Excess carbon in aquatic organisms and ecosystems: physiological, ecological, and evolutionary implications

Cells and organisms, both autotrophs and heterotrophs, commonly face imbalanced access to and uptake of elements relative to their requirements. C is often in excess relative to key nutrient elements like N or P in photoautotrophs. Likewise, one of the lessons from ecological stoichiometry is that the growth of consumers, especially herbivores and detritivores, is commonly limited by N or P such that they also experience C in excess in relative terms. ‘‘Excess’’ implies wastage, yet this definition, which is consistent with purely stoichiometric arguments, is by no means straightforward. In fact, many organisms put this apparently surplus C to good use for fitness-promoting purposes like storage, structure, and defense or mutualistic goals like symbiosis. Nevertheless, genuine excesses do occur, in which case the remaining ‘‘leftover C’’ must be disposed of, either in organic or inorganic form via increased metabolic activity and respiration. These fluxes of C in various forms have major effects on the C balance of organisms, as well as governing the energy flux and C pathways at the ecosystem level. We here discuss evolutionary and ecological implications of ‘‘excess C’’ both at the organism and ecosystem level.