The Opinion: Coral algal eviction and uptake under the spotlight?
Updated: Jun 25, 2020
Hey everyone! This article marks the start of a new series called 'The Opinion', where I analyse the latest research conducted in the field of the earth and natural sciences, giving my opinion on what I think overall and what this means for the future of our Earth. (The underlined sections are my own interpretations and ideas about certain points I make in the articles)
The symbiotic relationship between algae and coral is like no other, with their unique interdependent partnership supporting a 250,000 square kilometre ecosystem and sustaining more than 2 million marine species. However, this bond is threatened by increasing ocean temperatures, and unfortunately coral bleaching. Could there be a way to investigate the mechanics of coral bleaching by observing algal activity?
The coral provide the algae with a safe, protected environment in which they may also receive the necessary ingredients for photosynthesis (Carbon dioxide from coral aerobic respiration), while the algae then conduct photosynthesis, providing the coral with crucial biological molecules like 'oxygen, glucose, glycerol and amino acids'. This is great for the coral as they are not autotrophs like the algae and therefore really need these molecules to form proteins (with amino acid polypeptides), lipids (with fatty acids and the glycerol), aerobic respiration (with oxygen and glucose as reactants) and also to help construct the Calcium Carbonate 'skeleton'.
A phenomenal '90%' of those organic products (produced by the endosymbiotic algae, not all algae) are eventually transferred to the coral host. Irrespective of the fact that we know about the age of the origin of this symbiosis (Precambrian period), could the 90% statistic suggest that their interdependence has increased over time with percentage of organic products transferred to coral as an independent variable? Possibly from 6% to 30% to 60% to 90% as time progressed. Could there have been a logarithmic relationship?
It is believed that coral bleaching is caused by the increasing temperature, forcing corals to release their algae into the water out from their protective polyps, resulting in the loss of colour. The algae in the coral have different pigments that allow them to display their various, beautiful colours; the pigments absorb light, which makes sense because these specific algae are close to the surface, close to the sunlight to maximise the amount of light they receive for photosynthesis.
The aim of Hu et al's research is to collect samples from the 'Xenia' species of coral as well as its own algae from the 'Symbiodiniaceae family'. Hu got together a 'genome sequence' for Xenia, as well as the 'RNA profiles' of the cells in the polyps of Xenia. This allowed their team to find that Xenia had 'approximately 24,000 genes'. With this, Alvarado asks an important question, "which cells in the organism are responsible for recognizing the appropriate algal species and establishing the endosymbiosis?"
Cytometry is the measurement of the characteristics of cells from cell size and shape to position in the cell cycle. Hu et al used cytometry to separate the algae from within the symbiosomes which are membrane bound containers within coral which store the endosymbiotic algae. An RNA sequencing technique was also used to detect the quantity and presence of a specific genes being expressed in both algal and polyp cells. This technique allows the research team to see whether the same expressed genes are visible in both cells to show compatibility for symbiosis between the two cells. If a certain gene is present in both, then this provides solid evidence to Hu et al for their interdependence.
This study explains that the 'single cell RNA studies' used here only express a single snapshot in time that is recorded by the RNA, which makes it difficult for Hu et al to note the time of origin of the algal cells; to note when during their development did they start to coincide with the polyp of Xenia? Now this was an issue.
To tackle this, they tried the 'development on demand' technique like in lizards which is when the body reproduces a lost limb after it has been lost for whatever reason. In order for this to happen, the team amputated some tentacles off the Xenia polyps, Hu and colleagues used some methods such as RNA sequencing again and 'pulse-chase analysis'. This method is like using a radioactive tracer in the body, which can detect the position of the tracer from outside the body. But this technique works on a cellular level instead, where the cells are exposed to labelled compounds at one stage and then an unlabelled version of the same compound later in order to see the progression of movement of the cells over time.
The smart element of this study is that they investigate algal uptake as well as algal eviction from the coral. I think this is important as you can use the two to find an average of some sort of the two results, allowing for a more reliable answer that is accurate to a larger extent as well.
The methods stated above are used to detect the progress of the algal cells through time, from a parent algal cell to algal-uptake cells (this is for uptake) and then from mature algal cells to cells with no algae at all (for eviction). This allows Hu et al to observe changes between the most important stages of the cells as time increases. I think that if this were to be illustrated on a graph, it would be a negative quadratic:
As time increases (with increased ocean temperatures and bleaching factored in), the algal uptake increases up to a point, which then decreases during algal eviction as temperatures increase and coral bleaching is more abundant.
The definition of a differential gene expression is when changes to the gene expression during counts of presence of expression levels are statistically significant between two 'experimental conditions'. Hu et al put forth differentially expressed genes which pinpointed the different stages of the cells from uptake and eviction of algae as 'statistically significant'.
With the tests done and results awaiting, what now? How do these methods and results help our understanding of coral bleaching and the processes involved?
Well, you observe the different stages of the polyp cells in regeneration of tentacles with their different algal concentrations at different periods of time. Further to this, you may wish to investigate the mechanisms with which the algae are taken up, held and evicted by the corals; this provides some scope into genetic engineering possibilities by which recombinant DNA from another species could be inserted into the polyps, initializing change into their symbiosomes to retain the algae for longer periods of time. A difficult thing to achieve, I know, but it may only be able to be conducted in laboratory conditions with the right environmental settings to sustain the coral and algae.
Maybe this could be the step forward into reducing the volume of coral bleaching that occurs when temperatures do increase?
One may wish to investigate further the enzymes involved in the symbiosomes and the polyps when it comes to algal retention as they are then subject to higher temperatures, which disrupts the tertiary protein structure in the active sites, reducing specificity and increasing denaturing of those enzymes. I think it's worth a shot as you could try immobilising the enzymes to withstand those temperatures, but would they last in the wild?