A symbiotic system of algae-bacteria, and attached cultivation, are two ways to increase microalgae biomass, and beneficially effect wastewater treatment. However, the possible advantages of the algae-bacteria co-culture in attached cultivation, are still unclear. This paper investigates the effects of different morphologies of a phosphate solubilizing bacteria-Paenibacillus xylanexedens (bacteria supernatant, bacteria, broken bacteria), on the growth of microalgae-Chlorella pyrenoidosa and wastewater treatment in an attached co-culture system. The results show that the broken bacteria had the most significant effect, with the biomass and protein content of Chlorella pyrenoidosa increasing by 125.67% and 25.04%; and the removal rate of COD, NH4+-N and PO43− in wastewater increasing by 23.57%, 146.15% and 9.96% respectively. This indicates that the intracellular material of the Paenibacillus xylanexedens was more effective in promoting the biomass growth of Chlorella pyrenoidosa and the removal rates of COD, NH4+-N and PO43−, compared to the algae growing without the bacteria. The algae-bacteria symbiotic attached mode was superior to the suspended mode, in terms of both Chlorella pyrenoidosa biomass enhancement and effective wastewater treatment. The addition of different morphologies of Paenibacillus xylanexedens significantly enlarged the difference between the two culture modes. This study provides a new method for coupled algae-bacteria co-cultures for wastewater treatment, based on the symbiotic effect. Intracellular substances of Paenibacillus xylanexedens significantly promoted the growth of Chlorella pyrenoidosa and wastewater treatment effect. Display omitted •Intracellular substances of Paenibacillus xylanexedens have a significant effect.•The biomass and protein of Chlorella pyrenoidosa increased by 125.67% and 25.04% respectively.•COD, NH4+-N and PO43− removal rate increased by 23.57%, 146.15% and 9.96% respectively.•The algae-bacteria co-culture enlarged the differences between attached and suspended cultures.