The oceans are the largest carbon sink in the world and regulate the climate change through carbon fixation, the exchange of materials and energy with the atmosphere. As the global climate change becomes intensified, ocean carbon sink has become a popular research topic and one of major means to reduce global warming. Marine microbes play a pivotal role in ocean carbon fixation and ocean carbon cycling, makes an important meaning to ocean carbon sink. This paper summarizes the significance of unicellular marine protists in ocean carbon fixing and storage with an emphasis on members of Labyrintholomycota and their roles in marine carbon recycling and ocean secondary production. For better understanding of the processes and mechanisms for ocean carbon sink, several important topics and potential solutions were discussed. Hopefully, it will provide some useful information and guidelines for the biological aspects of ocean carbon sink.