In the austral summer of 2020, record high temperatures were registered in the Western Antarctic Peninsula. This offered a unique opportunity to evaluate the effect of extreme sea surface temperature and natural heatwaves on the metabolic balance (i.e. the balance between production and respiration), lipid damage and the possible change in lipid composition of coastal Antarctic (Potter Cove, King George/25 de Mayo Island, South Shetlands) microbial communities. Two marine heatwaves, one in January and a second one in February 2020 showed mean temperatures of 1.8 °C above the 20-year climatology values. During the first heatwave, a rapid and strong (60%) decrease in microbial community biomass was observed, Criptophyceae were replaced by unidentified nanophytoflagellates and other heterotrophic groups. The community experienced an increase in heterotrophic metabolism via an increase in community respiration (CR, 12.79 mmolO2 m−3d−1) and a negative net community production (NCP, −14.9 ± 0.31 mmolO2 m−3d−1), leading to a production:respiration (P:R) rate < 1. Additionally, the most representative fatty acids (FAs: 14:0, 16:0, 18:0, 16:1ω9, 18:1ω9, 18:2ω6 and 18:3ω3) decreased, except for the monounsaturated FAs (MUFA) 16:1ω9 and 18:1 ω9, which increased during this first heatwave. In the second marine heatwave, total biomass dropped to the minimum values reported during the entire study. Here, CR was at its maximum (25.09 mmolO2 m−3d−1), but NCP was also positive (1.96 mmolO2 m−3d−1) and P:R > 1, associated with an active autotrophic community. Again, significant lipid damage, a decrease in saturated FAs and ⍵6 polyunsaturated FAs, and an increase in MUFAs occurred. This field study validates previous experimental results on changes in natural plankton composition and physiology under global warming scenarios.
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