Abstract: The role of marine bacteria playing in the marine ecosystem and the biogeochemical cycles of carbon, and the advances in microbial loop research were reviewed in this paper. The community structure and function and especially the bacterial activity in marine environment and the ecological function of phycosphere bacteria in the frequent occurrence area of Harmful algal blooms (HABs) were discussed as well. The importance and urgency to carry out the study on the coupling effect of “microbial loop - HABs - key microbial groups” were suggested based on the need of frontier research topics.