## 2 Materials and Methods
### Intervention Design  
This study developed a game-based AR learning environment for education for sustainable development (ESD). The learning environment was developed as an application for tablets, which can be used in classrooms by small working groups. An IT agency specialized in game design assisted the process, as well as educators from all involved subjects and game-based learning experts. Following the design-based research approach, the development was split into iterations in which elements were created and iteratively tested with students. Three iterations tested (1) the overall game mechanics, (2) the digital elements, and finally (3) the overall learning environment as well as the applied instruments. Guided group interviews were transcribed and evaluated using qualitative content analysis. By combining AR with a game-based approach, the objective of this learning environment was to improve pre-service teachers' K on microplastics and CSE beliefs about using digital media. The aspect of managing and lowering technostress leads to integrating playfulness into AR learning environments [66; 67]. This could help lower the threshold for using and handling new technologies. To systematically incorporate game-based learning, we oriented our design on the eight game elements outlined in the framework's design guidelines for AR [16]: (1) goals and rules, (2) conflict and challenge, (3) control, (4) assessment, (5) action language, (6) human interaction, (7) (game) environment, and (8) story. The game-based interventions A and C therefore were equipped with a main playful storyline narrating an everyday scenario. In this setting, the game's antagonist, the plastic monster, must be defeated. This was accompanied by a set of rules that challenged players to make decisions and take action, influencing the game's course and outcome. The conflict was to avoid accumulating harmful plastic points--this was carried out by making decisions on consumption and answering quiz questions. There was no definitive right or wrong decision; players were rather obliged to consider the consequences, reflecting real-life sustainability dilemmas. To contrast and analyze the effects of both AR and game-based designs, we applied a 2 \(\times\) 2 design [68] with four different intervention groups: AR in a game-based environment (the original), AR without a game-based environment, a game-based environment without AR, and an environment without either an AR or game-based approach. For future reference, we call them A (AR + GAME), B (AR + non-GAME), C (non-AR + GAME), and D (non-AR + non-GAME). AR was applied to interventions A and B, which displayed subject-specific content via AR for biology, chemistry, and engineering. In contrast, C and D displayed the same content in a conventional manner using worksheets, microscopes, and physical material. AR was applied in so-called \"breakout rooms\", each dedicated to one of the three subjects with three specific thematic emphases. In the biology breakout room, different types of microplastics were depicted and displayed the variety of origins as well as their particle forms, sizes, and shapes. Here, the participants learned about the intake of particles via the intestinal epithelium. This breakout room concluded with the dangers and health risks associated with potential microplastic absorption in the human body. Chemistry experimented with an everyday issue (textile abrasion through washing) and in this breakout room, participants explored methods to filter out microplastic particles. Participants also observed the extracted and filtered particles under an augmented microscope. The engineering breakout room provided an overview of different materials and used AR to augment their molecular structures. It concluded with the purpose and benefits of the particular material characteristics. GAME, used for interventions A and C, was rendered in a didactic approach.