Carbon isotopic composition of Trichodesmium spp. colonies off Bermuda: Effects of colony mass and season

Colonies of Trichodesmium spp. are conspicuous, macroscopic components of the life in tropical and subtropical oceans. The large size and the morphology of the colony raise questions regarding the mechanism of carbon supply for photosynthesis. Constraints on these mechanisms may be indicated by the stable carbon isotopic composition (δ¹³C) that reflects the balance between carbon supply and speciation, as well as the growth rate and colony size. The δ¹³C of Trichodesmium off Bermuda measured here revealed a strong correlation between size of individual colonies and season. The smallest colonies, 2-7 μg C colony^-1, showed the lightest δ¹³C composition (∼-19‰), increasing to asymptotic values of ∼-12‰ above 7 μg C colony^-1. The average δ¹³C of the colonies was lightest immediately after the onset of stratification in the Sargasso Sea, gradually increasing by ∼4‰ to heavier values during the summer. We propose that the mass effect is due to increased use of HCO₃^- by the larger colonies, whereas the seasonal influence may be related to changes in irradiance and pCO₂ affecting the internal carbon cycling.