RESUMEN
BACKGROUND: Brasilonema is a cyanobacterial genus found on the surface of mineral substrates and plants such as bromeliads, orchids and eucalyptus. B. octagenarum stands out among cyanobacteria due to causing damage to the leaves of its host in an interaction not yet observed in other cyanobacteria. Previous studies revealed that B. octagenaum UFV-E1 is capable of leading eucalyptus leaves to suffer internal tissue damage and necrosis by unknown mechanisms. This work aimed to investigate the effects of B. octagenarum UFV-E1 inoculation on Eucalyptus urograndis and to uncover molecular mechanisms potentially involved in leaf damage by these cyanobacteria using a comparative genomics approach. RESULTS: Leaves from E. urograndis saplings were exposed for 30 days to B. octagenarum UFV-E1, which was followed by the characterization of its genome and its comparison with the genomes of four other Brasilonema strains isolated from phyllosphere and the surface of mineral substrates. While UFV-E1 inoculation caused an increase in root and stem dry mass of the host plants, the sites colonized by cyanobacteria on leaves presented a significant decrease in pigmentation, showing that the cyanobacterial mats have an effect on leaf cell structure. Genomic analyses revealed that all evaluated Brasilonema genomes harbored genes encoding molecules possibly involved in plant-pathogen interactions, such as hydrolases targeting plant cell walls and proteins similar to known virulence factors from plant pathogens. However, sequences related to the type III secretory system and effectors were not detected, suggesting that, even if any virulence factors could be expressed in contact with their hosts, they would not have the structural means to actively reach plant cytoplasm. CONCLUSIONS: Leaf damage by this species is likely related to the blockage of access to sunlight by the efficient growth of cyanobacterial mats on the phyllosphere, which may hinder the photosynthetic machinery and prevent access to some essential molecules. These results reveal that the presence of cyanobacteria on leaf surfaces is not as universally beneficial as previously thought, since they may not merely provide the products of nitrogen fixation to their hosts in exchange for physical support, but in some cases also hinder regular leaf physiology leading to tissue damage.
RESUMEN
Elucidating the role of cyanobacteria in the biotransformation of arsenic (As) oxyanions is crucial to understand the biogeochemical cycle of this element and indicate species with potential for its bioremediation. In this study, we determined the EC50 for As(III) and As(V) and evaluated the biotransformation of As by Synechococcus sp. through high-performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and X-ray absorption fine structure spectroscopy (XAFS). Synechococcus sp. exhibited higher sensitivity to As(III) with an EC(50, 96 h) of 6.64 mg L(-1) that was approximately 400-fold lower than that for As(V). Even though the cells were exposed to concentrations of As(III) (6 mg L(-1)) approximately 67-fold lower than those of As(V) (400 mg L(-1)), similar intracellular concentrations of As (60.0 µg g(-1)) were observed after 30 days. As(V) was the predominant intracellular As species followed by As(III). Furthermore, organic As species such as monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) were observed in higher proportions after exposure to As(III). The differential toxicity among As oxyanions indicates that determining the redox state of As in the environment is fundamental to estimate toxicity risks to aquatic organisms. Synechococcus sp. demonstrated potential for its application in bioremediation due to the high accumulation of As and production of As organic compounds notably after exposure to As(III).
