Primary productivity in the Ross Sea region is characterized by intense phytoplankton blooms whose temporal and spatial distribution are driven by changes in environmental conditions as well as interactions with the bacterioplankton community. However, the number of studies reporting the simultaneous diversity of the phytoplankton and bacterioplankton in Antarctic waters are limited. Here, we report data on the bacterial diversity in relation to phytoplankton community structure in the surface waters of the Ross Sea during the Austral summer 2017. Our results show partially overlapping bacterioplankton communities between the stations located in the Terra Nova Bay (TNB) coastal waters and the Ross Sea Open Waters (RSOWs), with a dominance of members belonging to the bacterial phyla Bacteroidetes and Proteobacteria. In the TNB coastal area, microbial communities were characterized by a higher abundance of sequences related to heterotrophic bacterial genera such as Polaribacter spp., together with higher phytoplankton biomass and higher relative abundance of diatoms. On the contrary, the phytoplankton biomass in the RSOW were lower, with relatively higher contribution of haptophytes and a higher abundance of sequences related to oligotrophic and mixothrophic bacterial groups like the Oligotrophic Marine Gammaproteobacteria (OMG) group and SAR11. We show that the rate of diversity change between the two locations is influenced by both abiotic (salinity and the nitrogen to phosphorus ratio) and biotic (phytoplankton community structure) factors. Our data provide new insight into the coexistence of the bacterioplankton and phytoplankton in Antarctic waters, suggesting that specific rather than random interaction contribute to the organic matter cycling in the Southern Ocean.

Bicarbonate ions are the primary source of inorganic carbon for autotrophic organisms living in aquatic environments. In the present study, we evaluated the short-term (hours) effects of sodium bicarbonate (NaHCO₃) addition on the growth and photosynthetic efficiency of the green algae Chlorella sorokiniana (211/8k). Bicarbonate was added to nonaxenic cultures at concentrations of 1, 2, and 3 g L⁻¹ leading to a significant increase in biomass especially at the highest salt concentration (3 g L⁻¹) and also showing a bactericidal and bacteriostatic effect that helped to keep a reduced microbial load in the algal culture. Furthermore, bicarbonate stimulated the increase in cellular content of chlorophyll a, improving the photosynthetic performance of cells. Since microalgae of genus Chlorella spp. show great industrial potential for the production of biofuels, nutraceuticals, cosmetics, health, and dietary supplements and the use of bicarbonate as a source of inorganic carbon led to short-term responses in Chlorella sorokiniana, this method represents a valid alternative not only to the insufflation of carbon dioxide for the intensive cultures but also for the production of potentially bioactive compounds in a short period.

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.

A multidisciplinary survey was carried out on the quality of water and sediments of the estuary of the Sele river, an important tributary of the Tyrrhenian Sea, to assess anthropogenic pressures and natural variability. Nine sediment sites were monitored and analyzed for granulometry, morphoscopy, benthic foraminifera and ostracod assemblages, heavy metals, and polycyclic aromatic hydrocarbons. Surface water was assayed for ionic composition and phytoplankton biomass. Total organic carbon (TOC) and total nitrogen (TN) in sediments were higher in the inner part of the estuary (IE), up to 12.7 and 0.7% because of anthropic influence. In waters, N-NH₄, N-NO₃, and P_tot. were high, with loads of P_tot in IE exceeding ~fourfold the limit. Here, it was also observed that the highest primary production was Chl-a, 95.70 µg/L, with cryptophytes, 37.6%, and diatoms, 33.8%, being the main phytoplanktonic groups. The hierarchical analysis split the estuary into two areas, with marked differences in anthropic pollution. Waters were classified as poor–bad level with respect to the content of nutrients. Sedimentological assay reveals littoral erosion and poor supply of river sandy sediments. The erosion environment is confirmed by the presence of meiobenthic recent marine forms intrusion inside the river. All these data reveal the fragility of the estuary and the need of urgent remediation actions.

The Antarctic Circumpolar Current (ACC) is the major current in the Southern Ocean, isolating the warm stratified subtropical waters from the more homogeneous cold polar waters. The ACC flows from west to east around Antarctica and generates an overturning circulation by fostering deep-cold water upwelling and the formation of new water masses, thus affecting the Earth’s heat balance and the global distribution of carbon. The ACC is characterized by several water mass boundaries or fronts, known as the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and South Antarctic Circumpolar Current Front (SACCF), identified by typical physical and chemical properties. While the physical characteristics of these fronts have been characterized, there is still poor information regarding the microbial diversity of this area. Here we present the surface water bacterioplankton community structure based on 16S rRNA sequencing from 13 stations sampled in 2017 between New Zealand to the Ross Sea crossing the ACC Fronts. Our results show a distinct succession in the dominant bacterial phylotypes present in the different water masses and suggest a strong role of sea surface temperatures and the availability of Carbon and Nitrogen in controlling community composition. This work represents an important baseline for future studies on the response of Southern Ocean epipelagic microbial communities to climate change.

We collected live mixed natural samples from the northeastern Ross Sea during the austral summer of 2017 and isolated a novel Prorocentrum sp. (Dinophyceae) associated with mucilaginous Phaeocystis antarctica (Coccolithophyceae) colonies. The haptophyte P. antarctica is a key species of the phytoplankton community in the Ross Sea, where blooms are subjected to iron limitation and/or co-limitation with other micronutrients (e.g., vitamin B₁₂) during the summer. We first performed preliminary genetic analyses to determine the specific identity of the novel Prorocentrum sp., which indicated that it represented a previously undescribed species. The formal description of this new species is in process. To further assess its relationship with P. antarctica, we obtained their monospecific and mixed cultures and evaluated their responses to different irradiance levels and iron and vitamin B₁₂ limitation. Our results indicated differential susceptibility of the two species to iron limitation and differential photosynthetic plasticity under high irradiance. Iron limitation reduced colony formation in P. antarctica and decreased the chlorophyll-a content in Prorocentrum sp., whereas B₁₂ limitation did not affect growth or photosynthetic efficiency in either species. In addition, P. antarctica could photosynthesize efficiently under different irradiance levels, due to its ability to modulate the light adsorption cross-section of PSII, whereas Prorocentrum sp. exhibited lower photosynthetic plasticity and an inability to modulate both the maximum photochemical efficiency and effective adsorption cross-section of PSII under high irradiance. The trophic interaction between Prorocentrum sp. and P. antarctica could present ecological implications for the food webs and biogeochemical cycles of the Antarctic ecosystem. Considering the predicted climate-driven shifts in global ocean surface light regimes and changes in iron or vitamin B₁₂ transfer, which are most likely to impact changes in the phytoplankton community structure, our results present implications for carbon export to deeper waters, ecological functioning, and associated biogeochemical changes in the future.