Photosynthesis in dimly lit ocean depths produces a unique geochemical signature

What is it about?

Photosynthetic microbes (phytoplankton) perform most of the carbon fixation in the ocean. The sunlit depths that they inhabit can extend as deep as 150 meters below the ocean surface. Our work examines the variations in photosynthetic organisms that occur in the high-light upper euphotic zone and lower-light lower euphotic zone at an open-ocean site, finding for the first time that important geochemical signatures (varying carbon isotope ratios: 13C/12C, or δ13C value) arise from these photosynthetic variations. Using an array of complementary hydrographic and biological data, we modeled the potential origin of this pattern in the context of existing understanding of how CO2 uptake efficiency and photosynthetic rates lead to carbon isotopic fractionation during photosynthesis. However, previous models were based primarily on cultures that simulated surface ocean conditions, and we found that these models could only partially explain the large variation we observed. Consequently, we suggest that light intensity and depth-related variations in the photosynthetic community have been largely overlooked in the interpretation of carbon isotopic data.

Featured Image

Photo by Cristian Palmer on Unsplash

Photo by Cristian Palmer on Unsplash

Why is it important?

The close integration of geochemical and microbiological sampling and analysis within our project allowed us to characterize naturally occurring conditions (co-varying light, nutrient, and community composition) that are not reproduced in culture studies. These conditions are present in open ocean environments globally, and the isotopic signature we observed arising from low-light photosynthesis may be as well, as suggested by our previous, global compilation of δ13C values of total organic particles. Thus, applications that utilize the carbon isotope composition of phytoplankton (e.g., biogeochemical models, food web studies, reconstructions of past carbon dioxide concentrations in the ocean) are currently not considering this source of variation in those frameworks.