Inorganic nutrients
Inorganic nutrients, in particular the macronutrients phosphorous and nitrogen, are needed as building blocks for
biomass production. Only symbiotic corals can take up
inorganic nutrients, as this is a process mediated by the
symbionts.
Both internal and external nitrogen sources are available
to corals. Indeed, urea and ammonia, two forms of nitrogen that are by-products of coral host metabolism, are not
excreted and released by the symbiotic association, but are
reused by the symbionts and transformed into photosynthates (Muscatine & D’Elia 1978). The uptake of sea water
inorganic nitrogen is also mediated by the symbionts,
although the coral host has adapted to transport these molecules through its membranes (Groveret al.2002; Godinot
et al.2009). The availability in nitrate (NO
3
) and ammonium (NH
þ
4
), which are the two main forms of sea water
inorganic nitrogen, is critical for coral growth and is usually
a limiting nutrient in oligotrophic coral reef environments
(concentrations