Microbial activity during a coastal phytoplankton bloom on the Western Antarctic Peninsula in late summer

Abstract

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, 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-/gamma-Proteobacteria 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.