Increasing evidences showed that cryptic biogeochemical cycles are wildly distributed in natural environments. (Definition of cryptic biogeochemical cycles: changes in concentrations of a certain redox species cannot be measured, but the element cycle rapidly turnover and connects to other element cycles). In a lake, cryptic microbial iron cycling in the low iron water column should be present, because there are two types of microorganisms, including the photoferrotrophic iron oxidizers and the heterotrophic iron reducers. Please explain how cryptic iron cycle is coupled to the carbon cycle in this system.