[Background] The bacterial biofilm has gained widespread application in waste water treatment field. However, the bacterial strains used in marine aquaculture are lack of the adapation to the high salt environment. Marichromatium gracile YL28 isolated mangrove is a true salt-requirement marine bacterium. Strain YL28 not only exhibits effective inorganic nitrogen removal ability, moreover, it has better biofilm-forming traits in response to illumination. [Objective] This work aims to investigate the characterizations of biofilm formation in marine purple sulfur bacterium Marichromatium gracile YL28 with the basis for the development and application of biofilm bioreactor in marine aquaculture. [Methods] The characteristics of biofilm formation and nitrogen removal of YL28 were examined by measuring biomass, dehydrogenase activity, polysaccharide content, protein content and inorganic nitrogen removal in simulated seawater system under anaerobically in the light condition. [Results] The planktonic bacterial biomass in culture suspension increased gradually with increasing time at 4 000 lx and followed by a rapid decrease in biomass before stationary period, and the biofilm-forming bacterial biomass reached a stabilization after stationary period, indicating that the planktonic bacteria had phototaxis and formed biofilm on the vessel wall. The biofilm forming rate of YL28 was highest (71.21%) under lower light illumination condition (500 lx) for 4 d. The light illumination in the range from 1 000?4 000 lx benefited for biofilm formation (0.60?0.80 mg/cm2 dry weight) although its biofilm forming rate (54.64%?68.66%) was not higher than low light condition. The light illumination of 5 000 lx had negative effect on the dehydrogenase activity of biofilm-forming bacteria. The activity of dehydrogenase of both the biofilm-forming bacteria and planktonic bacteria increased with the increasing light intensity, no significant difference in dehydrogenase activity between biofilm-forming bacteria and planktonic bacteria. The biofilm formation limited the illumination penetration into the vessel reactor, however, dehydrogenase activity of planktonic bacteria had not been greatly affected, indicating that the planktonic bacteria in culture suspension mainly grow on the surface of the biofilm and diffused freely to the culture fluid. With the changes in light intensity (1 000?5 000 lx) and incubation time (4?10 d), protein contents in extracellular polymeric substances (EPS) varied significantly. The highest protein content was observed at 3 000 lx, while polysaccharide content had little change. At 4 000 lx, the removal efficiency to ammonium and nitrite had no obvious difference between biofilm-forming bacteria and planktonic bacteria in culture suspension, whereas the nitrate removal efficiency by biofilm-forming bacteria was higher than that by planktonic bacteria. [Conclusion] YL28 had better biofilm-forming traits, and the mechanism of biofilm formation was that bacteria grew on the vessel wall in response to illumination. The biofilm-forming bacteria were capable of efficiently removing amonium and nitrite. This study shows great application potential for eliminating nitrogen pollution in marine aquaculture.