The paper reports a study aimed at investigating tertiary education students' engagement and interactions in the traditional face-to-face learning environment and the sequentially applied distance online learning environment imposed by the sudden upsurge of a worldwide health emergency, the COVID-19 pandemic in Spring 2020. The study took place in four distinct science learning communities formed by a total of 347 undergraduate students attending three different academic majors (Chemistry, Environmental Science, and Food Science and Technology) and 13 postgraduate students attending a Masters program related to Chemistry Education, in two Greek universities. The majority of the measured variables were shown to depend on the institution, the academic major, and the semester of study, although to a varying degree. Data analysis provided evidence for a statistically significant lower level of emotional engagement in the online relative to the traditional learning environment among the students of all three undergraduate learning communities. Multiple regression analysis showed that this documented decrease in students' emotional engagement is largely explained by the concurrent decrease at the level of human interaction (either student-student or student-instructor) upon the passage from the traditional to the online learning environment.