To assess whether bacteria influence the biogeochemical cycling of arsenic by laboratory cultures of the marine phytoplankton Dunaliella tertiolecta, the arsenic species produced by D. tertiolecta were compared in “operationally sterile” and bacteria spiked cultures. It was observed that glycerol (Gly-) arsenoriboside (41–78 %), phosphate (PO4-) arsenoriboside (7–38 %) and arsenate (As(V)) (15–21 %) were the major water-soluble arsenic species in all D. tertiolecta cultures irrespective of whether cultures were operationally sterile or contained added bacteria. PO4-riboside (46–74 %) and Gly-riboside (24–36 %) were also the major arsenic species in hydrolysed lipid extracts of D. tertiolecta irrespective of whether cultures were operationally sterile or contained bacteria. In addition to similarities in the arsenic species produced, total arsenic concentrations and culture growth did not differ relative to whether cultures were operationally sterile or not. Similar bacterial strains were identified in all D. tertiolecta cultures irrespective of whether bacteria were added or not. Consequently, it is evident that the presence of “foreign” or “added” bacteria in D. tertiolecta has minimal influence on the metabolism and cycling of arsenic by phytoplankton. Thus, the use of laboratory phytoplankton cultures containing bacteria may be appropriate means to investigate arsenic biogeochemical cycling unlike previously believed.