Subantarctic phytoplankton communities encounter climate change: Incubations to track the molecular underpinnings of the measured physiology

In complex, natural communities, ecosystem resilience rarely depends on a single species. Overlapping ecological niches provide functional redundancy and increase resilience to environmental change. Due to these cross-taxonomic interactions, it is imperative to study communities in situ to truly understand how an ecosystem will respond to environmental disturbances. We followed a natural phytoplankton community in 200L mesocosm incubations in the subantarctic Southern Ocean (SO) using conventional biochemical and biophysical measurements and metaproteomics to understand community responses to projected climate change. Incubations simulated current (control) SO conditions and future climate change (2100) conditions (i.e., increased temperature, pCO₂, iron, and light, decreased nutrients) for 7 days. After an acclimation period of 3 days, the response of the communities diverged: the 2100 conditions led to short-term ~3-fold increase in phytoplankton biomass that was arrested by day 6 as the community encountered resource limitation. The two mesocosm communities revealed diverging physiology after day 4 with changes in chlorophyll concentration and photosynthesis-related peptides between control and climate change conditions. These combined results – from molecular proteomics to realized physiology – suggest that photosynthetic processes are enhanced in natural Southern Ocean phytoplankton communities in their short-term responses to future climate conditions, but that these transient gains are unsustainable in the absence of external resource inputs.