Endolithic algae: The coral holobiont has lungs

ACRS Research Award

Boring algae | Oxygen evolution | Coral skeleton

Endolithic algae: The coral holobiont has lungs

By Francesco Ricci The University of Melbourne

Oxygen is an essential molecule in coral reefs ecosystems that mediates the effect of many stressors such as temperature and acidification. Algae that live inside the limestone skeleton of corals, called boring algae, produce oxygen through photosynthesis, which contributes to the total oxygen balance of coral reefs. The boring algae oxygen input into the reef's ecosystem is a biological process that is not yet fully understood and this process may be controlled by more than just light availability. The aim of this project is to understand the factors which control the amount of oxygen produced by these boring algae that have chosen the inside of a coral skeleton as their habitat.

We are using two methods to address our question. The first method is chemical imaging (Figure 1), which allows us to quantify the oxygen produced by the algae residing inside the coral colony in a natural environment. The coral is cut into two halves (Figure 2), a layer of nano-sensors that bind to oxygen molecules is spread onto the cut side of the coral, and by using a modified digital camera we can visualize the fluctuations in oxygen molecules (Figure 3). The second method relies on a metabolic chamber (Figure 4), which is used to quantify the oxygen produced in limestone boring algae under different light intensities. This method relies on patches coated with nano-sensors that bind to oxygen, and connected via optic fibres to a reading system.

The project preliminary results show that the oxygen produced by the algae living inside the coral

Figure 1: Chemical imaging setup, showing: (A) the modifed DSLR camera used to acquire the image, (B) the LED excitation light, (C) tank containing the sectioned coral and (black arrow) the oxygen nano-sensor layer, (D) light source used to mimic sunlight in the lab experiment, and (E) the computer used to control the setup. © Francesco Ricci

“The distribution of oxygen produced by algae living inside the coral skeleton appears to be controlled by more than just light”

skeleton can be irregular. Some samples do not consistently respond to light, lacking oxygen production when illuminated. We found that the oxygen measured inside the coral skeleton is very low compared to what would be expected. A number of reasons could determine such a low oxygen presence, e.g. high microbial respiration or low presence of carbon to be fixed. Although we have also observed that the algae increase oxygen production in response to increasing light intensity, this oxygen is not uniformly distributed inside the coral colony. Interestingly, the oxygen shows peaks in correspondence to the green layer of the coral skeleton, which is usually given by the presence of algae.

Acknowledgments

I would like to thank the Australian Coral Reefs Society, and the Ecological Society of Australia for providing the funding necessary for the field trip of this study. My supervisors, Associate Professor Heroen Verbruggen, Professor Linda Blackall and Professor Michael Kuhl are also a big and essential part of this project, their constant mentoring and advice are at the base of its success. And I would also like to thank my colleagues Alexander Fordyce, Marisa Pasella and Alison Waskowicz, for helping me setting up the experiment, conducting it and analyzing the data.

Figure 4: Metabolic chamber used to measure oxygen production of algae growing inside limestone chips. The yellow vials containing the algae are kept in a water bath during the experiment to keep the temperature constant. © Francesco Ricci

Figure 2: Alexander Fordyce (left) and Francesco Ricci (right) during the coral cutting. © Francesco Ricci Figure 3: (A) Cross-section of Porites lutea cut along its vertical axis. (B) Imaging of the O2 gradient within the skeleton shown in (A) and graph values taken along the dotted line. The cut surface of the skeleton was illuminated with 50.3 PAR for 240 minutes. Calibration bar in image B represents O2% on air saturation. © Francesco Ricci