Theoretical constraints on Trichodesmium colony size: The role of carbon dioxide and light

Size is an important trait among marine phytoplankton as it influences a vast range of physiological, ecological, and evolutionary processes. For Trichodesmium, a cyanobacterial diazotroph important for the global nitrogen cycle, size is a flexible trait that can change because of its ability to form colonies. Trichodesmium colonies can persist from 10 µm to greater than 1 mm in the natural environment. Despite this known ability, we still do not know whether a maximum size limit exists for Trichodesmium colonies and, more importantly, whether any mechanisms regulate this limit. In this paper, we use a theoretical metabolic model to investigate the role of two factors known to influence Trichodesmium colony size: carbon dioxide and light availability. The greater the availability of carbon dioxide or the average light availability, the greater the potential colony size of Trichodesmium. Carbon limitations have a much stronger effect on colony sizes than light limitations. Higher respiratory costs, perhaps due to higher water temperatures, do not appear to limit maximum colony sizes unless they consume nearly all the carbon that is fixed. Our theoretical model highlights several scenarios that likely assert some control over the ecology of Trichodesmium in the global ocean. It also implies that natural colonies must have mechanisms to escape from carbon limitations. To achieve sizes like 1 mm, Trichodesmium colonies must be highly porous (>91%) or live in environments with a nutrient flux 12 times greater than what molecular diffusion can provide.