Proliferation of purple sulphur bacteria at the sediment surface affects intertidal mat diversity and functionality

Microbial Community Composition during a Bloom of Purple Bacteria in Intertidal Sediments in Vigo (Northwest Spain)

In summer 2019, a large, bright pink microbial mat was visible on top of macroalgal deposits in muddy sediments of an urban beach (Playa do Adro, Vigo). In order to characterize the dominant organisms in these colored mats, results from microscopic observations, photosynthetic pigments, and molecular analysis were gathered. Light microscopy examination revealed pinkish microbial aggregates with minor contributions of larger protists and cyanobacteria. High-performance liquid chromatography (HPLC) pigment analysis documented the dominance of bacteriochlorophyll a and carotenoids whose spectra were compatible with those described in photosynthetic purple bacteria. 16S rRNA gene amplicon sequencing confirmed that the vast majority of reads belonged to Proteobacteria (73.5%), and among them, nearly 88% of those reads belonged to purple sulfur bacteria (Gammaproteobacteria). A single family, Chromatiaceae, constituted the bulk of this assemblage, including the genera Thiohalocapsa (32%), Marichromatium (12.5%), Phaeochromatium (5%), and Halocromatium (2%) as main contributors. Nonetheless, a considerable number of sequences could not be assigned to a particular genus, stressing the large biological diversity in these microbial mats and the potential presence of novel taxa of purple sulfur bacteria. IMPORTANCE Urban beaches are valuable recreational areas particularly vulnerable to human disturbance. In these areas, the intertidal sediments harbor a diverse community of microorganisms, including virus, bacteria, fungi, and protozoa. In this sense, pollution events can introduce pathogenic allochthonous microbes which may constitute a human health risk. Visual and sensory observations, such as a weird color or bad smell, are usually appreciated as a warning by beachgoers and authorities, as indeed was the case at do Adro beach in 2019. The observed proliferation seems to be common in summertime, but its dimension alerted beachgoers and media. The obtained results allowed for the identification of purple sulfur bacteria as responsible for the pink-violet top layer staining the intertidal zone. These blooms have never been associated with public health risks. Beyond solving the sanitary concern, other important findings were its diversity and large proportion of novel taxa, illustrating the complexity of these ecosystems.

Groundwater microbial communities and their connection to hydrochemical environment in Golmud, Northwest China

Groundwater microbial community normally co-varies with the associated geochemical transect in some hydrogeological sections along flowpath. However, in hydrogeological section with similar geochemical transect (e.g., salinity, ion compositions) how microbial community in groundwater varies are poorly understood. In this study, groundwater samples were collected at six boreholes vertically and horizontally along a generalized groundwater flowpath in the Golmud area, Qaidam Basin, northwest China. High-throughput sequencing and multivariate statistical analysis were applied to explore the underlying relationships between microbial community structure and hydrogeochemical environment. The result showed that microbial communities changed considerably at both horizontal and vertical scales, although the groundwater samples were of relatively stable ionic compositions and hydrochemical types. The dominant bacterial phyla in groundwater varied from Alphaproteobacteria, Betaproteobacteria and Flavobacteriia in 'phreatic and phreatic-like' groundwater in the recharge area to Gammaproteobacteria in the confined groundwater in the lacustrine plain. At both vertical and horizontal scale, Gammaproteobacteria increased while Alpha- and Betaproteobacteria decreased as the function of distance. Genera Roseateles, Aquabacterium, Sphingomonas, Acinetobacter, Acidovorax and Flavobacterium presented in phreatic groundwater, while Pseudomonas, Hydrogenophaga and Perlucidibaca presented in confined groundwater. Spatial distribution of microbial community was highly affected by the pH (for 'phreatic and phreatic-like' groundwater) and ORP (for confined groundwater) of groundwater that had similar salinity or ion compositions. This research extends our knowledge about microbial communities' variation along groundwater flowpath in studied area and similar arid or semi-arid areas.

A Nanoscale Study of Carbon and Nitrogen Fluxes in Mats of Purple Sulfur Bacteria: Implications for Carbon Cycling at the Surface of Coastal Sediments

Mass blooms of purple sulfur bacteria growing seasonally on green stranded macroalgae have a major impact on the microbial composition and functionality of intertidal mats. To explore the active anoxygenic phototrophic community in purple bacterial mats from the Roscoff Aber Bay (Brittany, France), we conducted a combined approach including molecular and high-resolution secondary ion mass spectrometry (NanoSIMS) analyses. To investigate the dynamics of carbon and nitrogen assimilation activities, NanoSIMS was coupled with a stable isotope probing (SIP) experiment and a compound specific isotope analysis (CSIA) of fatty acid methyl ester (FAME). Sediment samples were incubated with ¹³C- and/or ¹⁵N-labeled acetate, pyruvate, bicarbonate and ammonium. NanoSIMS analysis of ¹³C - and ¹⁵N -incubated samples showed elevated incorporations of ¹³C - and ¹⁵N in the light and of ¹³C -acetate in the dark into dense populations of spherical cells that unambiguously dominated the mats. These results confirmed CSIA data that ranked vaccenic acid, an unambiguous marker of purple sulfur bacteria, as the most strongly enriched in the light after ¹³C -acetate amendment and indicated that acetate uptake, the most active in the mat, was not light-dependent. Analysis of DNA- and cDNA-derived pufM gene sequences revealed that Thiohalocapsa-related clones dominated both libraries and were the most photosynthetically active members of the mat samples. This study provides novel insights into the contribution of purple sulfur bacteria to the carbon cycle during their seasonal developments at the sediment surface in the intertidal zone.

Environmental microbiology as a mosaic of explored ecosystems and issues

Microbes are phylogenetically (Archaea, Bacteria, Eukarya, and viruses) and functionally diverse. They colonize highly varied environments and rapidly respond to and evolve as a response to local and global environmental changes, including those induced by pollutants resulting from human activities. This review exemplifies the Microbial Ecology EC2CO consortium's efforts to explore the biology, ecology, diversity, and roles of microbes in aquatic and continental ecosystems.