Summary Discovery of red‐shifted chlorophyll d and f in cyanobacteria has opened up new avenues to estimate global carbon fixation driven by far‐red light. Shaded habitats in humid subtropical forest ecosystems contain an increased proportion of far‐red light components relative to residual white light. After an extensive survey of shaded ecosystems within subtropical forests, wide occurrence of red‐shifted chlorophyll‐producing cyanobacteria was demonstrated by isolated Chl f‐producing and Chl d‐containing cyanobacteria. Chl f‐producing cyanobacteria were classified into the genera of Aphanocapsa and Chroococcidiopsis and two undescribed genera within Leptolyngbyaceae. Newly isolated Chl d‐containing Acaryochloris sp. CCNUM4 showed the closest phylogenetic relationship with Acaryochloris species isolated from marine environments. Acaryochloris sp. CCNUM4 produced Chl d as major photopigment, and Chl f‐producing cyanobacteria use Chl a under white light conditions but Chl a + f under far‐red light conditions. Their habitats are widely distributed in subtropical forest ecosystems and varied from mosses on limestone to macrophyte and freshwater in the streams and ponds. This study presents a significant advance in the knowledge of distribution and diversity of red‐shifted chlorophyll‐producing cyanobacteria in terrestrial ecosystems. The results suggest that Chl f‐producing and Chl d‐containing cyanobacteria might be important primary producers in far‐red light dominant niches worldwide.

Cyanobacteria are foundational drivers of global nutrient cycling, with high intracellular iron (Fe) requirements. Fe is found at extremely low concentrations in aquatic systems, however, and the ways in which cyanobacteria take up Fe are largely unknown, especially the initial step in Fe transport across the outer membrane. Here, we identified one TonB protein and four TonB-dependent transporters (TBDTs) of the energy-requiring Fe acquisition system and six porins of the passive diffusion Fe uptake system in the model cyanobacterium sp. strain PCC 6803. The results experimentally demonstrated that TBDTs not only participated in organic ferri-siderophore uptake but also in inorganic free Fe (Fe') acquisition.Fe uptake rate measurements showed that a TBDT quadruple mutant acquired Fe at a lower rate than the wild type and lost nearly all ability to take up ferri-siderophores, indicating that TBDTs are critical for siderophore uptake. However, the mutant retained the ability to take up Fe' at 42% of the wild-type Fe' uptake rate, suggesting additional pathways of Fe' acquisition besides TBDTs, likely by porins. Mutations in four of the six porin-encoding genes produced a low-Fe-sensitive phenotype, while a mutation in all six genes was lethal to cell survival. These diverse outer membrane Fe uptake pathways reflect cyanobacterial evolution and adaptation under a range of Fe regimes across aquatic systems. Cyanobacteria are globally important primary producers and contribute about 25% of global CO fixation. Low Fe bioavailability in surface waters is thought to limit the primary productivity in as much as 40% of the global ocean. The Fe acquisition strategies that cyanobacteria have evolved to overcome Fe deficiency remain poorly characterized. We experimentally characterized the key players and the cooperative work mode of two Fe uptake pathways, including an active uptake pathway and a passive diffusion pathway in the model cyanobacterium sp. PCC 6803. Our finding proved that cyanobacteria use ferri-siderophore transporters to take up Fe', and they shed light on the adaptive mechanisms of cyanobacteria to cope with widespread Fe deficiency across aquatic environments.

Summary Cyanobacteria are globally important primary producers and nitrogen fixers with high iron demands. Low ambient dissolved iron concentrations in many aquatic environments mean that these organisms must maintain sufficient and selective transport of iron into the cell. However, the nature of iron transport pathways through the cyanobacterial outer membrane remains obscure. Here we present multiple lines of experimental evidence that collectively support the existence of a novel class of substrate‐selective iron porin, Slr1908, in the outer membrane of the cyanobacterium Synechocystis sp. PCC 6803. Elemental composition analysis and short‐term iron uptake assays with mutants in Slr1908 reveal that this protein is primarily involved in inorganic iron uptake and contributes less to the accumulation of other metals. Homologues of Slr1908 are widely distributed in both freshwater and marine cyanobacteria, most notably in unicellular marine diazotrophs. Complementary experiments with a homologue of Slr1908 in Synechococcus sp. PCC 7002 restored the phenotype of Synechocystis knockdown mutants, showing that this siderophore producing species also possesses a porin with a similar function in Fe transport. The involvement of a substrate‐selective porins in iron uptake may allow cyanobacteria to tightly control iron flux into the cell, particularly in environments where iron concentrations fluctuate.

Cyanobacteria are globally important primary producers and abundant in many iron-limited aquatic environments. The ways in which they take up iron are largely unknown, but reduction of Fe is an important step in the process. Here we report a special iron permease in Synechocystis, cFTR1, that is required for Fe uptake following Fe re-oxidation. The expression of cFTR1 is induced by iron starvation, and a mutant lacking the gene is abnormally sensitive to iron starvation. The cFTR1 protein localizes to the plasma membrane and contains the iron-binding motif "REXXE". Point-directed mutagenesis of the REXXE motif results in a sensitivity to Fe-deficiency. Measurements of iron ( Fe) uptake rate show that cFTR1 takes up Fe rather than Fe . The function of cFTR1 in Synechocystis could be genetically complemented by the iron permease, Ftr1p, of Saccharomyces cerevisiae, that is known to transport Fe produced by the oxidation of Fe via a multicopper oxidase. Unlike yeast Ftr1p, cyanobacterial cFTR1 probably obtains Fe primarily from the oxidation of Fe by oxygen. Growth assays show that the cFTR1 is required during oxygenic, photoautotrophic growth but not when oxygen production is inhibited during photoheterotrophic growth. In cyanobacteria, iron reduction/re-oxidation uptake pathway may represent their adaptation to oxygenated environments.

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