous network to a highly condensed form. The condensed nuclei can be identified by fluorescence microscopy, when the cells are stained it is easy to tell apart chromatin from non-apoptotic cells (Abcam, 2021). Chromatin condensation is one of the most important standards used when identifying apoptotic cells. (Oberhammer, 1994). Condensed chromatin is then fragmented by a specific nuclease called CAD which leads to generating fragments of approximately 200 base pairs, or DNA ladders. The most familiar way to detect DNA ladders is by genomic DNA fragmentation. If DNA fragmentation or CAD is lost it will led to significant increase in spontaneous or induced gene mutation, since these two are responsible for taking in genomic DNA during apoptosis (Yan, 2005). There is recent news that supports mitochondria’s involvement in cell death, newly identified mitochondrial electron transport chain, F0F1 ATP synthases, cardiolipins and ATPase. Chloroplasts don’t play as big a role as mitochondria but suggest they possibly contribute by producing reactive oxygen species in the presence of light. There are two different studies saying that mitochondria and chloroplasts could cooperatively execute PCD but could also operate in parallel to one another (Aken, 2015). Pyrenoid relation to apoptosis doesn’t seem to correlate much. Pyrenoid is a microcompartment within the chloroplasts of algae and hornworts, it is known for its function to promote photosynthetic CO2 fixation, so since chloroplasts play a large role in apoptosis you can indirectly say pyrenoids do as well since they are found within the chloroplasts (Zhan, 2018).
Pandorina, Chlorella, Rhodochorton and Volvox are four different types of algae. Volvox and Pandorina are found in freshwater habitats. These two and Chlorella are classified as phylum chlorophyta, which means they are green algae. Rhodochorton is a red algae so it doesn’t fit into this phylum chlorophyta like the other three. Rhodocorton is adapted to low light levels and is most commonly found in freshwater caves. Chlorella chloroplasts contain chlorophyll-a and –b and are without flagella. In most favorable conditions chlorella will multiply rapidly. Chlorella is considered to be a food source because of its high protein levels and essential nutrients. Volvox is composed of up to thousands of cells from two different cell types: flagellate somatic cells and germ cells that lack in soma. They contain flagella facing outwards, which is considered to be a special characteristic of them resulting in asexual reproduction. Volvox is found in deeper ponds and ditches and sometimes in shallow puddles. Pandorina is composed of 8, 16 and sometimes 32 cells. Each cell has two flagella with a large chloroplast with at least one pyrenoid. Pandorina is important since it shows the beginnings of polarity in colonies and differentiates Volvox from them since they have larger eyespots in the anterior cells. All four algae types are unique in their own ways but all play similar roles in our environment today. (Wikipedia Contributors, 2021a; Wikipedia Contributors, 2021b; Wikipedia contributors, 2020; Wikipedia contributors, 2021c)
Taphonomy experiments were performed on the four species of algae: Chlorella, Volvox aureus, Pandorina morum and Rhodochorton. The algae were then euthanized to prevent autolysis, the destruction of cells or tissues by their own enzymes, and allowed to decay in their natural habitats, either freshwater or seawater. Parallel experiments were performed under both oxic (with oxygen) or anoxic (without oxygen) conditions. These results showed no difference in patterns of decay and negligible effects on the decay of algae however, anoxic conditions seemed to work at slower rates. (Carlisle, 2021)
There were six different criteria being measured in all four algae species: nucleus visible, chloroplast holes/thinned, pyrenoid visible, cell collapsed, chloroplast holes and chloroplast collapsed. The first criteria, nucleus visibility, it appears that 70% of the cells showed nucleus visibility in the volvox to begin and then dipped below 20% of the cells and then to 30% three different times before slowly decreasing over time. For Pandorina, about all the cells showed nucleus visibility before dipping almost immediately to about 30% of cells and then for the most part decreased over time with a few peaks here and there ending around 10% of cells showing. Chlorella seemed to start around 30% of cells before increasing to a little above 60% after 3 days and then sort of declined and increased to about 25% at the end of the experiment. Rhodocorton was the most like volvox because it appears that they showed the minimum number of cells that showed nucleus visibility which was around 2-3% of cells. Moving to the next criteria where we will only look at the three green algae: volvox, Pandorina and chlorella. For chloroplast holes/thinned, volvox starts around 50% of cells and drops but then increases and remains steady to about 100% of cells. Pandorina is similar but different, starting at 0% and then slowly increases with a few dips and ends around 75% of cells. Chlorella started at 0% cells chloroplast holes/thinned and then rises somewhat steadily and then around day 21 goes to 100% cells and remains steady all the way through. The next criteria are pyrenoid visibility, volvox, Pandorina and chlorella have similar trends, Pandorina starts highest but they all show similar decreasing patterns, volvox ends up being lower ending at 0% of cells showing pyrenoid visibility. The only algae to have almost all cells collapsed immediately is Pandorina. Rhodocorton trends for chloroplast holes shows that they start low at 0% and then increases to 18% before dropping down on the 10th day but then increases for the most part to above 40% to end after the 45th day. Their trends for chloroplasts collapsed starts low and then increases to 90% by the 7th day before dropping to 40% before increasing back towards 90% of cells whose chloroplasts collapsed. (Carlisle, 2021)
Examining and analyzing the plant cells in the fossils, chloroplasts were visible in a fossil of a Zelkova leaf. A royal fern’s stem from Jurassic deposits shows a clear nucleus in each of their cells. I think it is interesting that nuclei are still present in the fossil because since they are the central and most important part of the cell, I feel they should be present and the least likely to break down easily. In the picture of the holotype of R. chitrakootensis, intracellular structures were originally interpreted to be pyrenoids but now are considered unlikely to be so. Since pyrenoids are microcompartments within chloroplasts, their main function is to act as centers of CO2 fixation by maintaining and generating CO2 rich environments, it makes sense for them not to be preserved in fossils since they most likely break down when CO2 is not present (Wikipedia Contributors, 2021d). In picture H & I, Shuiyousphaeridium and Dictyosphaera, early eukaryotes show structures that are considered to be “putative” nuclei or chloroplasts. The nucleus being present in all these fossils of different plant specimens makes sense and I am not surprised that they seem to be showing up in fossil records, being that they are such an important part of the cell. (Carlisle, 2021)
In result, oxic vs. anoxic conditions when the experiment was conducted showed similar results. When the algae were tested under anoxic conditions the only difference was that it worked at slower rates. All four types of algae, especially the three types of green algae, seemed to line up more similarly in terms of the six criteria that was analyzed and observed throughout the experiments.
Works Cited
Abcam. (2021). Nuclear condensation, DNA fragmentation and membrane disruption during apoptosis. Abcam, https://www.abcam.com/kits/nuclear-condensation-dna-fragmentation-and-membrane-disruption-during-apoptosis
Carlisle, E., Jobbins, M., Pankhania, V., Cunningham, J., and Donoghue, P. (2021). Experimental taphonomy of organelles and the fossil record of early eukaryote evolution. Science Advances 7, eabe9487.
Oberhammer, F. A., Hochegger, K., Fröschl, G., Tiefenbacher, R., & Pavelka, M. (1994). Chromatin condensation during apoptosis is accompanied by degradation of lamin A+B, without enhanced activation of cdc2 kinase. The Journal of Cell Biology, 126(4): 827–837.
Yan, B., Wang, H., Peng, Y., Hu, Y., Wang, H., Zhang, X., Chen, Q., Bedford, J., Dewhirst, M., Li, C. (2005). A unique role of the DNA fragmentation factor in maintaining genomic stability. Proceedings of National Academy of Sciences of the United States of America, https://www.pnas.org/content/103/5/1504/?tab=related#:~:text=Inhibition%20or%20loss%20of%20the,in%20primary%20mouse%20cells%20and.
Aken, O., and Van Breusegem, F. (2015). Licensed to Kill: Mitochondria, Chloroplasts, and Cell Death. Trends in Plant Science Vol. 20, Issue 11, 754-766.
Zhan, Y., Marchand, C. H., Maes, A., Mauries, A., Sun, Y., Dhaliwal, J. S., Uniacke, J., Arragain, S., Jiang, H., Gold, N. D., Martin, V., Lemaire, S. D., & Zerges, W. (2018). Pyrenoid functions revealed by proteomics in Chlamydomonas reinhardtii. Plos One Vol. 13, 13(2).
Wikipedia contributors. (2021a). Volvox, https://en.wikipedia.org/wiki/Volvox
Wikipedia contributors. (2021b). Pandorina, https://en.wikipedia.org/wiki/Pandorina
Wikipedia contributors. (2021d). Pyrenoid, https://en.wikipedia.org/wiki/Pyrenoid#:~:text=Pyrenoids%20are%20sub%2Dcellular%20micro,%2Dconcentrating%20mechanism%20(CCM).
Wikipedia contributors. (2020). Rhodocorton, https://en.wikipedia.org/wiki/Rhodochorton
Wikipedia contributors. (2021c). Chlorphyta, https://en.wikipedia.org/wiki/Chlorophyta