In all "real world" brown colored algae, brown color is a proof that special carotenoids which are specific for the group are involved in photosynthesis.
Of course this was part of trials related to global warming. Just the opposite is true. At higher temperatures nitrate will kill corals. High temperatures, elevated nitrate concentrations and nitrate and phosphate out of balance are the three stressors that seem to have synergistic effects in bleaching and killing corals.
Phosphate, ammonium and balanced or elevated phosphate concentrations are stabilizing symbiosis, also and especially important, under high temperatures.
