Case studies
The sample locations are situated in the axial summit caldera of the fast spreading (11 cm/yr full rate) ridge EPR at 9°50'N at a water depth of 2500 meters. In both case studies we use data from time series studies conducted in the 1990s. Venting temperatures are ≤55°C, and fluids issue from cracks in the seafloor or from lava pillars that are fissured near the base. Compositions of vent fluids from the two sites are reported in Von Damm and Lilley [13] and Von Damm [11]. These publications also present a detailed description of the geological setting and vent field characteristics, so we here highlight only the key features of these localities.
The northern area is characterized by the high temperature vents Bio9 and Bio9' and the associated diffuse flow sites BM9Riftia (BM9R), BM91o and BM12. The data set for this system (hereinafter referred to as Bio9 area) is the most detailed, because the site was the target of long-term measurements of fluid composition and temperature [25] and seismic activity [26]. Sohn et al. [26] documented a seismic swarm in 1995 in this area, followed by a temperature increase in the Bio9 vent with a delay of a few days [25]. Temperatures of the diffuse fluid samples range between 22.0 and 33.3°C (Table 1).
The southern area (Tube Worm Pillar, TWP) features high temperature venting through an 11-m high sulfide structure on top of a lava pillar. Discrete venting of 351°C fluid is restricted to the top of the chimney, while leakage of diffuse fluids is observed from around the base of the chimney. Eponymous for the site name, a large tubeworm colony inhabits the area of diffuse venting. The associated diffuse fluid samples were retrieved from Y vent, an adjacent broken-off lava pillar that issued fluids of temperatures between 20 and 25°C in 1992-1995, dropping to 18°C in 1997 and finally to 12°C in 2000.
Case study 1 - Bio9 area
Shank et al. [27] studied the change in the vent community during the time period from 1991 to 1995 at the vents in the Bio9 area. These authors report of a magmatic event in 1991, followed by venting of fluids high in hydrogen sulfide. These conditions boosted the establishment of a strong population of the tubeworm Riftia. During the following cruises in 1994, Shank et al. [27] observed the development of rusty spots that appeared within the Riftia colonies. In 1995, the rusty spots had spread and covered large areas of the Riftia population. In 1997, the Riftia population had broken down largely, while the rust had extended to cover much of the Riftia patch [13].
The temporal evolution of the fluid compositions in that time span reveals a decrease in hydrogen sulfide concentrations over the entire period after 1992 with a slight increase in November of 1995 (Figure 2). Before March of 1994, soluble iron follows the hydrogen sulfide concentration; afterwards the iron content increased and reached maximum concentrations during November of 1995. In November of 1997, the Fe concentration had dropped slightly, but was still much higher than during the beginning of the time series.
It has been suggested that the biological development of this area depends on the bioavailability of iron and H2S [13]. This interpretation is plausible, because Riftia live in symbiosis with sulfide oxidizing bacteria [28] and depend on the energy associated with sulfide oxidation. Also, Fe-oxidizing bacteria oxidize Fe2+ in the fluids to ferric hydroxide [29]. So the "rust" in the study area is an indicator that these microorganisms are thriving.
We determined the affinities for both catabolic pathways for the time period of critical geochemical and ecological changes (1991-1997) to improve the understanding of the biological evolution of the vent ecosystems. The calculations make use of the measured concentrations of iron, H2S and oxygen. Unfortunately concentrations of oxygen and the pH for diffuse fluids are not available; therefore, these values are estimated from conservative mixing. Calculated pH values for the fluids show a narrow range of 5.3 to 5.7; likewise, small variations are predicted for oxygen concentrations (92 - 96 μM). Both pH and O2 concentrations reflect the large fraction of seawater calculated from the silica mass balance. Depending on the mixing ratio of vent fluid and seawater, either one of the electron donors (Fe2+, H2S) or oxygen is the limiting reactant determining the amount of energy available per unit vent fluid based (Figure 2). Figure 2 shows the upper limit for iron and H2S oxidation based on an oxygen concentration of the East Pacific bottom seawater of circa 100 μM oxygen [30]. For H2S oxidation, O2 is the limiting reactant, while Fe-oxidation is limited by the availability of iron. An exception is the fluid sampled last in the time series; it exhibits exceptionally low sulfide concentrations and H2S is the compound limiting energy availability.
Figure 3 illustrates the normalized affinities for both reactions. It shows the consequence of limitation; sulfide oxidation has the highest affinity when the fraction of vent fluid in the mixture is lowest (Table 1), because then oxygen contents are greatest. In contrast, the normalized affinities for iron oxidation more closely mirror the iron concentration in the fluid. But affinities are also dependent on the vent fluid fraction, as increased pH favors ferric hydroxide precipitation from the mixed fluids.
The dynamic changes in the normalized affinities of sulfide and iron oxidation (Figure 3 and Additional File 1) can fully explain the ecological changes within the system. The incipient occurrence of rusty staining in November of 1994 correlates with an increased normalized affinity of iron oxidation, while the normalized affinity for sulfide oxidation remains at the same level. In November of 1995 a further increase in iron concentration in the fluid explains the continued spreading of the iron oxide staining. Tied to this change, the normalized affinity for Fe-oxidation almost quadrupled. The normalized affinity for hydrogen sulfide oxidation was only slightly decreased relative to 1994, which explains why the tubeworm colonies were still thriving, despite the increased development of rusty staining. Apparently, both metabolic pathways were favorable and were being exploited at that stage of system evolution. After 1995, the normalized affinity for hydrogen sulfide oxidation dropped as a consequence of the strongly decreased sulfide concentration in the fluid. Because of this drop in the affinity of sulfide oxidation, the tubeworm population, relying on favorable energetics for H2S oxidation, collapsed. Unlike sulfide oxidation, the normalized affinity for iron oxidation remains high, so organisms with the ability to gain energy from iron oxidation can still thrive. Since both reactions depend on oxygen, the reactions are in competition for that electron acceptor and the calculated affinities (Figure 3) are the predicted maxima.
The thermodynamic calculations presented here validate the interpretation by Von Damm and Lilley [13] and confirm that the ecological changes are driven by changes in fluid composition.
Case study 2 - Tube Worm Pillar (TWP)
The fluid compositions of the diffuse fluids issuing in TWP area have been proposed to reveal insights in the redox reactions in the subseafloor [13, 31]. Increased methane concentrations in the diffuse fluids led Von Damm and Lilley [13] to propose that hydrogenotrophic methanogenesis takes place in the subseafloor. Proskurowski et al. [31] could confirm this interpretation through carbon stable isotope measurements of methane and CO2 demonstrating that the carbon isotope ratios are consistent with active microbial carbon cycling in this area.
The compositional changes of diffuse fluid compositions relative to the concentrations predicted from conservative mixing are depicted in Figure 4. Throughout the time series, H2 concentrations are decreased by 1.5 to 2 orders of magnitude relative to the concentrations expected from conservative mixing (cf. Table 1). Methane, in contrast, is enriched by a factor of ten relative to the value predicted from conservative mixing in February-March of 1992. In October 1994 and November 1995 this enrichment is about 3-fold. By 2000, measured methane corresponds to those predicted from conservative mixing, and no methane excess can be observed (Figure 4). The methane excess in 1992-1995 is consistent with the decrease in hydrogen, and ratios of H2 depletion to CH4 excess between 3 and 6 are consistent with the stoichiometry of the hydrogenotrophic methanogenesis reaction, from which that ratio would be predicted to be 4. In 1997 and 2000, however, methane excess was minimal and H2 depletion was still significant, suggesting that other hydrogen-consuming reactions may have also played a role.
While methane enrichment and depletion of hydrogen are indicators for methanogenesis, some of the methane may be metabolized shallower in the system prior to venting by aerobic or anaerobic respiration (Figure 1). There are indications from the Guaymas Basin that anaerobic oxidation of methane (AOM) may take place in vent settings and at temperatures > 30°C [32]. Our calculations suggest that AOM is energetically feasible, so a loss of methane through AOM may be possible. If this reaction took place in the subseafloor, depletions of methane should be associated with increased hydrogen sulfide concentrations. This trend is not observed. AOM may still be taking place, but the rates are too small to affect the compositions of the diffuse fluids.
Affinity calculations for hydrogenotrophic methanogenesis and sulfate reduction in the modeled fluid (Figure 5) indicate strong driving forces for both reactions. The affinity for the reaction is strongly controlled by the hydrogen activity, which has a power of 4 in the relevant mass action equations. Hydrogen endmember concentrations increase from 6460 μM in February/March of 1992 to 8910 μM in March of 1994. Hydrogen concentrations then decrease in the following years to 2700 μM in April of 2000 (Table 1). Unfortunately, the time series contains three points with data lacking for either the diffuse or endmember fluid. In March of 1994, the highest hydrogen concentration in the endmember fluid was measured, but no diffuse fluid was sampled. Hence, in our calculations, the sample collected in October of 1994 has the highest predicted hydrogen content (Figure 4) and also the highest normalized affinity for methanogenesis with 34.0 Joule per kg vent fluid and electron transferred in reaction (J/kg e-) or of 41.5 J/kg e- for sulfate reduction in the predicted diffuse fluid (Figure 5). The fluids sampled in April of 2000 have the smallest fraction of vent fluid and the lowest hydrogen endmember concentration of 12.5 μM, yielding normalized affinities for methanogenesis of 8.9 J/kg e- and 11.0 J/kg e- for sulfate reduction. In these diffuse fluids, hydrogen concentrations are still lower than predicted from conservative mixing and they do not correlate with the endmember concentration or extent of mixing with seawater.
Overall, hydrogen concentrations decrease linearly within the first four years of the time series from 14.9 μM to 0.7 μM. Hydrogen concentrations then remain fairly constant in the range of 0.7 to 0.2 μM until 2000. Figure 5 shows the affinities of these reactions per mol electrons in the reaction and normalized to kg of vent fluid. The normalized affinities of sulfate reduction decrease from 1.4 J/kg e- to 0.05 J/kg e- in 1995 and rebounds to 0.08-0.09 J/kg e- in the following years. Hydrogenotrophic methanogenesis has an affinity of 1.0 J/kg e- in 1992 and decreases to 0.05 to 0.06 J/kg e- in 1995 - 2000 (Table 2).
The differences in the affinities (Table 2) reflect the variability in hydrogen concentrations, but the magnitude of these differences is quite small, because the intensive term (ΔrG°) in the Gibbs energy calculation is very large for both reactions. The calculated affinities for methanogenesis and sulfate reduction (Table 2) lie above the estimated energy limit of microbial metabolism 10 kJ/8 e- [33]. Minimum H2 concentrations required for microbial harnessing of hydrogen at the temperatures of diffuse venting (11.9 to 24.7°C) is on the order of 10-8 M [33], which is considerably lower than the measured concentrations (> 2 × 10-7 M). This result indicates that the microbial communities consume hydrogen but do not control its abundance. Apparently, hydrothermally driven influx of H2 into the system is overall greater than the rate at which H2 is metabolized. The constant hydrogen concentrations between 1995 and 2000 probably indicate some sort of steady-state between influx of hydrogen from below and hydrogen consumption in the subseafloor ecosystem. The early phase of decreasing hydrogen concentrations in the diffuse fluids is not related to changes in endmember compositions (steady between 250 and 350 μM), but may instead reflect the growth a hydrogenotrophic microbial community and increasing rates of consumption of H2 advected into the system by hydrothermal flow. In that early stage, there was a relation between H2 depletion and methane production, indicating that methanogenesis was responsible for both. In 1997 and 2000, H2 was still consumed, but the methane excess had disappeared. Instead, there was excess Fe in the fluids, suggesting that Fe-reduction was taking place, perhaps because it requires lower H2 activities than methanogenesis [33].