Keigan Thornton BIOL 293 – Biology Spring 2021

Cell and the Origins of Complex Life - Plant Cell Death and Decomposition

Cell death, also known as apoptosis, can help scientists uncover the mechanisms life and are key components in complex life forms. Compared necrosis, where cells are destroyed due to outside influences like temperature and chemical changes, apoptosis is actively participated in with the cell to allow the formation of new cells (FreeMedEduaction 2018). Three major organelles that have a key role in apoptosis. The first organelle that contributes to apoptosis is the nucleus. The nucleus is not only the source of DNA in a cell, but it also contains significant signals to start the process of apoptosis. The key qualities of nucleus induced apoptosis include condensing DNA and then fragmenting it to prevent future use in making new cells or proteins (Prokhorova et al, 2015). The second organelle would be the . Found in plants, and they key organelle to photosynthesis is also a major organelle in the process of apoptosis. However, unlike the nucleus changing the structure of DNA which causes the cycle of apoptosis, chloroplast is involved in amplifying reactive oxygen species (ROS) and other cellular reactions that cause a cell to die and no longer function (Ambastha, Tripathy, and Tiwari 2015). These processes also then signal to the cell to go through with apoptosis. The , the last organelle that causes apoptosis is more peculiar than the nucleus and the chloroplast. Found in or around the chloroplast within plants, this structure is very important for carbon fixation which then makes it crucial in sugar production (Lewin and Anderson, 2021). Pyrenoid induced cell death is somewhat unclear but their overproduction and abundance older cells stimulate the cascading effects of apoptosis, causing cell degradation and death (Linhong, Lanping, and Qinqin, 2011). Though other mechanisms cause the death of a cell, these organelles in particular form the basis of ending the life cycle of a cell. Within various forms of life these organelles can be found in various combinations depending on the type of organism that the cells make up. To look further into cell death and the complexities of life four different algae will be analyzed. These four algae are morum, aureus, mixed species of Chlorella, and mixed species of Rhodochorton. Figure 1 shows various traits among these organisms, including environment found in, and apoptosis involved organelles (‘Pandorina’, 2021; Pandorina morum (O.F. Müller) Bory 1826, 2021; Nazrul Kabir, 2021; ‘Chlorella’, 2021; ‘Chlorella pyrenoidosa’, 2021). In analyzing these algae types, those with the most in common include Pandorina morum and Volvox aureus, having multicell structures and the presence of both and . However, in looking over the latter two algae species more traits vary. Cholrella varies among the specific species, some lacking and some containing pyrenoids all the while being single celled. Somewhat similar however is Rhodochorton, but the multicell nature of this cell, lacking pyrenoids completely changes the variable in where pyrenoids appear. Overall, in understanding the complexities of life, and cell death the complexities of these seemingly simple algae must be looked at closer. Also, further study and understanding of the roles of nuclei, chloroplasts and pyrenoids should also be investigated to ask how pyrenoids play a role in cell death and overall, how, and why these complexities arose in the first place.

Algae Species What environment is Single celled or Chloroplasts, it found in? multicellular? Pyrenoids, or both present? Pandorina morum Freshwater Multicellular Chloroplast, and pyrenoid Volvox aureus Freshwater Multicellular Chloroplast, and pyrenoid Mixed Cholrella Marine/Saltwater Single celled Chloroplasts, pyrenoid presence is species- dependent Mixed Rhodochorton Marine/Saltwater Multicellular Chloroplasts only Figure 1. Algae type variations.

Current research on how cell death occurs is currently studied and thoroughly documented by researchers. Major findings can expand on how plant cells interact which could open a door in understanding other cell types in the future. Below to investigate major findings information from ‘Experimental taphonomy of organelles and the fossil record of early eukaryote evolution’ by Carlisle, et al., 2021 will be discussed.

Looking at Volvox aureus, Pandorina morum, Chlorella sp., and Rhodochorton sp. researchers analyzed how the cells decayed over time. Overall, the research suggested that the chloroplasts took a longer time to decay than the nuclei of the cells. Additionally, pyrenoids, mentioned earlier, would be less likely to be found in any fossilized algae but the analysis of spacing in the fossilized cells could allude to the possible previous presence of these structures. Carlisle et al. used 300 millimolar of Beta-mercaptoethanol (BME), a substance used to denature RNA by reducing form and destroying enzyme function, to euthanize the cells (2021). This would prevent the cells from autolysis or rupturing naturally. Growth of the cells did not occur in lab as they had immediately been taken from a stock source to be used for this experiment. In understanding cell decay the researchers split the cells into either environment with oxygen, ‘oxic’ or environments without oxygen ‘anoxic’. Though neither environment did nor did not prefer cell decay (Carlisle, et al., 2021), the oxygen-lacking environment seemed to show slower different results than other similar experiments had initially provided. The researchers note it could have been an experimental error with this environment, but regardless of it shows possibility that oxygen could play a part in cell degradation. Carlisle et al. noted 6 separate changes in the cells over time as they degraded from cell death including nucleus visibility, chloroplast thinning, visibility of pyrenoids, cell collapse, chloroplast holes, and chloroplast collapse (2021). The general trend for the disappearance and degradation of cell involved the chart below:

During First 6 Weeks - Cell colonies begin to deform, becoming missshapen or rupture entirely.

During First 6 Weeks - Pyreniods disppear, the grain ring of the structure is left behind.

After First 6 Weeks - Nuclei begin to disappear uniformly, chloroplast persist but are still heavily degraded.

After First 6 Weeks - Chlroroplasts disppear after preforations, thinning and decaying.

Figure 2. Flow chart of generalized cell organelle degradation based on information from Carlisle et al. 2021.

Immediately after the death of the cells, colony structure came apart. The notable Y- junction of P. morum did not persist and instead cells drifted apart from their signature structure (Carlisle, et al., 2021). Other cell colonies also experienced deformation or irregular clumping like the cells of V. aureus. The other close to immediate effect of cell death was the decay of the pyrenoid structure. Researchers noticed that though these were the least decay resistant in cells the ring surrounding the pyrenoid usually stayed as an isolated ring after other cell contents decayed (Carlisle, et al., 2021). However, Chlorella and Rhodochorton had dissimilar decay patterns with nuclei persisting well past the 6-week mark. For Rhodochorton the lack of pyrenoids in the cell were noted, alongside the persistence of both nuclei and chloroplasts as they degraded from the inside out leaving notable ring like structures (Carlisle, et al., 2021). Comparatively both P. morum and V. aureus show similar signs of degradation, while Chlorella and Rhodochorton had similar signs of degradation among the status of the nuclei in the cells. The overall trend of cell death shows that chloroplasts and nuclei persist well after the death of the cell. Though the timing of nucleus and chloroplast degradation can vary these tend to be the last structures left in a cell that has died. To see if this had been present in previous life forms the researchers from the journal analyzed fossil records to see if there had been a presence of degraded organelles or even the presence of pyrenoid starch grain rings (Carlisle, et al., 2021). In the fossils analyzed features of plant cells could theoretically be preserved rather quickly, with the phosphatization and silicification occurring within weeks of death keeping the structure of degrading nuclei, and chloroplasts. Fossil records depict various structures from cell walls and nuclei being present, supporting the researcher’s consensus on how though chloroplasts are the last organelle to decay the nucleus can still be found as well in the fossil record. Though this can be surprising depending on the environment, preservation and fossilization can indeed preserve the smallest details. From the feathers of a small dinosaur to ferns, it does not greatly surprise me that even small cellular components can be found. The details of the organelles are surely something that would be a surprise due to the loss of details in fossilization. Cell death is more detailed than I previously thought before reading this paper. I would have imagined that cells simply degrade on their own after death. But the discovery of the persistence of nuclei and even chloroplasts in a step-by-step process does indeed open the door on how death impacts complex life.

Citations Ambastha, V., Tripathy, B. C., & Tiwari, B. S. (2015). Programmed cell death in plants: A chloroplastic connection. Plant signaling & behavior, 10(2), e989752. https://doi.org/10.4161/15592324.2014.989752

Carlisle E.M., Jobbins M., Panhkhania V., Cunningham J. A., Donoghue P.C.J. (2021). Experimental taphonomy of organelles and the fossil record of early eukaryote evolution. 7(5), eabe9487. https://advances.sciencemag.org/content/7/5/eabe9487 ‘Chlorella’ (2021). Wikipedia. Available at https://en.wikipedia.org/wiki/Chlorella . (Accessed: 04 April 2021). ‘Chlorella pyrenoidosa’. (2021). Wikipedia. [online]. Available at https://en.wikipedia.org/wiki/Chlorella_pyrenoidosa. (Accessed: 04 April 2021). FreeMedEducation (2018). What is Necrosis Vs What is Apoptosis?. [online]. Available at: https://www.youtube.com/watch?v=1vaEVcMfa1E (Accessed: 04 April 2021).

G.M. Guiry in Guiry, M.D. & Guiry, G.M. (2021) Pandorina morum (O.F. Müller) Bory 1826. AlgaeBase. [online]. https://www.algaebase.org/search/species/detail/?species_id=27949. Accessed 04 April 2021.

G.M. Guiry in Guiry, M.D. & Guiry, G.M. (2021) Rhodochorton purpureum (Lightfoot) Rosenvinge 1900. AlgaeBase. [online]. https://www.algaebase.org/search/species/detail/?species_id=7. (Accessed 04 April 2021.) Lewin, R.A. and Andersen, R.A. (2021). Algae. Encyclopedia Britannica. [online]. Available at: https://www.britannica.com/science/algae (Accessed: 04 April 2021). Linghong, T, Lanping, D, and Qinqin, L. (2011). ‘Microscopic Observation of Pyrenoids in Order Ulvales () Collected from Qingdao Coast’. Journal of Ocean University of China, 10, 223. https://doi.org/10.1007/s11802-011-1777-6 Nazrul Kabir, A.K.M. (2021). Volvox: Characteristics, Structure, and Reproduction. [online]. Available at: https://biologyeducare.com/volvox/ . (Accessed 04 April 2021). ‘Pandorina’ (2021). Wikipedia. [online]. Available at: https://en.wikipedia.org/wiki/Pandorina. (Accessed: 04 April 2021). Prokhorova, E.A., Zamaraev, A.V., Kopeina, G.S., Zhivotovsky, B., & Lavrik, I.N. (2015). ‘Role of the nucleus in apoptosis: signaling and execution’. Cellular and Molecular Life Sciences: CMLS, 72(23). 4593–4612. https://doi.org/10.1007/s00018-015-2031-y