Marguerite S. Koch and Conall McNicholl
1Biological Science Department, Florida Atlantic University, Boca Raton, Florida, USA
Correspondence: Marguerite S. Koch, mkoch@fau.edu
Tropical reef macroalgae in varying light environments may be differentially utilizing CO2 versus HCO3– for photosynthesis. We hypothesized that macroalgae in low-light environments utilize CO2 due to energetic costs of taking up HCO3–. Further, we examined calcification strategies across irradiance gradients. Fleshy and calcified macroalgal species were collected (~200 individuals; 3 Phyla) from a shallow high-irradiance barrier reef (~1,500 μmol photons m-2 s-1; ~1 m), 2 mid-depth intermediate-irradiance reefs (~600 μmol photons m-2 s-1; ~17 m), 2 deeper wall reefs (~400 μmol photons m-2 s-1; ~25 m), and low-irradiance crevices (~15 μmol photons m-2 s-1; >15 m). The 13 δC of organic and inorganic tissue were analysed. The 15 δN, total N and C were also determined, as well as water column nutrients and irradiance. The 13 δC of organic tissue ranged from -11.8 to -25.5 with an average of -19.1 ± 3.2, indicating that all species likely utilized HCO3–. Ochrophytes had the most enriched organic 13δC signatures (-16.5), while chlorophytes and rhodophytes were similarly lighter (-20.6 and -20.9, respectively). Irradiance was a strong predictor of organic 13 δC (13 δC = 3.6 x 10-3 irradiance – 21.5; R2 = 0.91). Inorganic 13 δC of calcifiers was genus-specific with the chlorophytes ranging between 0.5 to 3.5, ochrophytes 2.9 to 5.2, and rhodophytes 1.7 to 3.4, except for Jania and Amphiroa that had negative signatures between -2.8 to -5.1. These data lead us to suggest that tropical macroalgae primarily utilize HCO3– for photosynthesis, with some constraints from irradiance, but taxonomic differences likely drive physiological mechanisms for calcification.