| 英文摘要: | Diatoms are important primary producers in the ocean, however their response to rising CO2 is uncertain. Now research shows how diatoms regulate their metabolism in response to changing CO2. Diatoms are unicellular algae that account for ~20% of global primary production1. Some of the carbon fixed is exported to the deep ocean, leading to a carbon sink. How carbon fixation by diatoms will respond to rising CO2 is unclear as most experiments so far have measured only the net carbon fixation and found conflicting results2. There is a need to understand the physiological and metabolic processes that underpin the diatom response to changes in CO2. Writing in Nature Climate Change, Gwenn Hennon and colleagues3 measure the expression of genes in diatoms that are acclimated to various CO2 concentrations, and use these data to elucidate the mechanism behind the physiological changes observed in their N-limited continuous cultures. They also probe a compilation of gene expression data from previous CO2 manipulation experiments and show that the same signalling pathway controls the regulation of genes that code proteins involved in the carbon concentrating mechanism (CCM) and photorespiration. Photosynthesis is rarely limited by CO2 in in the surface ocean, despite the concentration being well below that needed to saturate the photosynthetic CO2-fixing enzyme, Rubisco (Ribulose 1,5 bisphosphate carboxylase oxygenase). This is because diatoms and most other phytoplankton possess CCMs. These CCMs pump carbon — either in the form of CO2 or in the much more abundant form of bicarbonate — into the cell, elevating intracellular CO2 levels to a concentration that nearly saturates Rubisco. The use of CCMs by phytoplankton raises the question of whether they will show any response to rising CO2 emissions. CCMs are energy- and nutrient-intensive processes that require transporters and enzymes to pump carbon against a concentration gradient. Most of the surface ocean is nutrient limited, with nitrogen a common limiting factor, thus phytoplankton need to optimize the partitioning of energy and nutrients between a multitude of metabolic processes. Because of this optimization, it has been speculated that rising CO2 concentrations will alleviate some of the requirement for a CCM and thus free up resources for other processes, such as faster growth4. Hennon and colleagues3 attempt to uncover how diatom metabolism responds to long-term (>15 generations) growth at high CO2 by measuring the expression of genes in continuous cultures of the model diatom, Thalassiosira pseudonana, grown under nitrate limitation. The physiological response, which has been previously published5, showed that at constant growth rate (as set by the dilution rate of the continuous culture), there was a decrease in both gross photosynthesis and respiration when CO2 was increased from 350 μatm to 800 μatm, suggesting an overall reduction in general metabolism (Fig. 1). In the current study, the authors demonstrate an overall reduction in the expression of respiratory and photosynthetic genes, along with genes involved in general metabolism, for example, in transcription regulation and production of kinases. Interestingly, the expression of ribosomal components (structures that facilitate protein synthesis) was upregulated but the reason for this remains unknown.
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