Heterocyte production, gene expression, and phylogeography in Raphidiopsis (=Cylindrospermopsis) raciborskii.

Raphidiopsis (=Cylindrospermopsis) raciborskii was described as a subtropical-tropical cyanobacterium, later reported expanding into temperate regions. Heterocyte presence used to distinguish Cylindrospermopsis from the very similar Raphidiopsis, but recently the two genera were recognized as one and unified. This study aimed to investigate how heterocyte production is related to nitrogen (N) limitation in heterocytous and non-heterocytous strains of R.raciborskii. High N-concentrations did not inhibit heterocyte development in some strains, while prolonged N-starvation periods never stimulated production in others. RT-qPCR was used to examine the genetic background, through the expression patterns of nifH, ntcA and hetR. While gene expression increased under N-restriction, N-sufficiency did not suppress nifH transcripts as previously observed in other diazotrophyc cyanobacteria, suggesting that heterocyte production in R. raciborskii is not regulated by N-availability. Heterocytous and non-heterocytous strains were genotypically characterized to assess their phylogenetic relationships. In the phylogenetic tree, clusters were intermixed and confirmed Raphidiopsis and Cylindrospermopsis as the same genus. The tree supported previous findings of earlier splitting of American strains, while contesting the African origin hypothesis. The existence of two lines of Chinese strains, with distinct evolutionary patterns, is a significant addition that could lead to new hypotheses of the species biogeography.

Diatom-driven recolonization of microbial mat-dominated siliciclastic tidal flat sediments.

Modern microbial mats and biofilms play a paramount role in sediment biostabilization. When sporadic storms affect tidal flats of Bahía Blanca Estuary, the underlying siliciclastic sediment is exposed by physical disruption of the mat, and in a few weeks' lapse, a microbial community re-establishes. With the objective of studying colonization patterns and the ecological succession of microorganisms at the scale of these erosional structures, these were experimentally made and their biological recolonization followed for 8 weeks, with replication in winter and spring. Motile pennate diatoms led the initial colonization following two distinct patterns: a dominance by Cylindrotheca closterium in winter and by naviculoid and nitzschioid diatoms in spring. During the first 7 days, cell numbers increased 2- to 17-fold. Cell densities further increased exhibiting sigmoidal community growth, reaching 2.9-8.9 × 106 cells cm-3 maxima around day 30; centric diatoms maintained low densities throughout. In 56 days after removal of the original mat, filamentous cyanobacteria that dominate mature mats did not establish a significant biomass, leading to the rejection of the hypothesis that cyanobacteria would drive the colonization. The observed dominance of pennate diatoms is attributed to extrinsic factors determined by tidal flooding, and intrinsic ones, e.g. motility, nutrient affinity and high growth rate.

Diversity of the Potential 2-Methylisoborneol-Producing Genotypes in Thai Strains of Planktothricoides (Cyanobacteria)

ABSTRACT The genus Planktothricoides Suda & Watanabe is considered as a 2-methylisoborneol (MIB) producer, affecting water quality and aquatic animal products worldwide. To date, there is limited information about the diversity of this genus from Thailand. In this study, Thai Planktothricoides strains were isolated from fish ponds and reservoirs in North, Northeast and Central regions for morphological examination, phylogenetic analyses based on 16S rRNA, rbcLX and MIB synthase genes as well as GC/MS/MS analyses. The morphological results and the 16S rRNA and rbcLX phylogenies of Thai Planktothricodes strains enabled them to be designated as Planktothricoides raciborskii. Cell dimensions of Thai strains tested were in 1.86 to 5.96 µm length (L), 2.83 to 13.70 µm width (W), and the L/W ratio ranged from 1:6 to 1:1. Among Planktothricoides strains, the 16S rRNA phylogenies demonstrated that three subclades (A, B and C groups) were apparently divided. The similarity of 16S rRNA genes between subclades were 96-98%. From the detection of MIB synthase genes and GC/MS/MS analyses, some strains grouped into A group were considered as MIB-producers. In this study, most Thai Planktothricoides strains belonging to the A group were found in all three regions, while the strains forming the B and C groups were not distributed in the North region. To our knowledge, the present study is the first report investigation and characterization of the potential MIB-producing Planktothricoides from Thailand. Therefore, providing a valuable tool as a model for the early prediction and detection of taste and odor event is necessary.

CyDiv, a Conserved and Novel Filamentous Cyanobacterial Cell Division Protein Involved in Septum Localization.

Cell division in bacteria has been studied mostly in Escherichia coli and Bacillus subtilis, model organisms for Gram-negative and Gram-positive bacteria, respectively. However, cell division in filamentous cyanobacteria is poorly understood. Here, we identified a novel protein, named CyDiv (Cyanobacterial Division), encoded by the all2320 gene in Anabaena sp. PCC 7120. We show that CyDiv plays a key role during cell division. CyDiv has been previously described only as an exclusive and conserved hypothetical protein in filamentous cyanobacteria. Using polyclonal antibodies against CyDiv, we showed that it localizes at different positions depending on cell division timing: poles, septum, in both daughter cells, but also in only one of the daughter cells. The partial deletion of CyDiv gene generates partial defects in cell division, including severe membrane instability and anomalous septum localization during late division. The inability to complete knock out CyDiv strains suggests that it is an essential gene. In silico structural protein analyses and our experimental results suggest that CyDiv is an FtsB/DivIC-like protein, and could therefore, be part of an essential late divisome complex in Anabaena sp. PCC 7120.