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Temperature plays a fundamental role in determining phytoplankton community structure, distribution, and abundance. With climate models predicting increases in ocean surface temperatures of up to 3.2°C by 2100, there is a genuine need to acquire data on the phenotypic plasticity, and thus performance, of phytoplankton in relation to temperature. We investigated the effects of temperature (14–28°C) on the growth, morphology, productivity, silicification and macromolecular composition of the marine diatom Thalassiosira pseudonana. Optimum growth rate and maximum P:R ratio were obtained around 21°C. Cell volume and chlorophyll a increased with temperature, as did lipids and proteins. One of the strongest temperature-induced shifts was the higher silicification rates at low temperature. Our results reveal temperature-driven responses in physiological, morphological and biochemical traits in T. pseudonana; whereby at supra-optimal temperatures cells grew slower, were larger, had higher chlorophyll and protein content but reduced silicification, while those exposed to sub-optimal temperatures were smaller, heavily silicified with lower lipid and chlorophyll content. If these conserved across species, our findings indicate that as oceans warm, we may see shifts in diatom phenotypes and community structure, with potential biogeochemical consequences of higher remineralisation and declines in carbon and silicon export to the ocean interior. [ABSTRACT FROM AUTHOR]

Dimethylsulfoniopropionate (DMSP) is a ubiquitous organic sulfur compound that underpins sulfur cycling in the marine environment and is the precursor to the climatically active gas dimethylsulfide (DMS). Modelling studies have identified the Southern Ocean as a DMS hot spot during summer, yet except for the bloom forming haptophyte Phaeocystis, little is known about sulfur production by other important members of the marine microbial community. Here, we measured DMSP concentrations and DMSP lyase activity (DLA), with corresponding carbon, nitrogen and Chl a content, in 15 species of Antarctic phototrophic phytoplankton (14 microalgae species and one cyanobacterium) and one phagotrophic flagellate. We found that 11 of the 16 species were able to produce DMSP and eight possess DLA. DMSP content ranged from 0.06 to 73 fmol cell−1 and estimated DMSP production rates ranged from 0.008 to 12.42 fmol cell−1 day−1. As expected, Phaeocystis was amongst the highest producers, however, contrary to expectation DMSP concentrations were high in several pennate diatom species, with intracellular concentrations between 1.85 and 46.6 mM. Here we present the first evidence that the cyanobacterium Synechococcus may be a DMSP producer, with the potential to contribute significantly to the DMSP pool. This study has provided the first analysis of DMSP production and DLA in a suite of phototrophic and phagotrophic species isolated from Antarctica, revealing the variability in DMSP concentrations across multiple strains and within genera and delivered new evidence for potential DLA in diatoms. [ABSTRACT FROM AUTHOR]