Sulphide production and corrosion in seawaters during exposure to FAME diesel

Effects of sample preservation on marine microbial diversity analysis

Three replicate seawater samples were collected on three different days, filtered immediately and preserved with one of two guanidinium thiocyanate-based preservatives (DNAzol™ or RNA Lysis Buffer™ plus β-mercaptoethanol (RLA+)) and were kept frozen while being shipped to a lab. In parallel, a carboy of seawater was collected on each of the three days and maintained at ambient temperature while being shipped to a lab. Upon receipt the samples were filtered and treated in the same manner as for immediate preservation. Significantly more DNA was obtained from samples immediately preserved with DNAzol than the corresponding shipped samples for 2 of the 3 days. More DNA was extracted from DNAzol preserved samples but more RNA was obtained from RLA+ preserved samples. A protocol was designed to extract both DNA and RNA from split samples preserved with RLA+ and cDNA was synthesized from the RNA. Three high-throughput 16S rRNA gene libraries were constructed, one from DNA preserved with DNAzol, one from DNA preserved with RLA+ and one from cDNA (RLA+ preserved). Greater alpha diversity was found for libraries constructed from immediately preserved vs. shipped samples for both preservation types, with immediate preservation with DNAzol obtaining the highest level of diversity. Libraries constructed from immediately preserved (RLA+) DNA had greater alpha diversity than libraries constructed from shipped preserved (RLA+) DNA or cDNA. Unifrac measures of beta diversity showed clearer separation of sample types and a greater % variance explained for weighted than for unweighted principal coordinate analysis (PCoA) plots, indicating sample types varied more in their relative abundance of taxa than the presence/absence of particular taxa. We recommend immediate preservation of seawater samples, with DNAzol as the preferred preservative if quantification via qPCR will be performed or the highest alpha diversity is desired but preservation with RLA+ if RNA will be extracted.

Anaerobic biodegradation of biofuels and their impact on the corrosion of a Cu-Ni alloy in marine environments

Fuel biodegradation linked to sulfate reduction can lead to corrosion of the metallic infrastructure in a variety of marine environments. However, the biological stability of emerging biofuels and their potential impact on copper-nickel alloys commonly used in marine systems has not been well documented. Two potential naval biofuels (Camelina-JP5 and Fisher-Tropsch-F76) and their petroleum-derived counterparts (JP5 and F76) were critically assessed in seawater/sediment incubations containing a metal coupon (70/30 Cu-Ni alloy). Relative to a fuel-unamended control (1.2 ± 0.4 μM/d), Camelina-JP5 (86.4 ± 1.6 μM/d) and JP5 (77.6 ± 8.3 μM/d) stimulated much higher rates of sulfate reduction than either FT-F76 (11.4 ± 2.7 μM/d) or F76 (38.4 ± 3.7 μM/d). The general corrosion rate (r² = 0.91) and pitting corrosion (r² = 0.92) correlated with sulfate loss in these incubations. Despite differences in microbial community structure on the metal or in the aqueous or sediment phases, sulfate reducing bacteria affiliated with Desulfarculaceae and Desulfobacteraceae became predominant upon fuel amendment. The identification of alkylsuccinates and alkylbenzylsuccinates attested to anaerobic metabolism of fuel hydrocarbons. Sequences related to Desulfobulbaceae were highly enriched (34.2-64.8%) on the Cu-Ni metal surface, regardless of whether the incubation received a fuel amendment. These results demonstrate that the anaerobic metabolism of biofuel linked to sulfate reduction can exacerbate the corrosion of Cu-Ni alloys. Given the relative lability of Camelina-JP5, particular precaution should be taken when incorporating this hydroprocessed biofuel into marine environments serviced by a Cu-Ni metallic infrastructure.

Molecular methods resolve the bacterial composition of natural marine biofilms on galvanically coupled stainless steel cathodes

Navy vessels consist of various metal alloys and biofilm accumulation at the metal surface is thought to play a role in influencing metal deterioration. To develop better strategies to monitor and control metallic biofilms, it is necessary to resolve the bacterial composition within the biofilm. This study aimed to determine if differences in electrochemical current could influence the composition of dominant bacteria in a metallic biofilm, and if so, determine the level of resolution using metagenomic amplicon sequencing. Current was generated by creating galvanic couples between cathodes made from stainless steel and anodes made from carbon steel, aluminum, or copper nickel and exposing them in the Delaware Bay. Stainless steel cathodes (SSCs) coupled to aluminum or carbon steel generated a higher mean current (0.39 mA) than that coupled to copper nickel (0.17 mA). Following 3 months of exposure, the bacterial composition of biofilms collected from the SSCs was determined and compared. Dominant bacterial taxa from the two higher current SSCs were different from that of the low-current SSC as determined by DGGE and verified by Illumina DNA-seq analysis. These results demonstrate that electrochemical current could influence the composition of dominant bacteria in metallic biofilms and that amplicon sequencing is sufficient to complement current methods used to study metallic biofilms in marine environments.

Anaerobic Biodegradation of Alternative Fuels and Associated Biocorrosion of Carbon Steel in Marine Environments

Fuels that biodegrade too easily can exacerbate through-wall pitting corrosion of pipelines and tanks and result in unintentional environmental releases. We tested the biological stability of two emerging naval biofuels (camelina-JP5 and Fischer-Tropsch-F76) and their potential to exacerbate carbon steel corrosion in seawater incubations with and without a hydrocarbon-degrading sulfate-reducing bacterium. The inclusion of sediment or the positive control bacterium in the incubations stimulated a similar pattern of sulfate reduction with different inocula. However, the highest rates of sulfate reduction were found in incubations amended with camelina-JP5 [(57.2 ± 2.2)-(80.8 ± 8.1) μM/day] or its blend with petroleum-JP5 (76.7 ± 2.4 μM/day). The detection of a suite of metabolites only in the fuel-amended incubations confirmed that alkylated benzene hydrocarbons were metabolized via known anaerobic mechanisms. Most importantly, general (r(2) = 0.73) and pitting (r(2) = 0.69) corrosion were positively correlated with sulfate loss in the incubations. Thus, the anaerobic biodegradation of labile fuel components coupled with sulfate respiration greatly contributed to the biocorrosion of carbon steel. While all fuels were susceptible to anaerobic metabolism, special attention should be given to camelina-JP5 biofuel due to its relatively rapid biodegradation. We recommend that this biofuel be used with caution and that whenever possible extended storage periods should be avoided.

Impact of organosulfur content on diesel fuel stability and implications for carbon steel corrosion

Ultralow sulfur diesel (ULSD) fuel has been integrated into the worldwide fuel infrastructure to help meet a variety of environmental regulations. However, desulfurization alters the properties of diesel fuel in ways that could potentially impact its biological stability. Fuel desulfurization might predispose ULSD to biodeterioration relative to sulfur-rich fuels and in marine systems accelerate rates of sulfate reduction, sulfide production, and carbon steel biocorrosion. To test such prospects, an inoculum from a seawater-compensated ballast tank was amended with fuel from the same ship or with refinery fractions of ULSD, low- (LSD), and high sulfur diesel (HSD) and monitored for sulfate depletion. The rates of sulfate removal in incubations amended with the refinery fuels were elevated relative to the fuel-unamended controls but statistically indistinguishable (∼50 μM SO4/day), but they were found to be roughly twice as fast (∼100 μM SO4/day) when the ship's own diesel was used as a source of carbon and energy. Thus, anaerobic hydrocarbon metabolism likely occurred in these incubations regardless of fuel sulfur content. Microbial community structure from each incubation was also largely independent of the fuel amendment type, based on molecular analysis of 16S rRNA sequences. Two other inocula known to catalyze anaerobic hydrocarbon metabolism showed no differences in fuel-associated sulfate reduction or methanogenesis rates between ULSD, LSD, and HSD. These findings suggest that the stability of diesel is independent of the fuel organosulfur compound status and reasons for the accelerated biocorrosion associated with the use of ULSD should be sought elsewhere.

Molecular tools to track bacteria responsible for fuel deterioration and microbiologically influenced corrosion

Investigating the susceptibility of various fuels to anaerobic biodegradation has become complicated with the recognition that the fuels themselves are not sterile. Bacterial DNA could be obtained when various fuels were filtered through a hydrophobic teflon (0.22 μm) membrane filter. Bacterial 16S rRNA genes from these preparations were PCR amplified, cloned, and the resulting libraries sequenced to identify the fuel-borne bacterial communities. The most common sequence, found in algal- and camelina-based biofuels as well as in ultra-low sulfur diesel (ULSD) and F76 diesel, was similar to that of a Tumebacillus. The next most common sequence was similar to Methylobacterium and was found in the biofuels and ULSD. Higher level phylogenetic groups included representatives of the Firmicutes (Bacillus, Lactobacillus and Streptococcus), several Actinobacteria, Deinococcus-Thermus, Chloroflexi, Cyanobacteria, Bacteroidetes, Alphaproteobacteria (Methylobacterium and Sphingomonadales), Betaproteobacteria (Oxalobacteraceae and Burkholderiales) and Deltaproteobacteria. All of the fuel-associated bacterial sequences, except those obtained from a few facultative microorganisms, were from aerobes and only remotely affiliated with sequences that resulted from anaerobic successional events evident when ULSD was incubated with a coastal seawater and sediment inoculum. Thus, both traditional and alternate fuel formulations harbor a characteristic microflora, but these microorganisms contributed little to the successional patterns that ultimately resulted in fuel decomposition, sulfide formation and metal biocorrosion. The findings illustrate the value of molecular approaches to track the fate of bacteria that might come in contact with fuels and potentially contribute to corrosion problems throughout the energy value chain.