In the marine environment, the exchange of the substance between bacteria and algae is an important way of the migration and transformation of biogenic elements in the natural world. In order to obtain a further understanding of the biogeochemical cycles of biogenic elements, the transformation of nutrients (nitrogen and phosphorus) and organic matter between marine bacteria and two types of algae (Skeletonema costatum and Prorocentrum donghaiense) were studied in a co-culture or single-culture system of algae and bacteria based on a laboratory-simulated experiment. The changes of nutrient and organic carbon were tracked and the biomass of bacteria and the two algae were calculated. In addition, the concentration ratio [(DOC/DON)_a] of dissolved organic carbon (DOC) to dissolved organic nitrogen (DON) in different culture systems was analyzed to determine the sources of organic matter. The results indicated that bacteria may inhibit the growth of S. costatum, but have little effects on growth of P. donghaiense in the co-culture system. On the contratry, S. costatum accelerated the growth of bacteria, whereas P. donghaiense suppressed the growth of bacteria, which is probably related to the different sizes of the two algae. During the exponential growth phase of the two algae, bacteria promoted the uptake of ammonia (NH₄-N) by the algae, however NH₄-N was mainly released at the end stage of algae growth. The concentrations of nitrate (NO₃-N) and the biomass of algae showed a nearly negative correlation, but NO₃-N increased slightly during the decline phase of algae, indicating that the regeneration of NO₃-N took a longer time. Bacteria uptook little NO₃-N, but contributed to the regeneration of NO₃-N. Bacteria and S. costatum competed for phosphate (PO₄-P) and bacteria affected the uptake of PO₄-P by S. costatum. There were not obvious different between concentrations of PO₄-P in co-culture of P. donghaiense and bacteria from that in single culture of P. donghaiense, which was in contrast to the results of the culture with S. costatum. In the course of the culture, the concentration of DOC varied in different cultural systems. DOC increased rapidly in the co-culture systems of bacteria and algae, whereas the concentration of DOC increased slowly in the pure algal systems, and decreased slowly in the single-culture bacterial system. The analysis of the concentration ratio of DOC and DON showed that the method of tracing the source of particulate organic carbon (POC) can also be used to trace the source of DOC.