The corals consume the food, not the byproduct.
Do we know this for sure?
We do know that high nitrates can result in excess production of zooxanthellae...it stands to reason then that zooxanthellae takes nitrogenous waste as a growth input.
I wonder how much nitrate zooxanthellae metabolize while going about the day to day zooxanthellae life cycle....and if a tank stocked to the rim with corals is in fact easier to keep at low nitrogen levels....
then there is this....I will just leave this here:
Discovery of Symbiotic Nitrogen-Fixing Cyanobacteria in Corals
Lesser, et al.
Science 13 August 2004: 997-1000.
DOI:10.1126/science.1099128
"The δ 15N stable isotope data show a strong nitrogen fixation signal in the zooxanthellae fraction of corals with cyanobacterial symbionts suggesting that zooxanthellae use fixed nitrogen products."
Then this:
The diversity, specificity and flexibility of
Symbiodinium symbioses.
Ove Hoegh-Guldberg
Centre for Marine Studies, University of Queensland, St Lucia 4072 QLD Australia
"Within their host invertebrates, Symbiodinium spp. photosynthesize at rates comparable to free-living dinoflagellates but pass over 90% of the newly fixed carbon to their hosts. In exchange, Symbiodinium receives access to the normally limiting inorganic nitrogen and phosphorous."
"Nitrogen acquisition and assimilation
The relative roles of zooxanthellae and the host in the acquisition and assimilation of nitrogen have been contentious. The database has identified a number of the genes encoding crucial transport proteins and enzymes in this process. As a consequence we can now look at the expression of nitrate and ammonium transporters in zooxanthellae, the subsequent reduction of nitrate to ammonium and its assimilation into glutamine by glutamine synthase. This represents a significant advance in our knowledge and provides the capacity to determine how the expression of these genes changes under different nitrogen regimes."
