Sponges recycle carbon to give life to coral reefs
Coral reefs support some of the most diverse ecosystems on the planet, yet they thrive in a marine desert. So how do reefs sustain their thriving populations?
Marine biologist Fleur Van Duyl from the Royal Netherlands Institute for Sea Research is fascinated by the energy budgets that support coral reefs in this impoverished environment. According to van Duyl’s former student, Jasper De Goeij, Halisarca caerulea sponges grow in the deep dark cavities beneath reefs, and 90% of their diet is composed of dissolved organic carbon, which is inedible for most other reef residents. But when De Goeij measured the amount of carbon that the brightly coloured sponges consumed he found that they consume half of their own weight each day, yet they never grew. What were the sponges doing with the carbon? Were the sponges really consuming that much carbon, or was there a problem with De Goeij’s measurements? He had to find out where the carbon was going to back up his measurements and publishes his discovery that sponges have one of the fastest cell division rates ever measured, and instead of growing they discard the cells. Essentially, the sponges recycle carbon that would otherwise be lost to the reef. De Goeij publishes his discovery on November 13 2009 in The Journal of Experimental Biology at http://jeb.biologists.org.
The sponges were shedding the newly divided cells, which other reef residents could now consume. ‘Halisarca caerulea is the great recycler of energy for the reef by turning over energy that nobody else can use [dissolved organic carbon] into energy that everyone can use [discarded choanocytes],’ explains De Goeij.
In other words… Anthropogenic CO2 emissions help feed the critters that build coral reefs.
Dissolved Inorganic Carbon (CO2, bicarbonate, etc.) are consumed by shell building organisms to build shells (bicarbonate) and photosynthesis in the photic zone (CO2). DIC constitute about 97% of the carbon in the oceans.
Dissolved Organic Carbon (non-colloidal bits of carbohydrates, proteins, etc.) are the mostly the product of photosynthesis. DOC can come from land or, marine sources. This is consumed by sponges which secrete food for reef building organisms.
Both DIC and DOC are part of the carbon cycle.
Anthropogenic carbon emissions (primarily CO2) constitute about 3% of the Earth’s carbon budget (~6 Gt/yr).
More CO2 in the atmosphere leads to something called “CO2 fertilization.” In an enriched CO2 environment, most plants end to grow more. The fatal flaw of the infamous “Hockey Stick” chart was in Mann’s misinterpretation of Bristlecone Pine tree ring chronologies as a proxy for temperature; when in fact the tree ring growth was actually indicating CO2 fertilization as in this example from Greek fir trees…
Enriched atmospheric CO2 “feeds” reefs in two ways: 1) Enhanced photosynthesis for the symbiotic algae; and 2) More DOC to feed the sponges that also feed reef builders as the result of enhanced photosynthesis of land and marine vegetation.
Coral reefs can only grow in the photic zone of the oceans because zooxanthellae algae use sunlight, CO2, calcium and/or magnesium to make limestone.
The calcification rate of Flinders Reef has increased along with atmospheric CO2 concentrations since 1700…
As the atmospheric CO2 concentration has grown since the 1700’s coral reef extension rates have also trended upwards. This is contrary to the theory that increased atmospheric CO2 should reduce the calcium carbonate saturation in the oceans, thus reducing reef calcification. It’s a similar enigma to the calcification rates of coccoliths and otoliths.
In all three cases, the theory or model says that increasing atmospheric CO2 will make the oceans less basic by increasing the concentration of H+ ions and reducing calcium carbonate saturation. This is supposed to reduce the calcification rates of carbonate shell-building organisms. When, in fact, the opposite is occurring in nature with reefs and coccoliths – Calcification rates are generally increasing. And in empirical experiments under laboratory conditions, otoliths grew (rather than shrank) when subjected to high levels of simulated atmospheric CO2.
In the cases of reefs and coccoliths, one answer is that the relatively minor increase in atmospheric CO2 over the last couple of hundred years has enhanced photosynthesis more than it has hampered marine carbonate geochemistry. However, the otoliths (fish ear bones) shouldn’t really benefit from enhanced photo-respiration. The fact that otoliths grew rather than shrank when subjected to high CO2 levels is a pretty good indication that the primary theory of ocean acidification has been tested and falsified.
In the field of geology, when we falsify a hypothesis or a theory, we trend to start looking for a new hypothesis or theory. That’s why we rely very heavily on Chamberlain’s Method of Multiple Working Hypotheses. In the junk science of ocean acidification and anthropogenic global warming, it appears that the process is to simply discard any data that deviate from the ruling theory.