Methanogenesis produces strong 13C enrichment in stromatolites of Lagoa Salgada, Brazil: a modern analogue for Palaeo-/Neoproterozoic stromatolites?

Holocene stromatolites characterized by unusually positive inorganic δ(13) CPDB values (i.e. up to +16‰) are present in Lagoa Salgada, a seasonally brackish to hypersaline lagoon near Rio de Janeiro (Brazil). Such positive values cannot be explained by phototrophic fixation of CO2 alone, and they suggest that methanogenesis was a dominating process during the growth of the stromatolites. Indeed, up to 5 mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing δ(13) C values between -15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, (13) C-depleted hopanoids diplopterol and 3ß-methylated C32 17ß(H),21ß(H)-hopanoic acid (both -40‰) are preserved in lagoon sediments and are most likely derived from aerobic methanotrophic bacteria thriving in the methane-enriched water column. Loss of isotopically light methane through the water column would explain the residual (13) C-enriched pool of dissolved inorganic carbon from where the carbonate constituting the stromatolites precipitated. The predominance of methanogenic archaea in the lagoon is most likely a result of sulphate limitation, suppressing the activity of sulphate-reducing bacteria under brackish conditions in a seasonally humid tropical environment. Indeed, sulphate-reduction activity is very low in the modern sediments. In absence of an efficient carbonate-inducing metabolic process, we propose that stromatolite formation in Lagoa Salgada was abiotically induced, while the (13) C-enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, (13) C-enriched stromatolitic deposits also appear in the geological record of prolonged periods in the Palaeo- and Neoproterozoic. Lagoa Salgada represents a possible modern analogue to conditions that may have been widespread in the Proterozoic, at times when low sulphate concentrations in sea water allowed methanogens to prevail over sulphate-reducing bacteria.

Salmonella enterica serovar Typhimurium DT104 displays a rugose phenotype.

Rugose phenotypes, such as those observed in Vibrio cholerae, have increased resistance to chlorine, oxidative stress, and complement-mediated killing. In this study we identified and defined a rugose phenotype in Salmonella enterica serovar Typhimurium DT104 and showed induction only on certain media at 25 degrees C after 3 days of incubation. Incubation at 37 degrees C resulted in the appearance of the smooth phenotype. Observation of the ultrastructure of the rugose form and a stable smooth variant (Stv), which was isolated following a series of passages of the rugose cells, revealed extracellular substances only in cells from the rugose colony. Observation of the extracellular substance by scanning electron microscopy (SEM) was correlated with the appearance of corrugation during development of rugose colony morphology over a 4-day incubation period at 25 degrees C. In addition, the cells also formed a pellicle in liquid broth, which was associated with the appearance of interlacing slime and fibrillar structures, as observed by SEM. The pellicle-forming cells were completely surrounded by capsular material, which bound cationic ferritin, thus indicating the presence of an extracellular anionic component. The rugose cells, in contrast to Stv, showed resistance to low pH and hydrogen peroxide and an ability to form biofilms. Based on these results and analogy to the rugose phenotype in V. cholerae, we propose a possible role for the rugose phenotype in the survival of S. enterica serovar Typhimurium DT104.

Improved plating media for the detection of Salmonella species with typical and atypical hydrogen sulfide production.

The Salmonella detection ability of 2 surfactant-supplemental media, xylose-lysine-tergitol (Nia-proof) 4 (XLT4) and Miller-Mallinson (MM) agar, was compared against that of several commonly used plating media. XLT4 and MM appeared to be the most efficient in detecting Salmonella in meat products and food animal environments. MM was superior to XLT4 in detecting those increasingly more prevalent strains of Salmonella possessing weak to ultraweak H2S production characteristics.