PARTICLE AGGREGATION AND THE BIOLOGICAL REACTIVITY OF COLLOIDS

The colloidal fraction of dissolved organic carbon in seawater is one of the largest reservoirs of organic carbon on the planet, outweighing phytoplankton or the bacteria by a considerable margin. Even though t his colloid-sized material is a carbon reservoir of global significanc e, it is not easily accessible to the bacteria and may escape extensiv e biological degradation by virtue of its particle size characteristic s. However, when larger colloids (between 0.2 and 2.0 mum in diameter) are incorporated into microaggregates (that are tens of mum to mm acr oss), colloidal organic carbon (COC) is broken down as the aggregates become bioreactors for organic material. For example, the aggregation of colloids and bacteria by surface coagulation triggers a brief (2 to 4 h) episode of bacterial respiration. The bioreactive nature of aggr egates is confirmed in their development as sites of intense bacterial exoenzyme activity, even though the organic material released by this activity remains largely uncoupled from bacterial growth. The degrada tion of COC in aggregates is a process that is missing from current mo dels of carbon transport and ocean productivity. In addition, while th e COC caught up into aggregates may be more bioreactive than previousl y suspected, respiration of the aggregated material persists for only a few hours. Realistic measurements of respiration should take this sh ort-lived, but intense, response to aggregation into account.