Algebra holds an important role in school mathematics, and teachers must recognize the broad range of algebraic ideas and develop a deep understanding of these concepts to teach algebra conceptually. This study aims to investigate two pre-service middle school mathematics teachers' (PSTs) conceptualization of algebraic thinking before and after participating in an intervention designed to support PSTs' algebraic thinking and noticing of student algebraic thinking using a Seeds of Algebraic Thinking (AT) Perspective. This study was conducted as part of a mathematics methods course offered for middle school mathematics teachers in the 2023-2024 academic year at a large public university in the United States. To investigate PSTs' initial conceptions of algebraic thinking as well as the change in their conceptions, interviews were administered before and after the intervention. The data sources for this study were the pre- and post-interviews of the two participating PSTs. The data were analyzed qualitatively. The findings showed that both PSTs widened their algebraic thinking conceptualization after they participated in the video-based intervention designed using seeds of AT perspective. The findings showed that the intervention fostered their algebraic thinking conceptualization in different ways. After the intervention, PST1 broadened her conceptions to include other formal algebraic thinking ideas, such as generalizing and formalizing, using algebra as a tool, and reasoning about and with representations; PST2 deepened her conceptualization in terms of "early" algebraic thinking, such as balance, comparison, and proportionality. [This paper was published in: "EJER Congress 2024 International Eurasian Educational Research Congress Conference Proceedings," edited by Senel Poyrazli, Ani Publishing, 2024, pp. 46-53.]