Fig 1.
Photographs of the Beggiatoa mat.
Underwater picture of patches of white mat on the mangrove sediment (a). Sample of colorless filaments observed under dissecting microscope (b): Two morphotypes are visible; black arrowheads indicate morphotype 1, and white arrows indicate morphotype 2.
Fig 2.
Structure and identification of the two Beggiatoa morphotypes.
Light microphotographs of morphotype 1 (a) and morphotype 2 (d), respectively. White arrows highlight the white sheath, the black arrows point out the membranes separating two bacterial cells, and the dotted arrows highlight the sulphur vesicles. The apex of the morphotype 1 filament is marked by a black star. The right identification of the two morphotypes is confirmed by the positive hybridization with the specific probes (BEG572F for morphotype 1 and BEG282F for morphotype 2) designed from each bacterial sequence obtained in this study (b and e are morphotypes 1 and 2, respectively). NONEUB probe was used as negative control (c and f for morphotypes 1 and 2, respectively).
Fig 3.
Ultrastructure of the two Beggiatoa morphotypes.
SEM microphotographs of morphotype 1 (a-b). These images highlighted small vesicles (white arrows) absent from the membranes separating two adjacent cells (dotted arrows). On higher magnification (b), some of these vesicles are fractured (white arrows), and appeared linked to the membranes. TEM microphotographs of the morphotype 1 (c) and morphotype 2 (d) highlight a large central empty space with all the cytoplasmic content postponed on the external membranes. The small vesicles (black arrows) also appear empty due to the loss of sulphur during dehydration process. They are absent from the membranes (dotted arrows) separating two adjacent cells.
Fig 4.
Sulphide metabolism of the Beggiatoa spp.
The EDX spectra of morphotypes 1 (a) and 2 (b) obtained from non-dehydrated samples observed under an ESEM showed that the bacteria contain elemental sulphur. The sulphur mapping localizes this element (marked in red) inside the cells within the cytoplasmic granules (c and d).
Fig 5.
Neighbor joining (NJ) tree displaying the phylogenetic relationships between the Candidatus Beggiatoa sp. Guadeloupe FWI and Candidatus Isobeggiatoa sp. Guadeloupe FWI (in bold) with other colorless large sulphur bacteria.
Phylogenetic tree based on the analysis of 16S rDNA gene sequences of 925 nucleotides. Node robustness was assessed by performing 1000 bootstrap replicates. Only bootstrap values more than 49% are shown at each node. Leucothrix mucor, Thiothrix nivea, and Achromatium spp were used as outgroup. The scale bar corresponds to 0.02 changes per nucleotide.
Table 1.
Distribution of fatty acid methyl esters (FAME) in both morphotypes in % as determined by GC-MS.
Fig 6.
Vertical distribution of sulphide and oxygen in mangrove sediment under mesocosm conditions.
Representative (square) total sulphide and (triangle) oxygen microgradients measured into the sediment on the mesocosm system (a) without mat and (b) with mat. Concentrations are expressed in μM. Error bars = one standard deviation of the mean.