This study explores how preservice primary teachers became engaged in meaning-making mathematics teaching when participating in online discussions within learning environments integrating video-clips of mathematics teaching. We identified different modes of participation in the online discussions and different levels of knowledge-building. The data can be explained by the type of task set in the learning environments and by a gradually improved shared understanding of the teaching. The structure of the learning environments supported inquiry into meaningful topics and knowledge-building through activities like identifying, interpreting and designing. These findings suggest that the different types of task and conditions of online discussion in the learning environments influence the nature of the interaction showing the correspondence between synergistic interaction and higher-order thinking.