Picture: The remarkable diversity and life of a coral reef all because of the capacity to farm algae.
By Stuart Kininmonth, Heron Island Research Station.
Following the disastrous bleaching event at the start of the year, the coral reefs in the southern Great Barrier Reef are now in a state of recovery. Some reefs have died and are being colonized by algae, which will disintegrate, forming substrate for new corals. Others have shown signs of stress but have found a way to recover and are returning to normal. These resilient individuals, and collectively their species, are the winners and will rapidly shape the future character of the reefs. So, what is driving these massive changes?
While the metabolic pathways are very complicated and intricate, we have a good understanding of how corals manage to ‘farm’ the dinoflagellate algae known as zooxanthellae. The coral host must ensure the right amount of dissolved carbon dioxide is available to the algae, which in turn provides this molecule to the remarkable engine of chlorophyll. Combined with some other nutrients, the carbon dioxide is converted into sugar and oxygen through the Calvin–Benson cycle. This is beneficial for the coral when all goes well, but too much free radical oxygen can cause significant stress, and the coral must manage the farm effectively.
Three factors alter the performance of the zooxanthellae. Firstly, the amount of sunlight: on those cloudless, hot summer days, the chlorophyll can become overactive and cause stress, especially if the coral is situated in the shallow parts of the reef. Secondly, the temperature of the water can accelerate the growth and multiplication of zooxanthellae; excessive temperatures can lead to an unmanageable farm. Thirdly, the availability of nutrients is critical for both the maintenance and regulation of the zooxanthellae. Coral hosts essentially regulate the growth of the zooxanthellae, similar to how farmers manage their livestock. When conditions are just right and the coral is situated in the ideal spot on the reef, this mutual arrangement between an animal and an alga is remarkable. The animal no longer has to chase its food source or contend with food shortages. Consequently, coral polyps are undoubtedly the most successful of all animals that have inhabited planet Earth, with over 205,000 mapped coral reefs.