Castles Made of Sand: Coral Reefs and their Symbionts

A Nomadic Lifestyle

Unlike corals, many symbiodiniaceans can survive without their hosts for prolonged times and Cláudio is interested in this nomadic, free-living lifestyle. More specifically, Cláudio is studying a process through which free-living symbiodiniaceans can build their own protective ‘castles’ around them by making deposits of calcium carbonate. Why is this important? Well, Cláudio and his team think that the construction of this so-called endolithic stage might greatly improve the survival of free-living symbiodiniaceans and produce an endolithic seedbank in reef sands from which coral larvae and bleached coral can recruit symbionts (see figure above).

An Acidic Ocean

If the hypothesis holds true – that the constructed endolithic habitat in reef sands acts as a temporary lodging for symbiodiniaceans – then disruption to the underlying calcification process could greatly reduce the availability of symbiodiniaceans for the establishing of symbiotic partnerships.
With climate change, such a disruption may become a reality within only a few decades. “The increase in carbon dioxide in the atmosphere mean that more carbon dioxide is being absorbed by the ocean. Consequently, the ocean becomes more acidic and is known as ocean acidification.” He continues “It is predicted that by 2050 the pH will drop to a level such that sands in many coral reefs will enter a net dissolving state and this could mean that symbiodiniaceans might not be able to calcify anymore”.

Into the Field

Awarded a scholarship for his research, Cláudio journeyed to Lizard Island, Australia to study the distribution and diversity of endolithic symbiodiniaceans across different reef zones. Cláudio explains “we collect our sand samples mainly by scuba diving and use onsite laboratory facilities to assess a whole range of parameters on life and freshly prepared samples. Samples undergo washing and digestion steps to access the algae that reside inside the sand grains, followed by DNA extractions for next generation sequencing and qPCR analyses, pigment analyses, and several other analytical techniques.”