Abstract
Diatoms are one of the most successful phytoplankton groups in our oceans, being responsible for over 20% of the Earth’s photosynthetic productivity. Their chimeric genomes have genes derived from red algae, green algae, bacteria and heterotrophs, resulting in multiple isoenzymes targeted to different cellular compartments with the potential for differential regulation under nutrient limitation. The resulting interactions between metabolic pathways are not yet fully understood.
We previously showed how acclimation to Cu limitation enhanced susceptibility to overreduction of the photosynthetic electron transport chain and its reorganization to favor photoprotection over light-harvesting in the oceanic diatom Thalassiosira oceanica (Hippmann et al., 2017). In order to understand the overall metabolic changes that help alleviate the stress of Cu limitation, we generated comprehensive proteomic datasets from the diatom Thalassiosira oceanica grown under Cu-limiting and -replete conditions. The datasets were used to identify differentially expressed proteins involved in carbon, nitrogen and oxidative stress-related metabolic pathways and to predict the proteins cellular location.
Metabolic pathway analysis showed integrated responses to Cu limitation in diatoms. The up-regulation of ferredoxin (Fdx) was correlated with up-regulation of plastidial Fdx-dependent isoenzymes involved in nitrogen assimilation as well as enzymes involved in glutathione synthesis thus integrating nitrogen uptake and metabolism with photosynthesis and oxidative stress resistance. The differential regulation of glycolytic isoenzymes located in the chloroplast and mitochondria enables them to channel both excess electrons and/or ATP between these compartments. Additional evidence for chloroplast-mitochondrial cross-talk is shown by up-regulation of chloroplast and mitochondrial proteins involved in the proposed malate shunt.