Different carbon isotope fractionation patterns during the development of phototrophic freshwater and marine biofilms

Abstract. Natural phototrophic biofilms are influenced by a broad array of abiotic and biotic factors and vary over temporal and spatial scales. Different developmental stages can be distinguished and growth rates will vary due to the thickening of the biofilm, which is expected to lead to a limitation of light or mass transport. This study shows that variation in CO_2(aq) availability leads to a fractionation shift and thereby affects δ¹³C signatures during biofilm development. For phototrophic freshwater biofilms it was found that the δ¹³C value became less negative with the thickening of the biofilm, while the opposite trend was found in marine biofilms. Modeling and pH profiling indicated that the trend in the freshwater system was caused by an increase in CO_2(aq) limitation resulting in an increase of HCO₃⁻ as C-source. The opposite trend in the marine system could be explained by a higher heterotrophic biomass and activity causing a higher carbon recycling and thereby lower δ¹³C values. We conclude that δ¹³C was more related to the net areal photosynthesis rate and carbon recycling, rather than to the growth rate of the biofilms.