Marine colloids could be an important source of nitrogen for bacteria and photoplankton. But elevated concentration of colloids may stimulate algal growth and lead to red tides in coastal waters. The effects of colloidal organic carbon (COC) concentration on the growth of photosysthetic bacteria (PSB) were investigated under different colloidal treatments in the laboratory. The PSB growth was inversely proportional to COC concentration and was restricted by high-molecular-weight (HMW) colloids (>10KDa) in treatments with non-nutrient or just inorganic nutrient with low COC concentration (≤5μMC). However, the PBS growth was enhanced in the presence of HMW colloids in the treatment with inorganic nutrient and high COC (127 and 255μMC) or with both inorganic nutrient and low-molecular-weight organic matter. Both bacteria number and bacteria growth ratio increased significantly when the concentration of COC was ≥5μMC. Our results suggest that marine colloids can be utilized by bacteria and might regulate primary productivity in coastal waters.