Effects of attached bacteria on organic aggregate settling velocity in seawater

We investigated whether attached bacteria affect the settling velocity of organic aggregates via modifications of the physical properties of aggregates, including density, porosity, and morphology. Model aggregates, prepared by mixing 2 different polysaccharides (fucoidan and chitosan), were incubated in coastal seawater passed through either 0.8 mu m (AGG(0.8)) or 0.2 mu m (AGG(0.2)) filters. After incubation for 48 h, AGG(0.8) were much more densely (3.2- to 10.1-fold) colonized by bacteria than AGG(0.2). Based on median settling velocities (W-50), as determined by laser in situ scattering and transmissometry, the W-50 of AGG(0.8) was lower (1.6- to 4.5-fold) than that of AGG(0.2) for a size class of 62 to 119 mu m. Stokes model analyses indicated that this reduction in W-50 could be largely attributed to the higher porosities of AGG(0.8) (0.932-0.981) than those of AGG(0.2) (0.719-0.929). Our results support the notion that the modification of aggregate structure by attached bacteria (porosity enhancement) can be an important factor controlling the settling velocity of marine particles.