Phytoplankton biomass during the austral summer is influenced by freezing and melting cycles as well as oceanographic processes that enable nutrient redistribution in the West Antarctic Peninsula (WAP). Microbial functional capabilities, metagenomic and metatranscriptomic activities as well as inorganic ¹³C- and ¹⁵N-assimilation rates were studied in the surface waters of Chile Bay during two contrasting summer periods in 2014. Concentrations of Chlorophyll a (Chla) varied from 0.3 mg m⁻³ in February to a maximum of 2.5 mg m⁻³ in March, together with a decrease in nutrients; however, nutrients were never depleted. The microbial community composition remained similar throughout both sampling periods; however, microbial abundance and activity changed with Chla levels. An increased biomass of Bacillariophyta, Haptophyceae and Cryptophyceae was observed along with night-grazing activity of Dinophyceae and ciliates (Alveolates). During high Chla conditions, HCO₃⁻ uptake rates during daytime incubations increased 5-fold (>2516 nmol C L⁻¹ d⁻¹), and increased photosynthetic transcript numbers that were mainly associated with cryptophytes; meanwhile night time NO₃⁻ (>706 nmol N L⁻¹ d⁻¹) and NH₄⁺ (41.7 nmol N L⁻¹ d⁻¹) uptake rates were 2- and 3-fold higher, respectively, due to activity from Alpha-/Gammaproteobacteria and Bacteroidetes (Flavobacteriia). Due to a projected acceleration in climate change in the WAP, this information is valuable for predicting the composition and functional changes in Antarctic microbial communities.

Current warming in the Western Antarctic Peninsula (WAP) has multiple effects on the marine ecosystem, modifying the trophic web and the nutrient regime. In this study, the effect of decreased surface salinity on the marine microbial community as a consequence of freshening from nearby glaciers was investigated in Chile Bay, Greenwich Island, WAP. In the summer of 2016, samples were collected from glacier ice and transects along the bay for 16S rRNA gene sequencing, while in situ dilution experiments were conducted and analyzed using 16S rRNA gene sequencing and metatranscriptomic analysis. The results reveal that certain common seawater genera, such as Polaribacter, Pseudoalteromonas and HTCC2207, responded positively to decreased salinity in both the bay transect and experiments. The relative abundance of these bacteria slightly decreased, but their functional activity was maintained and increased the over time in the dilution experiments. However, while ice bacteria, such as Flavobacterium and Polaromonas, tolerated the increased salinity after mixing with seawater, their gene expression decreased considerably. We suggest that these bacterial taxa could be defined as sentinels of freshening events in the Antarctic coastal system. Furthermore, these results suggest that a significant portion of the microbial community is resilient and can adapt to disturbances, such as freshening due to the warming effect of climate change in Antarctica.

Summer sea ice cover in the Arctic Ocean has declined sharply during the last decades, leading to changes in ice structures. The shift from thicker multi-year ice to thinner first-year ice changes the methane storage transported by sea ice into remote areas far away from its origin. As significant amounts of methane are stored in sea ice, minimal changes in the ice structure may have a strong impact on the fate of methane when ice melts. Hence, sea ice type is an important indicator of modifications to methane pathways. Based on measurements of methane concentration and its isotopic composition on a drifting ice floe, we report on different storage capacities of methane within first-year ice and ridged/rafted ice, as well as methane supersaturation in the seawater. During this early melt season, we show that ice type and/or structure determines the fate of methane and that methane released into seawater is a predominant pathway. We suggest that sea ice loaded with methane acts as a source of methane for polar surface waters during late spring.

The concentration of greenhouse gases, including nitrous oxide (N2O), methane (CH4), and compounds such as\ud total dimethylsulfoniopropionate (DMSPt), along with other oceanographic variables were measured in the icecovered\ud Arctic Ocean within the Eurasian Basin (EAB). The EAB is affected by the perennial ice-pack and has\ud seasonal microalgal blooms, which in turn may stimulate microbes involved in trace gas cycling. Data collection\ud was carried out on board the LOMROG III cruise during the boreal summer of 2012. Water samples were\ud collected from the surface to the bottom layer (reaching 4300 m depth) along a South-North transect (SNT),\ud from 82.19°N, 8.75°E to 89.26°N, 58.84°W, crossing the EAB through the Nansen and Amundsen Basins. The\ud Polar Mixed Layer and halocline waters along the SNT showed a heterogeneous distribution of N2O, CH4 and\ud DMSPt, fluctuating between 42-111 and 27–649% saturation for N2O and CH4, respectively; and from 3.5 to\ud 58.9 nmol L−1 for DMSPt. Spatial patterns revealed that while CH4 and DMSPt peaked in the Nansen Basin,\ud N2O was higher in the Amundsen Basin. In the Atlantic Intermediate Water and Arctic Deep Water N2O and\ud CH4 distributions were also heterogeneous with saturations between 52% and 106% and 28% and 340%,\ud respectively. Remarkably, the Amundsen Basin contained less CH4 than the Nansen Basin and while both basins\ud were mostly under-saturated in N2O. We propose that part of the CH4 and N2O may be microbiologically\ud consumed via methanotrophy, denitrification, or even diazotrophy, as intermediate and deep waters move\ud throughout EAB associated with the overturning water mass circulation. This study contributes to baseline\ud information on gas distribution in a region that is increasingly subject to rapid environmental changes, and that\ud has an important role on global ocean circulation and climate regulation

Methane (CH₄) is one of the most powerful greenhouse gases with the capacity to influence the Earth's radiative budget as well as contribute to atmospheric chemistry. Natural oceanic production makes up to ∼4% of the overall global CH₄ emissions, however, there is uncertainty around the accuracy of this value due to a lack of accurate measurements. Such is the case in the Subtropical Eastern South Pacific Ocean (SESP), a region with pronounced chlorophyll‐a and oxygen gradients, which in turn affect the microbial CH₄ cycling. This study was conducted during spring‐summer (2014–2016) in the SESP. The region (∼17°–37°S/71°–110°W) is separated into (i) eutrophic, (ii) mesotrophic, and (iii) oligotrophic areas, according to oceanographic and biogeochemical criteria. The SESP presents high CH₄ zonal variability with levels ranging from 0.63 to 33.4 nmol L⁻¹, corresponding to 29% and 1,423% saturation, respectively. High CH₄ concentrations (>1,000% saturation) are observed in the narrow eutrophic area subjected to coastal upwelling. These conditions clearly differ to those observed in the extended oligotrophic subtropical gyre (∼100% saturation). Furthermore, CH₄ also tends to accumulate in the mesotrophic area (with upto 1,423% saturation), where oceanographic conditions as stratification, mesoscale eddies and island mass effect could trigger the presence of a microbial biomass that may be able to induce CH₄ regeneration. The CH₄ efflux is estimated to be between 0.13 and 19.1 µmol m⁻² d⁻¹ (mean ± SD = 4.72 ± 4.67) and the SESP has an emission rate of ∼87.9 Gg CH₄ yr⁻¹.