text.skipToContent text.skipToNavigation

Castles Made of Sand: Coral Reefs and their Symbionts

Explore Life Science

  • Nature
  • Essay

Towering coral, vibrant with color, and teeming with life is a common image when thinking of coral reefs. Scores of fish, sharks, sea turtles, and other exotic creatures form part of these richly diverse underwater gems of nature. But, did you know that reef ecosystems such as the Great Barrier Reef (GBR) would not exist without an intricate symbiosis that evolved between corals and single-celled algae, too small to be visible with the human eye?

Coral symbionts belong to a diverse group of dinoflagellate species from the family Symbiodiniaceae and the study of their role as symbionts has been a focus of marine biologists for decades. Performing fieldwork at Lizard Island, a remote location in the northern section of the GBR, Ph.D. candidate Cláudio Brandão from the University of Aveiro (CESAM/Functional Phycology Lab), is uncovering new aspects of the ecology of symbiodiniaceans during a free-living phase in their life cycle. Here, Cláudio explains how these tiny organisms can branch out on a solo mission to construct an alternative refuge to their coral homes.

The Legendary Marine Explorer

Cláudio explains how Jacques Cousteau – known as the pioneer of scuba diving gear and famed for his exploration of the Earth’s oceans – inspired him to pursue a career in marine biology.

“My father had the whole collection of the Jacques Cousteau movies and I watched these several times over. When I was young, I wanted to be like Cousteau – to study what is underneath the surface. I wanted to understand why certain organisms behave the way they do, how nature works, and how everything is interconnected.” Cláudio Brandão, University of Aveiro, Portugal

“I joined my current scientific group first as a volunteer during my bachelor’s degree in marine biology.” Being exposed to the team´s research on microalgae and especially coral symbionts, Cláudio then went on to do his master’s thesis on aspects of the free-living life history of symbiodiniaceans
and got so hooked that “after my master’s degree, I knew I wanted to know more and more about the ecology of symbiodiniaceans.”

Eviction Notice

In a healthy reef, symbiodiniaceans and their coral hosts engage in a mutually beneficial relationship. However, climate change and the associated rise in sea surface temperature severely stress the partners and one-way coral react to this stress is by evicting their resident symbionts. The expulsion
of their algae transforms the coral from colorful to white – in a process that has been aptly named coral bleaching. Unfortunately, bleaching is not a viable solution for coral to deal with temperature stress, as without their symbiotic residents coral eventually die, with dire consequences for the entire reef ecosystem.

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.”

“My Eppendorf pipettes have always been a reliable tool, even under challenging conditions in the field. I use Eppendorf because I know the consumables and equipment are top quality and I trust that they’re precise.” Cláudio Brandão, University of Aveiro, Portugal

Pipetting with Precision

“Many of my project tasks both in the field and back in the home laboratory require the precise pipetting of small reagent volumes and so my personal Eppendorf Research® Plus pipettes, which I won in a competition with Eppendorf, have travelled with me to Lizard Island and back.”

Related links