Ecologists often compartmentalize natural communities into autotroph-based “green” food webs versus detritus-based “brown” food webs.  Much theory exists for each of these webs independently, but we are still in the early stages of building conceptual and theoretical models that adequately link these two different ways of processing energy and materials.  Exudation of energy-rich compounds by healthy autotrophs – a direct, rapid shunt of fixed carbon from primary producers to heterotrophic microbes – is an important, but frequently overlooked, link between the green and brown webs.  Carbon exudates have been studied extensively in individual ecosystem types (e.g., grasslands, forests, marine benthos, pelagic plankton), but to date no systematic comparison across ecosystems has been made.  We reviewed the literature on carbon exudation to assess the ubiquity and patterning of this rapid green-to-brown link across ecosystems, and to develop expectations for how exudation might affect ecosystem properties such as nutrient cycling and secondary production.  Studies were selected by examining the abstracts of all publications returned from searches on ISI Web of Science (1965-2007) and using a distinct set of search terms for each of three coarse categories of ecosystem type:  terrestrial, aquatic benthic, and aquatic pelagic.

Results/Conclusions

Our review provides a number of key findings about the magnitude and potential functions of autotroph carbon (C) exudation. 

(1) We found a median percent extracellular release (PER) of recently fixed C of 10% across a variety of terrestrial, freshwater, and marine ecosystems, and a range of 0-80%. 

(2) There are critical differences in the magnitude of PER among ecosystem (and primary producer) types.  Measurements of PER are generally lower in terrestrial ecosystems than in aquatic ecosystems.  This could be due to biological differences, such as a greater allocation of photosynthate to structure in terrestrial plants as compared to plankton and algae.

(3) Released C may play a number of critical roles in ecosystems by affecting autotrophs, microbes, and ecosystem parameters.  For example, exudates may increase nutrient concentrations through a variety of mechanisms, including increased bacterial activity, increased microbivory, spatial translocation of nutrients by microbes or microbial predators, and/or altered microbial assemblages.

(4) Trophic studies of the green web that ignore this large and rapid coupling between autotrophs and microbes are susceptible to dramatic miscalculations of secondary productivity, biomass flow, and nutrient cycling.