The bleaching of corals and coral reefs has become a common phenomenon. When conditions for living aren’t ideal, the coral begin to expel their Zooxanthellae [3]. Since the Zooxanthellae are the source for color in the coral, when they are expelled it creates a white coral. Not only is the color lost, but the ability for the coral to sustain itself is hurt greatly. At the E.O. Smith Aquaculture lab (Figure 1), an experiment was designed to see if lighting intensity affected the concentration of the Zooxanthellae present in the corals. Three groups of the Rhodactis sp. were put into three separate tanks under different intensities of light.

Relation To Zooxanthellae

Zooxanthellae are single celled, symbiotic algae that reside within the tissues of many coral species [2]. With the corals, the Zooxanthellae share a symbiotic relationship. The coral provides a safe living habitat and organic compounds the algae utilize for photosynthesis. The Zooxanthellae produces nutrients, specifically carbohydrates during photosyntheis, along with other organics needed by the coral [3]. The relationship between the algae and coral facilitates a tight recycling of nutrients in nutrient-poor tropical waters. In fact, as much as 90 percent of the organic material photosynthetically produced by the Zooxanthellae is transferred to the host coral tissue. This is the driving force behind the growth and productivity of coral [5]. Corals that are stressed due to any of a number of factors commonly “bleach.” Bleaching, or the loss of colors in the corals, is common when the Zooxanthellae concentrations are expelled from the coral.

Rhodactis sp.

The Rhodactis sp., or mushroom coral as it is known due to the likeness of a mushroom cap, are a very hardy species of coral [1]. Unlike many corals, the mushroom coral variety grow as one coral “polyp” instead of as hundreds or thousands of combined polyps [4]. They aren’t very large as a result, and very few grow larger than about 5” in diameter.

Aquarium Parameters

The corals were split into three tanks of 15 gallons each. The three tanks are part of a larger, 120 gallon system. The specific gravity of the tank was maintained at 1.023 sg and at a temperature of 24° Celsius.

Aquarium Lighting

The systems were individually lit with LED aquarium lights at three different levels of intensity as measured by Photosynthetically Active Radiation (PAR, μmol photons m-2 second-1): A- 300, B- 600, and C- 900.


  • Divide the 15 corals into three test groups, and place them in their respective tanks
  • Adjust the lighting in the tanks to 900, 600, and 300 PAR, or μmol photons m-2 second-1
  • Each week, remove one coral from each group
  • Cut a disc from each coral using a typical drinking straw, and mix the piece with 1 ml of salt water for 60 seconds
  • Under medium power (F.O.V. 1600μm) in a microscope, count the zooxanthellae in 3 randomly selected areas and average the totals
  • Repeat the procedure for 10 weeks, rotating the corals.

Data and Analysis

In terms of both net Zooxanthellae change as well as percentage of growth on a weekly basis, group B, in 600 PAR lighting had the highest values. It appears that the coral population dramatically increased or decreased during the first week, setting up its growth for the rest of the experiment. This can be argued, because all of the Rhodactis sp. came from the same initial tank. If the trend for percentage growth per week is looked at for the A and C group, it can be seen that both seem to be oscillating around the 0-5% mark every week, after their initial stabilization in the first few weeks. A constant percentage growth rate implies a linear overall growth rate. The C group’s percentage growth almost seems sinusoidal, as if there is some natural phenomenon causing the coral a weekly repetitive change in its Zooxanthellae population. The data for group B’s appears to be trending up in a linear fashion, implying that there is a slightly exponential growth rate at which the population is growing. Though all of the coral’s percentage growth appears to be erratic, there looks to be a stabilization point around week 7 and 8 where the “jumps” in the percentage data appear to be much less dramatic. One might claim that the data is completely erratic, but that doesn’t seem to be the case at all upon further inspection.

Conclusion and Further Uses

To answer the question presented at the beginning, the B group at 600 PAR consistently had the highest average Zooxanthellae count. On top of that, the B group’s percentage growth seemed to be rising, implying an exponential growth rate to the actual population. The A and C groups, at 300 and 900 PAR respectively, had lower average populations of coral, and the percentage growth typically seemed to trend lower than the B group’s. This data is specific to the Rhodactis sp. Mushroom coral, and can’t be applied to another without an experiment along the lines of this one that would definitively prove it. That being said, it proves that the intensity of light, in PAR, which results in the highest concentration of Zooxanthellae in the Rhodactis sp. is 600 μmol photons m-2 second-1. These are the “proper conditions” under which the coral doesn’t expel Zooxanthellae, but allows the concentration to grow. This higher concentration of Zooxanthellae may mean that the coral is able to live a more “successful” life, where success is determined by the ability of the corals to grow and thrive. A higher population of the Zooxanthellae means that the color of the coral will continue to deepen, as opposed to the adverse effect of whitening. It is an interesting note that that all the corals started out as a brownish color with a slightly red tint. Throughout the ten weeks they developed a darker, more greenish brown color. Again with the B groups’ changes being the most prominent. The experiment turned out to be a relative success, as proof has emerged that the lighting intensities do effect the coral and Zooxanthellae’s symbiotic relationship. It would be interesting to perform the experiment with a different set of corals.