The Arctic and Antarctic regions constitute 14% of the total biosphere. Although they differ in their physiographic characteristics, both are strongly affected by snow and ice cover changes, extreme photoperiods and low temperatures, and are still largely unexplored compared to more accessible sites. This review focuses on microalgae and bacteria from polar marine environments and, in particular, on their physiological and molecular responses to harsh environmental conditions. The data reported in this manuscript show that exposure to cold, increase in CO₂ concentration and salinity, high/low light, and/or combination of stressors induce variations in species abundance and distribution for both polar bacteria and microalgae, as well as changes in growth rate and increase in cryoprotective compounds. The use of -omics techniques also allowed to identify specific gene losses and gains which could have contributed to polar environmental adaptation, and metabolic shifts, especially related to lipid metabolism and defence systems, such as the up-regulation of ice binding proteins, chaperones and antioxidant enzymes. However, this review also provides evidence that -omics resources for polar species are still few and several sequences still have unknown functions, highlighting the need to further explore polar environments, the biology and ecology of the inhabiting bacteria and microalgae, and their interactions.

Primary production in the Southern Ocean highly depends on phytoplankton and has been reported to be limited by the availability of the micronutrient iron. The aim of this review is to summarize the past and current knowledge on iron limitation in the Southern Ocean, and specifically how it affects primary producers, thus influencing the whole Southern Ocean community structure, carbon cycling, and large-scale ocean biogeochemistry. In this region, extensive variability exists between different areas regarding iron availability, but also between seasons. Moreover, co-limitations with other abiotic environmental factors exist, further complicating the assessment of the role of iron as limiting factor for phytoplankton productivity. Currently, climate change is altering the Southern Ocean environment. How these changes will affect resident phytoplankton is still not clear, possibly modifying the iron supply mechanisms. Existing projections point towards a possible partial relief of iron stress on phytoplankton, but the interactions between different environmental changes, and the cascade effects they will have, are still poorly understood, and some aspects understudied. Here we try to synthetize the available predictions and uncertainties concerning this topic.

Cold environments include deep ocean, alpine, and polar areas. Even if the cold conditions are harsh and extreme for certain habitats, various species have been adapted to survive in them. Microalgae are among the most abundant microbial communities which have adapted to live in low light, low temperature, and ice coverage conditions typical of cold environments by activating different stress-responsive strategies. These species have been shown to have bioactivities with possible exploitation capabilities for human applications. Even if they are less explored compared to species living in more accessible sites, various activities have been highlighted, such as antioxidant and anticancer activities. This review is focused on summarizing these bioactivities and discussing the possible exploitation of cold-adapted microalgae. Thanks to the possibility of mass cultivating algae in controlled photobioreactors, eco-sustainable exploitation is in fact possible by sampling a few microalgal cells without impacting the environment.

Salinity is among the main drivers affecting growth and distribution of photosynthetic organisms as Chlamydomonas spp. These species can live in multiple environments, including polar regions, and have been frequently studied for their adaptation to live at different salinity gradients. Upon salinity stress (hypersalinity is the most studied), Chlamydomonas spp. were found to alter their metabolism, reduce biomass production (growth), chlorophyll content, photosynthetic activity, and simultaneously increasing radical oxygen species production as well as lipid and carotenoid contents. This review summarizes the current literature on salt stress related studies on the green algae from the genus Chlamydomonas considering physiological and molecular aspects. The overall picture emerging from the data suggests the existence of common features of the genus in response to salinity stress, as well as some differences peculiar to single Chlamydomonas species. These differences were probably linked to the different morphological characteristics of the studied algae (e.g., with or without cell wall) or different sampling locations and adaptations. On the other hand, molecular data suggest the presence of common reactions, key genes, and metabolic pathways that can be used as biomarkers of salt stress in Chlamydomonas spp., with implications for future physiological and biotechnological studies on microalgae and plants.