High adenylyl cyclase activity and in vivo cAMP fluctuations in corals suggest central physiological role

Abstract

Corals are an ecologically and evolutionarily significant group, providing the framework for coral reef biodiversity while representing one of the most basal of metazoan phyla. However, little is known about fundamental signaling pathways in corals. Here we investigate the dynamics of cAMP, a conserved signaling molecule that can regulate virtually every physiological process. Bioinformatics revealed corals have both transmembrane and soluble adenylyl cyclases (AC). Endogenous cAMP levels in live corals followed a potential diel cycle, as they were higher during the day compared to the middle of the night. Coral homogenates exhibited some of the highest cAMP production rates ever to be recorded in any organism; this activity was inhibited by calcium ions and stimulated by bicarbonate. In contrast, zooxanthellae or mucus had >1000-fold lower AC activity. These results suggest that cAMP is an important regulator of coral physiology, especially in response to light, acid/base disturbances and inorganic carbon levels.

Figure 1. Phylogenetic tree of putative adenylyl cyclases identified in A. digitifera and P. damicornis.

Coral tmAC isoforms are colored in red and coral sAC isoforms are colored in blue.

Figure 2. Daily fluctuations of endogenous cAMP levels in the coral P. damicornis normalized to total protein.

N = 9 (3 fragments per time point per day on 3 different days spanning 2 months). Error bars indicate SEM; asterisk indicates statistical significance (p < 0.01).

Figure 3. Dose response curve of the effect of bicarbonate on cAMP production in coral tissue homogenates (P. damicornis, N = 5; A. youngei, N = 3).

Lines were drawn from a non-linear curve fit model. Error bars indicate SEM.

Figure 4. Production of cAMP in P. damicornis tissue homogenates as measured by A) ELISA (N = 5) or B) 2-column assay (N = 3) or C) freshly isolated zooxanthellae (N = 5) as measured by ELISA.

Note different scale for panel C. Treatments: Control = 40 mM NaCl; KH7 = 40 mM NaCl + 50 μM KH7; Bicarb = 40 mM NaHCO₃; Bicarb + KH7 = 40 mM NaHCO₃ + 50 μM KH7. Symbols indicate statistically significant differences. Error bars indicate SEM.

Figure 5. Cyclic AMP production in P. damicornis tissue homogenates in response to A) bicarbonate and/or calcium (N = 6) or B) forskolin (N = 4).

Treatments: Control = 40 mM NaCl; Bicarb = 40 mM NaHCO₃; Ca²⁺ = 2.5 mM CaCl₂; Bicarb + Ca²⁺ = 40 mM NaHCO₃ with 2.5 mM CaCl₂; 50 μM forskolin. Letters indicate statistically significant differences. Error bars indicate SEM.