Abstract
Aquatic environments are heavily impacted by human activities including climate warming and the introduction of xenobiotics. Due to the application of silver nanoparticles as bactericidal agent the introduction of silver into the environment strongly has increased during the past years. Silver ions affect the primary metabolism of algae, in particular photosynthesis. Mixotrophic algae are an interesting test case as they do not exclusively rely on photosynthesis which may attenuate the harmful effect of silver. In order to study the effect of silver ions on mixotrophs, cultures of the chrysophyte Poterioochromonas malhamensis were treated in a replicate design in light and darkness with silver nitrate at a sub-lethal concentration. At five time points samples were taken for the identification and quantitation of proteins by mass spectrometry. In our analysis, relative quantitative protein mass spectrometry has shown to be a useful tool for functional analyses in conjunction with transcriptome reference sequences. A total of 3,952 proteins in 63 samples were identified and quantified, mapping to 4,829 transcripts of the sequenced and assembled transcriptome. Among them, 720 and 104 proteins performing various cellular functions were differentially expressed after eight days in light versus darkness and after three days of silver treatment, respectively. Specifically pathways of the energy and primary carbon metabolism were differentially affected by light and the utilization of expensive reactions hints to an energy surplus of P. malhamensis under light conditions. The excess energy is not invested in growth, but in the synthesis of storage metabolites. The effects of silver were less explicit, observable especially in the dark treatments where the light effect could not mask coinciding but weaker effects of silver. Photosynthesis, particularly the light harvesting complexes, and several sulphur containing enzymes were affected presumably due to a direct interference with the silver ions, mainly affecting energy supply.
Highlights
Protist predators are the primary agents of top-down control of bacteria and primary producers, and key organisms determining the fate and transport of organic matter in nature [1]
Xie et al [60] used in a similar approach the isobaric tags for relative and absolute quantitation technique to investigate the cellular response of the algae Phaeodactylum tricornutum to sub-lethal concentrations of aluminium
They found aluminium-induced toxic effects on photosynthesis leading to an increase in reactive oxygen species (ROS) and energy production
Summary
Protist predators are the primary agents of top-down control of bacteria and primary producers, and key organisms determining the fate and transport of organic matter in nature [1]. Due to the immense diversity of protists and their small size the ecology of these organisms has, up to recently, largely been addressed on the level of broad functional groups. As e.g. small flagellates, comprise a large number of taxonomically quite different organisms with different growth and survival strategies ranging from mainly photosynthetic flagellates (e.g. Uroglena), that only take up very small numbers of bacteria, probably as vitamin source, to entirely heterotrophic flagellates (e.g. Spumella spp.). Between these extremes, there are organisms that utilize both photosynthesis and phagotrophy to different extents depending on the environmental conditions experienced Chrysophytes are of special interest because their abundance may increase during phases of lake re-oligotrophication [4, 5] and because they often dominate mixotrophic phytoplankton populations [6]
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