Since 2015, Antarctic sea ice has entered a period of persistent record-low extent after decades of strong regional variability and large swings. The speed and persistence of this decline point to a system that may be changing state, with implications for the Southern Ocean, Antarctic ecosystems, and the global climate system. What is driving this shift remains insufficiently understood. Atmospheric variability plays a role, but ocean processes, including subsurface heat release and changes in stratification, are increasingly recognized as key pieces of the puzzle, alongside feedbacks involving snow, ice shelves, clouds, and freshwater input. This white paper focuses on four areas where progress is needed. First, improved understanding is needed of sea ice mass balance and dynamics: how sea ice grows, melts, moves, and deforms. Second, exchanges of heat, mass, and momentum between ocean, atmosphere, sea ice, and ice shelves need to be quantified across seasons and regions. Third, the consequences for ecosystems and biogeochemistry are likely substantial. Sea ice supports productive microbial communities and carbon and nutrient cycling, so its decline will affect polar food webs and Southern Ocean carbon storage. Finally, climate feedbacks, from albedo changes to cloud and ocean interactions, remain poorly constrained but could amplify ongoing change. A recurring limitation is the lack of coordinated, year-round observations across Antarctica. Existing data are sparse and uneven in space and time. Antarctica InSync is designed to address this by bringing together satellite observations, autonomous platforms, field campaigns, coastal observatories, harmonized data products, and modelling across pack ice, the marginal ice zone, landfast ice, and polynyas. The white paper recommends standardized observations of essential sea ice, snow, ocean, atmospheric, ecosystem, and biogeochemical variables, integrated into interoperable data products and models. Addressing these gaps is essential to improve predictability and anticipate future Antarctic sea ice change.

Theme 1 addresses the Southern Ocean’s (south of 30°S) critical role in regulating Earth’s climate through circulation patterns that mediate global exchanges of heat, carbon, freshwater, and nutrients. The region has absorbed over 70% of anthropogenic heat and has contributed to about 40% of the global ocean uptake of human-emitted carbon dioxide (CO₂) while also controlling ice shelf stability and sea level rise. However, fundamental gaps in year-round observations, particularly during austral autumn, winter, and spring, severely limit our understanding of these processes and their responses to rapid climate change. This white paper establishes key knowledge gaps and high-priority recommendations that are tractable within the InSync timeframe through coordinated program execution with strong engagement from national operators and funding agencies. Critical needs include seasonal observations of the marginal ice zone where carbon, heat and nutrient-rich waters upwell, year-round continental shelf measurements where dense water formation and ice-ocean interactions occur, standardized air-sea flux measurements, and strategic monitoring of regional choke points. Success requires international resource sharing, coordinated deployments, and sustained commitment to both process studies and long-term monitoring.

Expedition cruise vessels travelling to Antarctica offer several educational opportunities, with Citizen Science (CS) projects being the most prominent way to engage participants in polar science. FjordPhyto is a CS project where travellers onboard expedition cruise vessels gather data and samples for five months (from November to March) during the Antarctic summer season to help researchers understand changes in microalgae communities in response to melting glaciers. Since its inception in 2016, FjordPhyto has involved over 8,000 Antarctic travellers. To understand travellers' perceptions engaging with the program during its first two years, 81 voluntary feedback surveys were collected from participants during 2017-2018 and 2018-2019 seasons. When analysing open-ended questions, three main themes associated with participant experience emerged: educational, enjoyable, and motivational. Results showed that 'educational' was the most frequent category in responses when describing how their experience was enriched by CS engagement. Participants also expressed an 'appreciation for scientific learning' or specifically, for 'learning about ecosystems and climate change'. Furthermore, 97% of respondents stated that participating in CS enriched their travel experience. This preliminary exploratory study provides a first understanding of how CS projects like FjordPhyto can have a positive impact on the polar tourism experience. This study sets a precedent for being a first assessment of the impact CS can have on Antarctic travellers and encourages further quantitative and qualitative studies to evaluate the effectiveness of CS to educate and raise awareness of environmental issues.

Naked dinoflagellates are a regular component of the Antarctic phytoplankton but remain poorly studied. In December 2016, the first massive bloom (9.5 × 106 cells · L-1) of small (~15 μm) naked dinoflagellates was recorded in the western Antarctic Peninsula. To identify these organisms, we performed Illumina next-generation sequencing analysis on field samples to obtain genetic information (SSU rDNA 18SV9 and 16SV4-V5). In addition, we performed polymerase chain reaction (PCR) amplification and Sanger sequencing using dinoflagellate-specific primers (LSU rDNA D1-D3 and ITS/5.8S), as well as traditional light and scanning electron microscopy observations. Phylogenetic analyses revealed that these organisms belonged to the Gymnodinium sensu stricto group and may represent an undescribed species. These analyses also indicated that the observed organisms were closely related to the species Gymnodinium dorsalisulcum, G. impudicum, Barrufeta bravensis, and B. resplendens, as well as to the genera Lepidodinium and Wangodinium. This work has provided the LSU rDNA gene sequence from an Antarctic species belonging to the Gymnodinium sensu stricto group along with a description of the observed morphology of these Antarctic blooming dinoflagellates. We compared the 18S V9 amplicon sequence variant (ASV) that dominated the bloom with global databases and observed that it is widely distributed in the Antarctic Peninsula as well as in the global ocean. This study highlights the need for further efforts to identify and describe the diversity of naked dinoflagellates in Antarctic waters.

The Gerlache Strait is a narrow channel that separates the western coast of the Antarctic Peninsula (WAP) from the Palmer Archipelago. This area is characterized by the presence of interconnected fjords, bays, islands, and channels that serve as a refuge for megafauna during summer. Through the framework of FjordPhyto – a citizen science collaboration with the International Association of Antarctica Tour Operators (IAATO) vessels – we assessed phytoplankton biomass and composition in surface waters of six under-explored nearshore areas connected to the Gerlache Strait (between 64° and 65° S) during three consecutive seasons, from November to March (2016–2019). During the first two seasons, we found significant differences in the phytoplankton community distribution and successional patterns to the north and south of the sampling area; the greatest differences were evidenced mainly in the months of high biomass, December and January. During December, cryptophytes bloomed in the north, while microplanktonic diatoms dominated in the south, and during January, small centric diatoms dominated in the north, while prasinophytes bloomed in the south. This spatial distinction in phytoplankton communities were mainly associated with the occurrence of a surface thermal front in the Gerlache Strait around 64.5° S. The presence of the front separating warm waters to the north and colder waters to the south, during the months of December to February, was confirmed by the analysis of 10 years of remote sensing data. By contrast, during the third season, low biomass prevailed, and no differences in the phytoplankton composition between the north and south areas were observed. The third season was the coldest of the series, with smaller differences in water temperature north and south of the usual front location. This study shows for the first time a complete overview of the phytoplankton composition throughout the entire growth season (November through March) in the nearshore areas of the WAP between 64° and 65° S. The results of this work contribute to the understanding of the phytoplankton community in relation to small scale physical features during the Antarctic austral summer.

Acidification of the ocean due to high atmospheric CO 2 levels may increase the resilience of diatoms causing dramatic shifts in abiotic and biotic cycles with lasting implications on marine ecosystems. Here, we report a potential bioindicator of a shift in the resilience of a coastal and centric model diatom Thalassiosira pseudonana under elevated CO 2 . Specifically, we have discovered, through EGFP-tagging, a plastid membrane localized putative Na + (K + )/H + antiporter that is significantly upregulated at >800 ppm CO 2 , with a potentially important role in maintaining pH homeostasis. Notably, transcript abundance of this antiporter gene was relatively low and constant over the diel cycle under contemporary CO 2 conditions. In future acidified oceanic conditions, dramatic oscillation with >10-fold change between nighttime (high) and daytime (low) transcript abundances of the antiporter was associated with increased resilience of T. pseudonana . By analyzing metatranscriptomic data from the Tara Oceans project, we demonstrate that phylogenetically diverse diatoms express homologs of this antiporter across the globe. We propose that the differential between night- and daytime transcript levels of the antiporter could serve as a bioindicator of a shift in the resilience of diatoms in response to high CO 2 conditions in marine environments.

PurposeThe purpose of this paper is to provide a conceptual framework for using citizen science – defined as a data collection method through which non-professionals engage in contributing to authentic scientific inquiry – within the expedition cruise industry to contribute significantly to the collection of environmental data from hard-to-access Arctic areas.Design/methodology/approachThe authors review trends in Arctic expedition cruise tourism and current needs in Arctic research and monitoring, and clarify where the expedition cruise tourism industry could have the most impact by providing data to the scientific community. The authors also compare the regulatory context in the Antarctic to that in the Arctic and discuss how these differences could affect the widespread use of citizen science. At last, the authors describe some general principles for designing citizen science programs to be successful on board, and highlight several existing programs that are being recognized for their contributions to a greater scientific understanding of the Arctic.FindingsThe authors find that citizen science data from the expedition cruise industry are underutilized as a tool for monitoring Arctic change. Numerous examples illustrate how citizen science programs on-board expedition ships can successfully collect robust scientific data and contribute to enhancing the knowledge and stewardship capacity of cruise passengers. Inclusion of citizen science data from the expedition cruise industry should be considered a critical part of international Arctic observing networks and systems.Social implicationsActive participation in Arctic citizen science by tourists on expedition cruise ships has many potential benefits beyond the collection of high quality data, from increasing passengers’ knowledge and understanding of the Arctic while on board, to affecting their attitudes and behaviors after they return home.Originality/valueThe potential for tourism to contribute to Arctic observing systems has been discussed previously in the scientific literature; the authors narrow the focus to citizen science programs in the expedition cruise industry, and provide concrete examples, in the hope that this will streamline acceptance and implementation of these ideas by researchers and tourism practitioners.

The Antarctic Peninsula is one of the fastest warming regions in the world, with over 87% of its glaciers in retreat. The resulting influx of glacial meltwater to the coastal ecosystems may influence the succession patterns of primary producers. It is critical to document these dynamics, gathering time-series data throughout the seasonal growth period, because any shifts in primary production can affect higher trophic level organisms in the nearshore food web. The Antarctic tourism industry maintains a fleet of vessels that visit the peninsula's nearshore waters throughout the austral summer, November to March. We developed a citizen science (CS) program- FjordPhyto-to leverage these vessels as platforms for gathering data about the region. Trained staff and travelers collect environmental data and biological samples that are sent to researchers. Preliminary results include phytoplankton species identification, cell abundance determination, carbon biomass estimates, and euphotic depth measurement at multiple sites. We show that CS is a valid tool that can enhance research in Antarctica, while also providing an enriching experience to travelers interested in learning more about science in polar environments.

Acidification of the ocean due to high atmospheric CO2 levels may increase the resilience of diatoms causing dramatic shifts in abiotic and biotic cycles with lasting implications on marine ecosystems. Here, we report a potential bioindicator of a shift in the resilience of a coastal and centric model diatom Thalassiosira pseudonana under elevated CO2. Specifically, we have discovered, through EGFP-tagging, a plastid membrane localized putative Na+(K+)/H+ antiporter that is significantly upregulated at > 800 ppm CO2, with a potentially important role in maintaining pH homeostasis. Notably, transcript abundance of this antiporter gene was relatively low and constant over the diel cycle under contemporary CO2 conditions. In future acidified oceanic conditions, dramatic oscillations of >10-fold change between nighttime (high) and daytime (low) in transcript abundances of the antiporter gene were associated with increased resilience of T. pseudonana . By analyzing metatranscriptomic data from the Tara Oceans project, we demonstrate that phylogenetically diverse diatoms express homologs of this antiporter across the globe. We propose that the differential between night- and daytime transcript levels of the antiporter could serve as a bioindicator of a shift in the resilience of diatoms in response to high CO2 conditions in marine environments. ### Competing Interest Statement The authors have declared no competing interest.

The western Antarctic Peninsula (WAP) is one of the most productive regions in the Southern Ocean. However, little is known about the phytoplankton composition in nearshore waters, in fjords and channels between 63º and 67°S, where Antarctic krill and baleen whales are conspicuous. This study represents the first attempt to describe spatial and temporal composition of the phytoplankton community (species, cell concentration, phytoplankton biomass) in twelve relatively unexplored nearshore sites of the WAP. Sampling was carried out in the frame of a Citizen Science project during late summer of 2016 and during the spring–summer 2016–2017. Species identification and enumeration were performed by light and scanning electron microscopy and phytoplankton carbon biomass was estimated by using cell-volume conversion. The highest phytoplankton abundance and biomass values were found in December-January, and were mainly represented by nanophytoflagellates (2–20 µm). Cryptophytes were more abundant in early summer and prasinophyceans in late summer. The abundance of large bloom-forming diatoms was unexpectedly low. Three blooming flagellated taxa were found during the sampling season, chronologically: Pyramimonas sp. in Neko Harbor (March 3, 2016, 1.4 × 10 6 cells L −1 , and 327 µgC L −1 ), cryptophytes in Wilhelmina Bay (December 14, 2016, 6.4 × 10 6 cells L −1 , and 97.5 µgC L −1 ) and unidentified unarmored dinoflagellates near Danco Island (December 18, 2016, 9.5 × 10 6 cells L −1 , and 1597 µgC L −1 ). The last one represents, as far as we know, the first record of a dinoflagellate bloom in the WAP. It is to note that blooming organisms, analyzed morphologically, do not coincide with previously described Antarctic species.

The FjordPhyto Citizen Science project is designed to engage the International Association of Antarctic Tour Operators and their Guests in hands-on science as they journey along the fjords of the west Antarctic Peninsula. The Antarctic Peninsula is one of the fastest warming regions in the world. Melting glaciers bring an influx of freshwater and nutrients into the fjords potentially altering the biology at the phytoplankton level. Phytoplankton play a critical role in regulating the atmosphere, drawing carbon dioxide into the ocean and producing over half the Earth’s oxygen. These microscopic drifting plants make up the foundation of the food system supporting whales, seals, and penguins. FjordPhyto aims to understand how glacial meltwater impacts phytoplankton communities among various fjords throughout the austral summer. Visitors will collect phytoplankton samples and photograph images using simple-to-operate tools. Equipment and educational material will be provided by the FjordPhyto research team as outlined in this Capstone Project. Citizen Science is a powerful tool bringing travelers and scientists together to answer critical science questions. FjordPhyto provides a fun and easy way to involve visitors in the legacy of research in Antarctica, while providing scientists with data that greatly expands the current knowledge of Antarctic fjord ecosystems.

Availability of genome-scale in situ hybridization data allows systematic analysis of genetic neuroanatomical architecture. Within the hippocampus, electrophysiology and lesion and imaging studies demonstrate functional heterogeneity along the septotemporal axis, although precise underlying circuitry and molecular substrates remain uncharacterized. Application of unbiased statistical component analyses to genome-scale hippocampal gene expression data revealed robust septotemporal molecular heterogeneity, leading to the identification of a large cohort of genes with robust regionalized hippocampal expression. Manual mapping of heterogeneous CA3 pyramidal neuron expression patterns demonstrates an unexpectedly complex molecular parcellation into a relatively coherent set of nine expression domains in the septal/temporal and proximal/distal axes with reciprocal, nonoverlapping boundaries. Unique combinatorial profiles of adhesion molecules within these domains suggest corresponding differential connectivity, which is demonstrated for CA3 projections to the lateral septum using retrograde labeling. This complex, discrete molecular architecture provides a novel paradigm for predicting functional differentiation across the full septotemporal extent of the hippocampus.