Sorption of organic matter from four phytoplankton species to montmorillonite, chlorite and kaolinite in seawater

The sorption of organic matter from four species of phytoplankton to clean mineral surfaces was studied using sequential adsorption experiments. The soluble intracellular components from four phytoplankton species, Phaeocystis globosa, Gymnodinium sanguineum, Scrippsiella trochoidea, and Ditylum brightwellii, were repeatedly exposed to three minerals; montmorillonite, kaolinite, and chlorite. The surface-reactive fraction of organic matter in phytoplankton dissolved organic carbon (DOC) solutions ranged from 47% to 85% of the total. Adsorption partition coefficients (Kd) ranged from 53 to 175 l kg−1. These partition coefficients are significantly higher than the partition coefficients commonly observed for sedimentary porewater organic matter (≤30 l kg−1), and suggest that phytoplankton exudates contain a component of organic matter with considerable surface reactivity. S. trochoidea and D. brightwellii consistently had the highest Kd values and the greatest reactive organic matter component relative to P. globosa and G. sanguineum. On average, the highest Kd values were associated with chlorite (average Kd of 119 l kg−1) and montmorillonite (average of 105 l kg−1) rather than kaolinite (average Kd of 89 l kg−1). Organic matter from all four phytoplankton species interacted most with montmorillonite (average=82% reactive), followed by chlorite (72% reactive) and kaolinite (63% reactive). Significant variation in both the extent of surface-reactive material and the partition coefficients of the reactive material using this matrix of source organisms and mineral surfaces suggest that both the mineral and organic matter sources are influential in the sorption of organic matter to sediment surfaces.