Rings track growth in algae, too

Concentric rings found on a type of marine algae are growth bands similar to those in the cross-section of tree trunks, Australian researchers have discovered.

A team led by Guillermo Diaz-Pulido of Griffith University in Queensland reports in the journal PLOS One that the coralline algae known as Porolithon onkodes, found on the Great Barrier Reef, produces “skeletal banding” as a result of the annual activity cycle of their reproductive chambers.

Each summer these chambers, called conceptacles, align to produce spores that are released into the ocean, and over time, consecutive rows of bands develop as a result. 

Coralline algae are an abundant calcifying marine organism found worldwide and an important component of coral reefs. However, Diaz-Pulido says banding structures have never been identified in a tropical species until now. 

The discovery of the bands and linking the activity of the conceptacles to their formation was only made possible by a sophisticated method that combined five different techniques: X-ray diffraction, mineralogy mapping, ultraviolet light imaging, micro-computed tomography and scanning electron microscopy. 

When P. onkodes specimens were exposed to ultraviolet light the conceptacle chambers fluoresced, revealing distinctive bands.  Mineral mapping revealed that the algal exoskeleton is a complex mix of magnesium, magnesium calcite, aragonite, dolomite and magnesite. 

By comparison, reef-building corals use only aragonite to construct their skeletons. An even more intriguing finding was that the individual bands possess a blend of minerals different to the rest of the exoskeleton. The researchers suggest they can be read as a record of changing environmental conditions, as well as the organism’s age and growth rate. 

Diaz-Pulido’s team collected specimens from right along the Great Barrier Reef. Further research will investigate whether the growth rings encode evidence of ocean acidification or climate change. 

“We hope understanding the complexities of the mineralogy of growth and the banding of the algae will allow us a better understanding of what is going to happen in the future, and especially where the reefs would be able to cope with the increased concentration of carbon dioxide and warming,” says Diaz-Pulido.  

The research emphasises the importance of coralline algae in shaping reef environments. Diaz-Pulido makes the analogy coral reefs are constructed like buildings. 

“The corals provide the building blocks and the coralline algae are the cement of the reef, so without that cement, what would we do without the building blocks, nothing,” says Diaz-Pulido. 

“When hurricanes and cyclones come through, the reef stands strong, so understanding how this coralline algae grows and its ecology is very important.”