You probably have two questions for me: 1) What is a rhodolith and why do I care? and once we’ve answered those, 2). How does a species of red algae grow to the size of the GBR?
Let’s start with the first question/s. When we think of calcium carbonate (CaCO3) bearing organisms – whether it be for an aesthetically pleasing SCUBA back drop, or you happen to be interested in the global carbon cycle – corals are often the first taxa to spring to mind. Not only are they visually stunning, but they are important components of the carbon cycle (and perhaps not in ways you would think, the precipitation of CaCO3 actually liberates CO2).
Rhodoliths are a species of coralline algae, typically pink, that secrete calcium carbonate in to their cell walls. They are found as encrusted nodules, rock like and sometimes smooth and spherical in shape. It is highly likely that if you have ever been to the intertidal zone, you would have seen a pink frilly fringe around your favourite rock pool. Well you’ve been looking at coralline algae. So to answer the question of why I care? Not including the intrinsic value in all living creatures, along with corals rhodoliths are also major players in the cycling of CaCO3 on our planet. Yes the C-word: Carbon.
In an article published in Plos One last week, a group of Brazilian researchers describe what might be “the most voluminous shallow-water benthic (bottom dwelling) coralline algae community ” on the planet. Okay so it is a community rather than a single organism. Located on the Abrolhos Shelf off the coast of Brazil these algal beds have an areal extent of 20,900 km2 putting them on the same scale as the GBR. Alone these beds contain ~5% of the global CaCO3 that is found in carbonate banks, precipitating 0.025 Gt CaCO3/year. The authors are quick to point out that these beds are vulnerable to increased sedimentation due to land use change, as well as the pervasive threat of ocean acidification. Their particular CaCO3 is high-magnesium calcite which is more susceptible to dissolution, and therefore most readily effected by decreasing oceanic pH.
So how do we end up with rhodolith beds the size of the GBR? As with all things the answer is with time. These particular beds have been radiocarbon dated between 7800-8200 BP. From a geological stand point this accumulation is rapid, however from an anthropocentric perspective it may seem slow. From a SCUBA divers perspective however the GBR might still be more enticing, unless you enjoys swimming over vast tracts of pink.
Access the full article here.