Physiological Characterization of the Coral Holobiont Using a New Micro-Respirometry Tool

Metabolic activity, which can be defined as the sum of all the organismal activity that involves energy, is critical to understanding the function and evolution of life on earth. On coral reefs, measures of metabolism are really important for describing patterns like symbiosis, or the process between a host and, for example, the symbiote Symbiodiniaceae inside the coral host tissue. And it can tell us not only about how the organism functions normally, but how that organism functions when climate stressors are at play, for example.

To begin the equipment and coral setup, connect the two water bath plates using blue polypipe and connectors. Ensure that the motor plate is visible through the transparent water bath plates when the respirometry chambers are not in place. Next, connect the base of the water bath to the base motor plate with the magnetic gears before connecting this assembly to a power source and turning on the power.

To assemble the respirometry chamber, add the magnetic bead to the glass chamber and place the opaque plastic flow through stand base into the glass chamber. Firmly place the chambers into the water baths, ensuring the glass chambers are in contact with the temperature controlled water for the experiment. Using the valve control knobs, modulate the water flow as required, applying continuous and gentle flow set at 75 liters per hour, with slow stirring at 30 rotations per minute.

Proceed to connect the oxygen fiber optic cables with the oxygen sensor spots, which detect and transmit signals from the chamber through the cable. Ensure to connect the fiber optics to the matching ports in the oxygen module. Insert the cables into the holes drilled in the side of the lid chambers, ensuring contact with the spots.

If required, attach the white, thin, self-sealing plumbing tape to make the cable fit snugly and to let it remain firmly inside the water chamber. Ensure that the individual coral can be seen with brown tentacles facing up inside the chamber. Finally, measure the respiration using the oxygen measurement software.

After importing the respirometry data using the R package respR, the inspect function was used to check for common anomalies like non-numeric or missing values, and plot the oxygen time series, and calculate a rolling rate. The rolling rate became stable after the time point of 3000. The declining oxygen became detectable only after row 200 in the full-time series.

The auto underscore rate function, which identifies linear regions of the data, was used to extract rates. A linear region from around 3, 000 to 5, 000 was identified as the most linear region. The adjust_rate function was used to determine the background rates from the control experiments.

Finally, the rates were converted to the desired output units using various parameters. The surface area specific rate, which is the absolute rate divided by the surface area of the specimen, was obtained as output. The results of respiration in the dark at the small specimen size tested indicated the efficiency of this method in capturing low signal threshold.

Further, the representative respiration values for blanks, as well as a control versus high temperature comparison for very small samples were obtained. Despite the significance, there is a lack of methods, and therefore a lack of data on key metabolic rates that have to do with coral reefs, especially as it pertains to coral offspring, which are often a very small size, making it hard to measure. So this is why we developed this low cost, very flexible physiological respirometry system that's designed to measure respiration rates, IE metabolic rates, in very small organisms, and in this case, coral juveniles.