COLLOID TRANSPORT AND BACTERIAL UTILIZATION OF OCEANIC DOC

A substantial amount of dissolved organic carbon (DOC) may exist as biologically-labile colloids in seawater, but the way in which bacteria gain access to this globally-significant reservoir of carbon remains unknown. Models of collision efficiency and the transport of colloids to bacteria by Brownian motion, turbulent shear and bacterial swimming suggest that a significant fraction of this carbon could escape degradation by virtue of its particle size characteristics. Collisions between bacteria and colloid-sized particles are influenced primarily by short-range surface and hydrodynamic forces, and bacterial cells are transported least efficiently to particles of their own diameter (approximately 1-mu-m). When the models are modified to admit the possibility that bacteria have developed specific strategies to enhance their collision with particles, the transport minimum is forced by bacterial swimming to smaller colloid sizes (between 0.1 and 0.3-mu-m in diameter). A comparison of modelled transport rates to those derived from respiration measurements suggests that, in natural seawater, the collision efficiency (E) between bacteria and colloid-sized particles falls between an upper limit, when E is equal to a maximum of one, and a lower limit, when E is controlled by short-range surface and hydrodynamic forces. This comparison highlights the importance of coagulation and the formation of bacterial-colloid aggregates before larger colloids can be utilized efficiently by the bacteria and recycled to CO2 by their respiration.