Microalgae are the core of global primary production, and the base of aquatic food webs in many ecological niches. As many other biological systems, they are exposed to climate changes, occurring through the increase of the atmospheric carbon dioxide (CO2) concentration because of anthropogenic activities. Climate changes drive the alteration of environmental physico-chemical parameters, with several direct and indirect consequences on microalgal photosynthesis. In fact, CO2 is a metabolic input of photosynthesis, therefore the increase in the atmospheric CO2 concentration is expected to directly impact the microalgal carbon (C) fixation rate. On the other hand, it contributes to the acidification of aquatic environments, impacting photosynthesis also indirectly. pH reduction impairs the efficacy of C concentration mechanisms indeed, which in turn curbs the photosynthetic C fixation rate. Another implication is the greenhouse effect which drives the increase in the average temperature of several ecological niches. Rising temperatures affect the fluidity of thylakoid membranes and alter the activity of C fixation pathways, impacting both light and dark reactions of photosynthesis, respectively. Overall, these phenomena drive to changes in the composition of microbial photosynthetic communities and interspecies interactions, which can influence the overall primary production rate with important implications on the functionality of whole natural ecosystems. To comprehensively understand these phenomena, multi-level investigations are essential, encompassing molecular to ecosystem-scale studies. In this chapter we provide evidence for the impact of climate change in the microalgal photosynthesis, highlighting the vulnerability of these primary producers and discussing their key role as buffer to mitigate climate alterations.