Cell Death and Complex Life
As a cell goes through the cell cycle, it has to pass checkpoints to move onto the next stage. If the cell fails one of those checkpoints, the cell is flagged to be destroyed so that it doesn’t affect the cell cycle or any other functions in the body. Apoptosis is the most common form of cell death in living organisms. It is mediated by a specific kind of enzyme called a caspase. Caspases kill specific proteins found in the nucleus and the cytoplasm to trigger cell death (Alberts B, Johnson A, Lewis J, et al., 2002). If apoptosis checkpoints are not working, a cancerous tumor can form (J. Grisham, 2014).
As the cell begins to shrink during apoptosis, chromatin in the nucleus, and the nucleus itself, condense to the point where you can see them on a microscope. Once condensed, the chromatin are chopped up into pieces to ensure that the mutated DNA sequence does not replicate and affect any other cells. The nuclear membrane then begins to “bleb”, or bulge, to detach from the remains of the cell. When it comes to programmed cell death, chloroplasts are looked at in relation to mitochondria. Mitochondria and chloroplasts are where most, if not all energy, created in a plant comes from. Guard cells regulate CO2 influx and water loss. These guard cells contain many mitochondria and many chloroplasts, making them a perfect place for apoptosis to initiate. Mitochondria and chloroplasts have been found to regulate the activation of caspases. Caspase activation ensures that all cellular components are degraded in a controlled manner and do not really affect surrounding tissues (“Caspase”). Pyrenoids are part of the chloroplasts and are known to promote photosynthetic CO2 fixation in a plant. This means that it directs the CO2 wherever it needs to go in an organism. It is important that apoptosis start where the mitochondria and chloroplasts are so that all cellular components are degraded in a controlled manner.
Algae is a very diverse group of simple organisms ranging from unicellular to multicellular. Though they lack stems, leaves, and other major organisms a normal plant might have, they contain chlorophyll and are thus autotrophic species. In this assignment we will be looking at four different types of algae: Pandorina morum, Volvox aureus, Chlorella, and Rhodochorton. All four kinds of algae can dwell in freshwater, however Rhodochorton is typically a marine species. Pandorina morum is a freshwater algae composed of 8 to 32 cells held together to form a colony, each cell having one chloroplast with one or multiple pyrenoids (“Pandorina”). Volvox aureus is a freshwater algae that also forms colonies, but these colonies are much larger than that of the Pandorina colonies at around 50,000 cells. Volvox aureus algae have chloroplasts that contain chlorophyll for photosynthesis (“Volvox”). Chlorella are a group of single celled freshwater organisms that contain chloroplasts with chlorophyll-a and -b. There are about thirteen species of algae within this group of algae (“Chlorella”). Rhodochorton, or red algae, are multicellular organisms that also contain chloroplasts with chlorophyll to make their own food (“Red algae”).
The article that we are reviewing to further our knowledge of cell life and death was written by Emily M. Carlisle, Melina Jobbins, Vanisa Pankhania, John A. Cunningham, and Philip C. J. Donoghue. The authors wanted to look at the taphonomy, or process of fossilization, of cellular organelles. More specifically, the authors are looking at six changes in decay displayed by Pandorina morum, Volvox aureus, Chlorella, and Rhodochorton. They were looking at this because it has just been accepted within the scientific community that cellular organelles, such as the nucleus, decay too quickly to fossilize. However, that had never been experimentally proven to be the case.
According to the materials and methods section, the researchers got the samples of algae from Sciento.co.uk. Upon arrival and after ensuring that the correct samples were sent in, the team euthanized the algae sample with beta-mercaptoethanol (BME), which is used for denaturing RNases. We can’t really look at decay and fossilization of algal cells if they are still alive. After euthanization of the algae, each species was separated into two experimental groups, an oxic group and an anoxic group. The oxic group had the top of the test tube removed and was filled with water so that the cells didn’t dry out. The anoxic group’s lid was left on until the researchers were ready for sampling. The results of the study showed that neither the oxygen’s absence or presence had a factor in the rates or the patterns of decay.
The six criteria that the authors looked at during the decay process were the visibility of the nucleus, the thinning of chloroplasts, holes showing up in chloroplasts, visibility of pyrenoids, complete collapse of chloroplasts, and eventually the complete collapse of the cell.
Initially after V. aureus is euthanized, the colonies tend to disperse. The chloroplasts begin to lose shape and develop holes at around day 7, and after about thirty days you aren’t able to see the pyrenoids anymore. Most nuclei in V. aureus were still visible by the end of the experiment. The decay of P. morum was very similar to that of V. aureus, in that their colonies disperse and chloroplasts lose shape. However one chloroplast was actually seen leaving the cell around day 12. Pyrenoids also decayed fairly quickly, leaving the nucleus in almost perfect shape. In the Chlorella species, the decay process is relatively similar. Chloroplasts begin to collapse around day 21, and pyrenoids dissolve very quickly. You can still see nuclei in some cells. Some chloroplasts and nuclei escaped the cell as it collapsed. In the Rhodochorton species, chloroplasts pull away from the cell wall and develop holes and collapse by day 19. You can still observe some nuclei even at day 31.
In the fossilized cell of a Zelkova leaf, we are able to see chloroplasts and a starch grain from where a pyrenoid dissolved. A chloroplast is a little surprising to see because in the decay processes, chloroplasts were mainly the first to deconstruct. In all of the fossilized cells, we can see very distinct images of nuclei. This is unsurprising because, as seen in the process of decay of all four algal species, you are still able to see some intact nuclei.
References:
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell, Fourth Edition (4th ed.). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK26873/#:~:text=Apoptosis%20is%20mediated%20by%20proteolytic,producing%20a%20proteolytic%20caspase%20cascade
Apoptosis (article) | Developmental biology. (n.d.). Retrieved from https://www.khanacademy.org/science/biology/developmental-biology/apoptosis-in-development/a/apoptosis
Julie Grisham Friday, M. 1. (2014, May 16). What Is Apoptosis? Retrieved from https://www.mskcc.org/news/what-apoptosis
Nuclear condensation, DNA fragmentation and membrane disruption during apoptosis. (2021, March 23). Retrieved from https://www.abcam.com/kits/nuclear-condensation-dna-fragmentation-and-membrane-disruption-during-apoptosis
Samuilov, V. D., Lagunova, E. M., Dzyubinskaya, E. V., Izyumov, D. S., Kiselevsky, D. B., & Makarova, Y. V. (2002). Involvement of Chloroplasts in the Programmed Cell Death of Plant Cells. Biochemistry (Moscow), 67(6), 627–634. https://doi.org/10.1023/a:1016138003183
Wikipedia contributors. (2020, October 1). Rhodochorton. Wikipedia. https://en.wikipedia.org/wiki/Rhodochorton
Wikipedia contributors. (2021a, February 26). Caspase. Wikipedia. https://en.wikipedia.org/wiki/Caspase#:%7E:text=Activation%20of%20caspases%20ensures%20that,such%20as%20pyroptosis%20and%20necroptosis.
Wikipedia contributors. (2021b, March 9). Chlorella. Wikipedia. https://en.wikipedia.org/wiki/Chlorella
Wikipedia contributors. (2021c, March 9). Volvox. Wikipedia. https://en.wikipedia.org/wiki/Volvox
Wikipedia contributors. (2021d, March 10). Pandorina. Wikipedia. https://en.wikipedia.org/wiki/Pandorina
Wikipedia contributors. (2021e, March 30). Red algae. Wikipedia. https://en.wikipedia.org/wiki/Red_algae