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 this colloid-sized material is a carbon reservoir of global significance, it is not easily accessible to the bacteria and may escape extensive biological degradation by virtue of its particle size characteristics. However, when larger colloids (between 0.2 and 2.0 μm in diameter) are incorporated into microaggregates (that are tens of μm to mm across), 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 aggregates 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 degradation of COC in aggregates is a process that is missing from current models of carbon transport and ocean productivity. In addition, while the COC caught up into aggregates may be more bioreactive than previously suspected, respiration of the aggregated material persists for only a few hours. Realistic measurements of respiration should take this short-lived, but intense, response to aggregation into account.