Kovacikia minuta sp. nov. (Leptolyngbyaceae, Cyanobacteria), a new freshwater chlorophyll f-producing cyanobacterium

Evolution of far-red light photoacclimation in cyanobacteria

Cyanobacteria oxygenated the atmosphere of early Earth and continue to be key players in global carbon and nitrogen cycles. A phylogenetically diverse subset of extant cyanobacteria can perform photosynthesis with far-red light through a process called far-red light photoacclimation, or FaRLiP. This phenotype is enabled by a cluster of ∼20 genes and involves the synthesis of red-shifted chlorophylls d and f, together with paralogs of the ubiquitous photosynthetic machinery used in visible light. The FaRLiP gene cluster is present in diverse, environmentally important cyanobacterial groups, but its origin, evolutionary history, and connection to early biotic environments have remained unclear. This study takes advantage of the recent increase in (meta)genomic data to help clarify this issue: sequence data mining, metagenomic assembly, and phylogenetic tree networks were used to recover more than 600 new FaRLiP gene sequences, corresponding to 51 new gene clusters. These data enable high-resolution phylogenetics and-by relying on multiple gene trees, together with gene arrangement conservation-support FaRLiP appearing early in cyanobacterial evolution. Sampling information shows that considerable FaRLiP diversity can be observed in microbialites to the present day, and we hypothesize that the process was associated with the formation of microbial mats and stromatolites in the early Paleoproterozoic. The ancestral FaRLiP cluster was reconstructed, revealing features that have been maintained for billions of years. Overall, far-red-light-driven oxygenic photosynthesis may have played a significant role in Earth's early history.

Microcystin production is important for toxic Microcystis to survive long-term nitrogen starvation

Toxic cyanobacterial blooms have expanded and intensified on a global scale. Although microcystins are known as the most abundant cyanotoxins released during cyanobacterial blooms, the physiological role of these toxic secondary metabolites has not been fully resolved. Here, we show that microcystin production is important for toxic Microcystis to maintain carbon metabolism under long-term nitrogen starvation and subsequent recovery. Compared to carbon metabolism in the nonmicrocystin-producing strains, toxic Microcystis could accumulate more carbon reserves under nitrogen limitation, which is important for the survival of cells under stressful conditions. Transcriptomic analysis revealed that the genes involved in microcystin synthesis were significantly up-regulated at the initial recovery phase, indicating their essential role in strengthening glycogen catabolism and fueling recovery. Flow cytometry analysis showed that compared to nontoxic strains, microcystin-producing Microcystis exhibited a higher survival and recovery rate after prolonged nitrogen starvation, which is consistent with the dominance of these species at the early stage of cyanobacterial blooms. The close genetic traits between Microcystis strains suggest that the strategies observed here might be highly conserved. Our results imply that toxic Microcystis establishes a competitive advantage over nontoxic species and provides insights into the seasonal succession of natural Microcystis populations.

Kovacikia euganea sp. nov. (Leptolyngbyaceae, Cyanobacteria), a new chlorophyll f producing cyanobacterium from the Euganean Thermal District (Italy)

Hot springs are considered modern terrestrial environments analogous to Archean continental surfaces, where photosynthetic life could have evolved. In this habitat cyanobacteria dominate thanks to the adaptations to high temperature and the capability to acclimate to low light intensity and far-red enriched spectra typical of microbial biofilms. The isolation and characterization of new cyanobacterial species from these environments is fundamental to discover genetic and physiological traits allowing them to thrive under such unfavorable conditions, giving useful information to understand the evolution and plasticity of oxygenic photosynthesis as well as to assess their metabolic biodiversity for biotechnological purposes. In this study, we present the polyphasic characterization of a filamentous cyanobacterium, denominated strain ETS-13, isolated from mud biofilms collected in the Euganean Thermal District (Italy). The area is known since ancient times for the presence of thermal springs and muds exploited for the beneficial properties linked to heat, electrolytes, and organic compounds produced by the microbiota. The ETS-13 genome was assembled and annotated, while phylogenetic analyzes were performed using a combined approach based on the 16S rRNA sequence and considering the 16S-23S ITS secondary structures. In addition, morphological, biochemical, and physiological features of the organism were investigated, allowing its classification as a new species of the Kovacikia genus, named Kovacikia euganea, which formed a cluster with other species of Leptolyngbyaceae from thermal environments. Interestingly, the strain was the first isolated in Italy capable of performing Far-Red Light Photoacclimation (FaRLiP) when exposed to far-red light, a feature found in other species of the same genus so far tested for this acclimation and isolated form geographically distant and different environments.

Widespread distribution of chlorophyll f-producing Leptodesmis cyanobacteria

Chlorophyll (Chl) f was reported as the fifth Chl in oxygenic photoautotrophs. Chlorophyll f production expanded the utilization of photosynthetically active radiation into the far-red light (FR) region in some cyanobacterial genera. In this study, 11 filamentous cyanobacterial strains were isolated from FR-enriched habitats, including hydrophyte, moss, shady stone, shallow ditch, and microbial mat across Central and Southern China. Polyphasic analysis classified them into the same genus of Leptodesmis and further recognized them as four new species, including Leptodesmis atroviridis sp. nov., Leptodesmis fuscus sp. nov., Leptodesmis olivacea sp. nov., and Leptodesmis undulata sp. nov. These cyanobacteria had absorption peaks beyond 700 nm due to Chl f production and red-shifted phycobiliprotein complexes under FR conditions. All but L. undulata produced phycoerythrin and showed varying degrees of a reddish-brown to dark green color under white light conditions. However, the phycoerythrin contents were sharply decreased under FR conditions, and these three Leptodesmis species appeared green. In summary, the Leptodesmis genus contains diverse species with the capacity to synthesize Chl f and is likely a ubiquitous group of Chl f-producing cyanobacteria.

Cyanobacteria Boring Limestones in Freshwater Settings-Their Pioneering Role in Sculpturing Pebbles and Carbonate Dissolution

In freshwater lakes and rivers, cyanobacteria belonging to the family Leptolyngbyaceae bore > 1 mm deep into limestone pebbles by dissolving carbonate at the tip of their 3-8 μm-thick filaments. The abundance of these borings decreases downward while it is so high at the rock surface that micrometric debris is formed. Moreover, the disintegrated material on the pebbles' surface can be easily removed, for instance, when pebbles are grinding against each other due to wave or current action or when insect larvae settle and scratch loosened grains from the surface while constructing their cases. After a larvae case has been abandoned, it decays with time and the surface benath it is colonized again by boring cyanobacteria. These processes can alternate repeatedly and lead to a sculptured appearance of the pebbles, especially because insect larvae tend to colonize already existing depressions where they are better protected from predation and where they can access suspended food more easily. In the sculptures entrenched by insect larvae, larvae of byssate bivalves like Dreissena polymorpha may settle. When growing, these bivalves also remove loosened carbonate from the bored surface. Thus, boring cyanobacteria play a pioneering, preconditioning role in the morphological evolution of limestone (pebble) surfaces by transforming an initially hard substrate into a firm- to softground that is subsequently colonized and structured by animals. Consequently, sculptured pebbles are the product of multiphase, preconditioned bioerosion. Ultimately, the synergistic effects of these bioerosive processes result in the dissolution of carbonate leading to a maximum take-up of approximately 0.5-0.8 kg CO2 per square meter and year, as a preliminary estimate indicates.

Chlorophyll f production in two new subaerial cyanobacteria of the family Oculatellaceae

Chlorophyll (Chl) f was recently identified in a few cyanobacteria as the fifth chlorophyll of oxygenic organisms. In this study, two Leptolyngbya-like strains of CCNU0012 and CCNU0013 were isolated from a dry ditch in Chongqing city and a brick wall in Mount Emei Scenic Area in China, respectively. These two strains were described as new species: Elainella chongqingensis sp. nov. (Oculatellaceae, Synechococcales) and Pegethrix sichuanica sp. nov. (Oculatellaceae, Synechococcales) by the polyphasic approach based on morphological features, phylogenetic analysis of 16S rRNA gene and secondary structure comparison of 16S-23S internal transcribed spacer domains. Both strains produced Chl a under white light (WL) but additionally induced Chl f synthesis under far-red light (FRL). Unexpectedly, the content of Chl f in P. sichuanica was nearly half that in most Chl f-producing cyanobacteria. Red-shifted phycobiliproteins were also induced in both strains under FRL conditions. Subsequently, additional absorption peak beyond 700 nm in the FRL spectral region appeared in these two strains. This is the first report of Chl f production induced by FRL in the family Oculatellaceae. This study not only extended the diversity of Chl f-producing cyanobacteria but also provided precious samples to elucidate the essential binding sites of Chl f within cyanobacterial photosystems.