Olivine-induced seasonal dynamics of eukaryotic microalgal and bacterial assemblages in mid-latitude nearshore marine ecosystems

Ocean alkalinization, especially through olivine addition, represents a promising strategy for reducing atmospheric CO₂ levels. The addition of olivine may have seasonal impacts on marine microalgal and bacterial communities, which have not been studied yet. In this study, controlled laboratory experiments were conducted in spring and autumn to measure the responses of microalgal and bacterial communities to olivine addition in different seasons. Our findings revealed that both communities demonstrated different seasonal response patterns. Olivine addition had no significant effect on Diatomea community succession in the spring experiment, while in autumn, it enhanced the competitive advantage of Thalassiosira under the availability of nitrogen and phosphate. The bacterial community structure exhibited minimal changes after olivine addition in spring, while significant shifts were observed, predominantly influenced by the succession of microalgae in autumn. Candidatus Actinomarina showed a strong correlation with the Thalassiosira, suggesting a possible link between the two microorganisms in terms of element utilization. Furthermore, bacteria (Methylobacterium-Methylorubrum in spring and Marivivens in autumn) with a tendency for particle adhesion increased following the addition of olivine in both seasons, however, the concomitant alterations around the olivine microenvironment might be deleterious to their growth. The present investigation underscores the significance of the application timing, which has important implications for olivine-based ocean alkalinization.