The assimilation of inorganic nutrients by phytoplankton strongly depends on environmental conditions such as the availability of nitrogen and temperature, especially warming. The acclimation or adaptation of different species to such changes remains poorly understood. Here, we used a multimethod approach to study the viability and physiological and biochemical responses of the marine diatom Chaetoceros pseudocurvisetus to different temperatures (15, 25, and 30 °C) and different N:P ratios. Nitrogen limitation had a greater effect than high temperature on cell growth and reproduction, leading to a marked elongation of setae, decreased phosphorus assimilation, increased lipid accumulation, and decreased protein synthesis. The elongation of setae observed under these conditions may serve to increase the surface area available for the uptake of inorganic and/or organic nitrogen. In contrast, high temperatures (30 °C) had a stronger effect than nitrogen deficiency on cell death, nitrogen assimilation, chlorophyll a accumulation, the cessation of setae formation, and cell lipid remodelling. Significant changes in thylakoid lipids were observed in cells maintained at 30 °C, with increased levels of digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol. These changes may be explained by the role of galactolipids in thylakoid membrane stabilization during heat stress.

The impact of the heavy metal cadmium on the lightly silicified marine diatom Cylindrotheca closterium was studied with regard to growth dynamics, organic matter production and morphological characteristics. C. closterium cultures were able to sustain positive growth in the tested range of 0–1000 μg Cd l^–1 with a significant decrease in specific growth rate in the exponential growth phase at the highest tested Cd concentration. The concentrations of both dissolved (DOC_cell) and particulate organic carbon per cell (POC_cell) in C. closterium cultures exposed to 500 and 1000 µg l^–1 Cd more than doubled compared with the control culture. Enhanced organic matter production as a feedback response to Cd exposure may thus contribute to cell detoxification, supported by the electrochemical data showing that Cd was not significantly accumulated by or associated with diatom cells except for the culture exposed to 1000 μg Cd l^–1. In order to gain more insight into the effects of cadmium on diatom morphological features, the cell surface of C. closterium was characterized at the nanoscale on a single cell level using atomic force microscopy (AFM). Changes induced by Cd were observed as irregular patterns of silica spheres on more silicified parts of the cell, girdle band and around the raphe. These changes clearly show that beside the physiological response, C. closterium also showed a morphological response which is probably due to the interference of Cd with the diatom biosilification process. This study overall contributes to better understanding of the effects of cadmium on diatoms and shows that morphological characteristics assessed by AFM may be a valuable indicator for metal contamination.

We present the characterization and vertical distribution of suspended particulate lipids containing C, H and O which have the potential to sequester carbon from the upper ocean when associated with sinking particles. Lipids have been shown to be valuable in a host of environments to provide insights into the sources and processing of organic materials in the oceans. Here we present, direct-infusion, high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with bulk lipid measures for marine lipid characterization. We present the water column distribution of free fatty acids, tri-, di- and monoacylglycerols from the surface layer to abyssopelagic depths (4800 m) for samples collected in the Northeast Atlantic at the Porcupine Abyssal Plain sustained observatory (PAP-SO) (49.0°N, 16.5°W). Triacylglycerols (TG) with even carbon number (TG) and odd carbon number (oddTG, reflecting bacterial origin), were analyzed, while free fatty acids were analyzed as unsaturated (UFA), branched (BrFA) and saturated (SAFA) fatty acids. The surface productive layer (euphotic zone) was characterized with the highest incidence of lipids that are not reported in the Nature Lipidomics Gateway database, especially lipids that are highly unsaturated (acyl chain unsaturation was on average 3.8 for TG, oddTG, UFA and diacylglycerols (DG)). Additionally, we observed high lipid degradation at epipelagic depths. Fatty acid markers indicate that diatoms and dinoflagellates were important contributors to the lipid pool. Depth-resolved lipid change includes decreased lipid abundance and molecular diversity together with substantial loss of unsaturation with increasing depth. The major lipid change occurs at upper mesopelagic depths. Unlike other observed lipids, the abundance of SAFA remained essentially constant down the water column whereas the number of SAFAs and their contribution to total lipids increased with depth. Thus, we demonstrate that lipid saturation affects the export of carbon from the atmosphere to the deep ocean