Marine particle formation and the subsequent export of organic carbon from the pelagic zone is an important mechanism for sequestering CO2. We studied the effect of increased CO2 concentrations and temperature on the coagulation efficiency of phytoplankton, a factor important for modeling aggregate formation. Dissolved and colloidal exudates derived from phytoplankton during blooms are enriched in acidic sugars and have recently been shown to be precursors for abiotically-formed gel particles operationally classified as transparent exopolymer particles (TEP). It has been suggested that TEP enhances aggregation by increasing effective particle size and collision frequency of particles, and also by serving as organic glue between cells. TEP abundance, dissolved and colloidal neutral and acidic sugar composition, and cellular abundances were measured for diatoms grown in mesocosms at elevated temperatures and for coccolithophores grown in chemostats under different temperatures and pCO2 concentrations. Coagulation efficiency of cells undergoing collisions by shear was determined experimentally using horizontal Couette devices. Here we report our findings and discuss their implications for marine particle dynamics and the marine carbon cycle in the future.