Many autotrophic microorganisms are likely to adapt to scarcity in dissolved inorganic carbon (DIC; CO 2 ϩ HCO 3 Ϫ ϩ CO 3 2Ϫ ) with CO 2 concentrating mechanisms (CCM) that actively transport DIC across the cell membrane to facilitate carbon fixation. Surprisingly, DIC transport has been well studied among cyanobacteria and microalgae only. The deep-sea vent gammaproteobacterial chemolithoautotroph Thiomicrospira crunogena has a low-DIC inducible CCM, though the mechanism for uptake is unclear, as homologs to cyanobacterial transporters are absent. To identify the components of this CCM, proteomes of T. crunogena cultivated under low-and high-DIC conditions were compared. Fourteen proteins, including those comprising carboxysomes, were at least 4-fold more abundant under low-DIC conditions. One of these proteins was encoded by Tcr_0854; strains carrying mutated copies of this gene, as well as the adjacent Tcr_0853, required elevated DIC for growth. Strains carrying mutated copies of Tcr_0853 and Tcr_0854 overexpressed carboxysomes and had diminished ability to accumulate intracellular DIC. Based on reverse transcription (RT)-PCR, Tcr_0853 and Tcr_0854 were cotranscribed and upregulated under low-DIC conditions. The Tcr_0853-encoded protein was predicted to have 13 transmembrane helices. Given the mutant phenotypes described above, Tcr_0853 and Tcr_0854 may encode a two-subunit DIC transporter that belongs to a previously undescribed transporter family, though it is widespread among autotrophs from multiple phyla.IMPORTANCE DIC uptake and fixation by autotrophs are the primary input of inorganic carbon into the biosphere. The mechanism for dissolved inorganic carbon uptake has been characterized only for cyanobacteria despite the importance of DIC uptake by autotrophic microorganisms from many phyla among the Bacteria and Archaea. In this work, proteins necessary for dissolved inorganic carbon utilization in the deep-sea vent chemolithoautotroph T. crunogena were identified, and two of these may be able to form a novel transporter. Homologs of these proteins are present in 14 phyla in Bacteria and also in one phylum of Archaea, the Euryarchaeota. Many organisms carrying these homologs are autotrophs, suggesting a role in facilitating dissolved inorganic carbon uptake and fixation well beyond the genus Thiomicrospira.KEYWORDS autotroph, bicarbonate transporter, carbon concentrating mechanism, carbon fixation, chemolithoautotroph, hydrothermal vent

The carbonate chemistry of freshwater systems can range from inorganic carbon‐limited to supersaturated with respect to the atmosphere, and the pH of these systems can vary temporally and spatially from alkaline to acidic. Determining how these heterogeneous systems respond to increases in atmospheric CO2 is critical to understanding global impacts of these changes. Here, we synthesize 22 studies from a variety of systems to explore the effects of elevated CO2 on freshwater chemistry and microalgae, which form the base of autotrophic food webs. Across the variability in freshwater systems, elevated CO2 significantly affected water chemistry by decreasing pH and increasing dissolved inorganic carbon. Microalgae were also affected by elevated CO2 with measured increases in (1) nutrient acquisition through microalgal carbon‐to‐nutrient ratios, (2) photosynthetic activity, and (3) growth. While these effects were measured from controlled experiments, the results indicate a wide range of potential freshwater ecosystem effects from elevated atmospheric CO2. Our synthesis also identified several knowledge gaps. Generally, larger sample sizes and studies of longer duration are needed for more robust analyses and conclusions. Additionally, more field experiments across a range of freshwater ecosystem types and studies involving benthic species and multiple trophic levels are needed to strengthen global predictions across the broad variability found within and among freshwater systems.

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