Macroalgae, increasingly recognized for their role in global carbon sequestration (blue carbon), are thought to have a negligible contribution from Antarctica according to global models. These models are based on limited data, particularly from high-latitude coastal regions. While studies highlight the significant contribution of Arctic macroalgae to blue carbon, Antarctic seaweed contributions remain poorly understood. Current paradigms suggest that Antarctica has limited suitable habitat for macroalgae, primarily due to light limitations in high-latitude, ice-covered waters. However, observations from the Antarctic Peninsula indicate macroalgal biomass comparable to some Arctic and temperate ecosystems, highlighting the potential for greater carbon sequestration. High rates of macroalgal material delivery to deep Antarctic waters also suggest a potential for long-term carbon storage. Climate change is rapidly altering Antarctic coastal environments through glacial retreat and changing sea-ice extent, impacting land-sea dynamics and potentially influencing macroalgal carbon sequestration. Light availability, determined by factors such as sea ice, suspended sediments, and phytoplankton blooms, is a crucial limiting factor for benthic macroalgae distribution and productivity. This study investigated the extent and depth distribution of Antarctic macroalgae to reassess their contribution to global carbon fixation and sequestration, considering the impacts of climate change.

Existing literature reveals a significant knowledge gap concerning the contribution of Antarctic macroalgae to global carbon sequestration. While studies acknowledge the importance of macroalgae in carbon sequestration globally and particularly in the Arctic, data on Antarctic macroalgae are scarce and largely confined to regions like the Antarctic Peninsula. Global models often predict minimal suitable habitat for Antarctic macroalgae due to light limitations imposed by sea ice and low solar angles. However, observations from the Antarctic Peninsula reveal surprisingly high macroalgal biomass, rivaling or even exceeding some Arctic and temperate regions. These macroalgae contribute significantly to benthic food webs and may play a crucial role in carbon sequestration. High rates of macroalgal material reaching the deep ocean, coupled with the potential for long-term carbon storage in cold polar environments, suggest that the contribution of Antarctic macroalgae might be significantly underestimated. Prior research emphasizes the influence of climate-driven changes such as glacial retreat and sea-ice dynamics on Antarctic coastal ecosystems and, consequently, on carbon sequestration pathways. The interplay between increased habitat availability from glacial retreat and potential disturbances from icebergs underscores the need for improved understanding of Antarctic macroalgal distribution and its role in carbon cycling.

The study conducted benthic surveys in the Ross Sea region of Antarctica during January and February of 2021 and 2023 using the New Zealand research vessel RV *Tangaroa*. Un-navigated coastal areas were mapped using a multibeam echosounder (MBES) to identify suitable benthic habitats. Remote seafloor imaging was performed using a Deep-Towed Imaging System (DTIS) across transects parallel to the shore, targeting depths between 40 and 250 m. The DTIS system recorded continuous high-definition video and still images. Five ice-free locations were sampled, totaling 25 individual sampling stations. Imagery processing involved taxonomic identification of macroalgae, distinguishing between attached and drift algae. Density calculations were performed by dividing abundance by the total transect area. Specimen collections were made in 2023 to confirm species identification and create reference specimens. Global modeling of seafloor light availability was used to estimate photosynthetically active radiation (PAR) reaching the seabed, combining global bathymetric models, surface radiation, and light attenuation estimates. The R-package "coastalLight" was employed to calculate irradiance at the seabed, addressing challenges in determining water clarity metrics. Net primary productivity (NPP) was estimated for three major Antarctic macroalgal species (*Himantothallus grandifolius*, *Palmaria decipiens*, and Crustose Coralline Algae (CCA)) using published photosynthetic parameters and modeled seabed light. The productivity-irradiance equation was employed to calculate NPP at every cell with available seabed light data. Antarctic-wide macroalgal carbon fixation was estimated by extrapolating NPP across the observed latitudinal ranges of the three species. These calculations involved adjusting for factors such as suitable substrata and growing season length. The resulting estimates were compared to global macroalgal carbon fixation estimates.

The study revealed dense populations of attached canopy-forming brown algae (*H. grandifolius*) at depths up to 74-95 m at multiple locations in the northern Victoria Land Coast of the Ross Sea. A diverse range of rhodophytes, chlorophytes, and phaeophytes were observed across different depths. Rhodophytes were found as deep as 99 m, CCA at 125 m, and *D. menziesii* and *H. grandifolius* between 75-95 m. Drift algae were common at depths exceeding 100 m, including *H. grandifolius* at depths greater than 200 m. Global seafloor light modeling indicated that light intensities at surveyed depths and latitudes were sometimes sufficient for photosynthesis, but frequently below expected thresholds. This discrepancy was attributed to the remarkable adaptation of Antarctic macroalgae to low light, frequent macroalgal transport to deeper waters, and variability in light availability. Light and metabolic modeling, using literature-derived photosynthetic parameters, suggested that *P. decipiens* could have net carbon gains at depths between 0 and 70 m, *H. grandifolius* to ~75 m, and CCA as deep as 125 m. Extrapolating from observed latitudinal ranges and densities, the study estimated that the three model species could fix a combined 2.7 Tg of carbon annually in the Ross Sea and 37 Tg across Antarctica. This represents a potential contribution of up to 2.8% of global macroalgal carbon fixation, with *H. grandifolius* and *P. decipiens* potentially contributing 1.1% and 1.2%, respectively. The findings highlight deeper verified records of macroalgae in the Antarctic, particularly *H. grandifolius*, and suggest that Antarctic macroalgae may be more abundant and reach greater depths than previously thought. High rates of deep-water transport of both attached and drift macroalgae support the significance of macroalgal carbon export and sequestration in the Antarctic region.

This study significantly advances our understanding of Antarctic macroalgal communities and their role in global carbon cycling. The discovery of dense macroalgal populations at unexpectedly deep depths challenges existing paradigms and highlights the limitations of current global models in accurately assessing Antarctic macroalgal contribution to carbon fixation. The observed distributions are largely in line with physiological expectations based on light requirements, but some observations of red algae extend beyond predicted limits, indicating potential adaptations or uncaptured factors influencing macroalgal depth distribution. The high frequency of drift algae suggests considerable transport to deeper waters, enhancing the potential for carbon sequestration. While there are uncertainties concerning the accuracy of modeled light estimates, the findings strongly suggest that Antarctic macroalgae are significantly more productive than previously assumed, potentially playing a much larger role in global carbon budgets than previously recognized. The observed macroalgal distributions, coupled with modeling results, provide substantial evidence that the contribution of Antarctic seaweed to global carbon fixation is higher than what global models suggest, highlighting the need for more comprehensive surveys and improved understanding of light penetration, macroalgal biomass, and the fate of macroalgae in these environments.

This research provides compelling evidence that Antarctic macroalgae are significantly more abundant and contribute more to global carbon fixation than previously understood. The discovery of deep-living macroalgal communities in the Ross Sea, coupled with modeling estimates, suggests a potential contribution of 0.9-2.8% to the global macroalgal carbon fixation budget. These findings underscore the importance of comprehensive surveys in understudied regions and the need for refined global models that account for the unique characteristics of Antarctic ecosystems. Future research should focus on further investigation of light penetration in Antarctic waters, accurate biomass assessments, and detailed studies of the fate of macroalgal biomass in the deep ocean to quantify carbon sequestration potential more precisely. Conservation efforts should prioritize the protection of these important Antarctic benthic habitats in light of their substantial role in global carbon cycling.

The study's reliance on global seabed light models presents a limitation, as these models may not fully capture the variability in light conditions due to factors such as ice cover, suspended sediment, and phytoplankton blooms. Furthermore, the extrapolation of carbon fixation from a limited number of surveyed sites to the entire Ross Sea and Antarctic region introduces uncertainty. The use of published photosynthetic parameters, rather than in situ measurements, also contributes to potential inaccuracies in productivity estimates. While the study provides significant insights into Antarctic macroalgal communities, further research with more extensive surveys and in situ measurements is essential to reduce uncertainties.