Environmental content is becoming increasingly important in schools, but teachers may have difficulty grasping accurate knowledge. This study investigated how student teachers construct a conceptual understanding of biodiversity loss in an online environment. Sixty-five first-semester primary student teachers at a Finnish university were given the task of writing a synthesis of text from online sources to prepare for a primary school science lesson. The learning environment included texts that were relevant, irrelevant, and fake. A content analysis of their syntheses showed that they generally managed to define the phenomenon and adequately explain its causes and implications. However, they struggled to find different perspectives and write a coherent synthesis, as they used irrelevant and fake content instead of integrating text from multiple relevant sources. These findings indicate a need to develop teacher education to support student teachers in recognising the validity of sources on environmental issues and synthesising them into a coherent whole.