The current study aims to explore how Thai preservice biology teachers make decisions upon and informally reason about two culture-based socioscientific issues, which are different yet contextually related. Participants included 46 fourth-year preservice biology teachers (12 males and 34 females) who completed a written questionnaire asking about personal engagement, opinions, and reasons regarding floating vessels into rivers and releasing lanterns into the sky as part of a cultural tradition. The data were inductively analysed using an iterative process of coding. The Wilcoxon signed-rank tests were also conducted to determine the significant differences between and within the two issues. The results indicate that the participants, while collectively considering both issues from multiple perspectives, tended to approach each issue differently. They tended to engage personally with and argue for the conservation of floating vessels, as this activity has stronger religious and cultural meanings and a clearer connection with modern science than the activity of releasing lanterns. The results are discussed based on the concept of collateral learning. In addition to technology-based socioscientific issues, it is recommended that culture-based socioscientific issues can be appropriately used to promote science learning and develop some of the characteristics of global citizens.