Orthophosphate uptake of planktonic microorganisms in Lake Balaton

Seasonal orthophosphate uptake of natural plankton has been studied in hypertrophic and mesotrophic basins of Lake Balaton. Initial uptake of carrier-free labelled phosphoric acid followed first order kinetics in accordance with a single two-compartment model. In isotope equilibrium from 1.2 to 9.7 per cent of radioactivity was found in the filtrate. Turnover times of orthophosphate in water were some minutes in summer and two orders of magnitude longer in winter and April. Turnover time of phosphorus in microorganisms has been estimated on the basis of equilibrium isotope distribution between water and particular phases and turnover time in water. It ranged from 28 to over 1 000 min. The shortest times were found in summer, and the longest in winter. Turnover time of orthophosphate in water increased in relation to phosphate additions according to Michaelis-Menten kinetics. The values of Michaelis-constant plus the ambient orthophosphate concentration (K + S_n) suggested an overestimation of real phosphate concentrations by chemical measurements, being much lower than soluble reactive phosphorus contents in most cases, as it is well-known in many other lakes. Both K + S_n values and maximum uptake velocities (V) were significantly higher in the hypertrophic basin than in the mesotrophic one, in contrast to essentially similar values of turnover times. Maximum uptake velocity per unit chlorophyll-a P _opt:V ratio showed extreme to moderate phosphorus deficiency for Lake Balaton plankton, which seemed to decrease somewhat in the course of eutrophication.     

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.A study of aerobic sediments from fifteen lakes, selected as representative of Danish lakes with respect to the sediment Fe and phosphorus content, showed that the release of soluble reactive phosphorus was negatively correlated with the surface sediment Fe:P ratio. Analysis of phosphate adsorption properties of surface sediment from 12 lakes revealed that the capability of aerobic sediments to buffer phosphate concentration correlated with the Fe:P ratio while the maximum adsorption capacity correlated with total iron. Thus, the Fe:P ratio may provide a measure of free sorption sites for orthophosphate ions on iron hydroxyoxide surfaces.The results indicate that provided the Fe:P ratio is above 15 (by weight) it may be possible to control internal P-loading by keeping the surface sediment oxidized. Since the Fe:P ratio is easy to measure, it may be a useful tool in the management of shallow lakes.     

Vertical variations in kinetics of alkaline phosphatase and P species in sediments of a shallow Chinese eutrophic lake (Lake Donghu)

The vertical distribution of the variables relevant to P forms in sediments were studied in a shallow Chinese freshwater lake (Lake Donghu) in 1997, 1998, 1999 and 2000, to assess the contribution of enzyme to P availability in sediment cores. Sediment P was fractionationd into iron-bound P, calcium-bound P, acid soluble organic P (ASOP) and hot NaOH extractable residual organic P. The former two species made the largest contribution to the sediment P pool. All P species exhibited significantly higher concentrations in different depths at Station I, compared with those found at Station II, except for ASOP. Coupled with these lower ASOP concentrations, the V _max data of alkaline phosphatase, measured on the same samples, were significantly higher at station I. Taken together, ASOP were probably important in supplying the enzymatic substrate (Phosphatase Hydrolyzable Phosphorus, PHP) into interstitial water. Dissolved orthophosphate and PHP concentrations were highly heterogeneous , but peaked in subsurface, paralleled by higher V _max and lower K _m values of alkaline phosphatase, throughout the sediment core. Sediment in the eutrophic lake is not only enriched in available P (iron-bound P), or stores residual P, but also tends to release PHP, thereby inducing the production of alkaline phosphatase and releasing o-P into water column by enzymatic hydrolysis. The latter process may also occur in relatively deep sediment layers.     

<Emphasis Type="Italic">Alona alsafadii</Emphasis> n. sp. from Yemen,a primitive,groundwater-dwelling member of the <Emphasis Type="Italic">A. karua</Emphasis>-group

The effect of algae on the production of musty-smelling compounds by actinomycetes was studied. Streptomyces spp., causing intensive musty odor, were isolated from hypertrophic Lake Kasumigaura and cultured in association with algae from the same lake. Isolate E and I effectively utilized the cyanobacteria, Microcystis aeruginosa and Anabaena spiroides, and the diatom, Synedra acus, as a carbon source and produced a musty-smelling 2-methylisoborneol in the shaken sediment cultures. High populations of algae and actinomycetes, and aerobic condition in the sediment seem responsible for the occurrence of musty odor in Lake Kasumigaura.     

Degradation rates of organic phosphorus in lake sediment

Phosphorus (P) binding groups were identified in phytoplankton, settling particles, and sediment profiles by ³¹P NMR spectroscopy from the Swedish mesotrophic Lake Erken. The ³¹P NMR analysis revealed that polyphosphates and pyrophosphates were abundant in the water column, but rapidly mineralized in the sediment. Orthophosphate monoesters and teichoic acids degraded more slowly than DNA-P, polyphosphates, and P lipids. Humic acids and organic acids from phytoplankton were precipitated from the NaOH extract by acidification and identified by ³¹P NMR spectroscopy. The precipitated P was significantly more recalcitrant than the P compound groups remaining in solution, but does not constitute a major sink of P as it did not reach a stable concentration with depth, which indicates that it may eventually be degraded. Since P also precipitated from phytoplankton, the origin of humic-P can not be related solely to allochthonous P.     

The role of <Emphasis Type="Italic">Campsurus notatus</Emphasis> (Ephemeroptera: Polymitarcytidae) bioturbation and sediment quality on potential gas fluxes in a tropical lake

About 30% of the total area of Lake Batata (Amazon) was impacted by the disposal of bauxite tailings originated from the process of washing bauxite. This effluent, composed by fine particles of clay and water, settled on top of the natural sediment, originating a new substratum with a different physical and chemical composition. This phenomenon created a new distinct habitat (impacted sediment) influencing the benthic community. The aim of this study was to evaluate the impact of bioturbation by Campsurus notatus (Ephemeroptera: Polymitarcytidae) on potential gas fluxes in the sediment of natural and impacted areas of the lake. The natural sediment had a significantly higher methane concentration when compared to the impacted one. In incubated sediment cores, the presence of C. notatus nymphs resulted in a significant increase in oxygen consumption and methane and carbon dioxide release to the water column. The effect of the presence of nymphs on methane was ambiguous. The C. notatus nymphs strongly decreased methane concentration in natural sediment samples, probably because of the enhancement of the oxic sediment area. However, this effect was not observed in impacted samples. Finally, the new substratum of Lake Batata decreased methane concentration in sediment and water column. C. notatus nymphs demonstrated to have a significant role on gas flux (methane and CO₂) from sediment to water column as well as on oxygen consumption in Lake Batata, consequently influencing the carbon cycle in this lake. Handling editor: S. M. Thomaz     

Overwintering of Microcystis aeruginosa Kutz. in a shallow lake

The standing crop and photosynthetic activity of Microcystisaeruginosa Kütz. in both the plankton and sediment wereinvestigated from November 1979 to May 1982 in Lake Kasumigaura,Japan. The number of planktonic colonies of this species decreasedfrom early autumn to early spring, but increased in the sedimentduring late summer and autumn. The overwintering colonies inthe sediment were –100–1000 times greater per unitarea than those in lake water. No photoinhibition of photosynthesiscould be observed in overwintering Microcystis. The values ofthe initial slopes of photosynthesis-light (P-I) curves weresimilar to those of the summer population, although the maximumphotosynthetic rate (P_max) measured at 20°C was lower thanthat of the summer planktonic population. In winter the valuesof initial slope of the P-I curve, and the ratio of phycobilinto chlorophyll a sorted from sediment were higher than in coloniesfrom the plankton.     

Causative microorganisms for musty odor occurrence in the eutrophic Lake Kasumigaura

The musty odor phenomenon in Lake Kasumigaura, Japan (a eutrophic lake that is used for various purposes, including water supply source) was examined. The causative microorganisms responsible for the production of the odor compounds, geosmin and/or 2-methylisoborneol (MIB), and the odor release mechanism were studied in vitro and in situ. The numbers of the filamentous P. tenue in the lake water column were closely correlated with the concentration of MIB, but not geosmin. The isolated monoxenic P. tenue in culture produced only MIB. Geosmin concentration was well correlated with the number of actinomycetes in the sediment. Forty isolates of actinomycetes from the sediment around the water supply intake tower produced both geosmin and MIB in culture. Furthermore, the average ratio of production of geosmin to MIB was 1.40–1.0. Actinomycetes in the sediment played an important role in geosmin production. We concluded that geosmin and MIB from actinomycetes in the sediment, and MIB from P. tenue in the water column were jointly responsible for the severe musty odor occurrence of the waters supply source at Lake Kasumigaura.     

Phosphorus sorption characteristics of sediment in the Simiyu and Kagera River basins: implications for phosphorus loading into Lake Victoria

As part of a larger study to assess the influence of land use on riverine and atmospheric phosphorus (P) loading to Lake Victoria, P sorption characteristics of eight composite bottom sediment samples from the Simiyu and Kagera rivers were determined using the Langmuir equation. The samples had low to medium Langmuir adsorption maxima (Γ_m), ranging from 107 to 201μg g^?1. Langmuir binding energy co-efficient (K) ranged from 60 to 181μg l^?1 and the equilibrium P concentration at zero sorption (EPC₀) from 0.1 to 2.75μg g^?1. By using Langmuir co-efficients derived from P sorption experiments and soluble reactive phosphorus (SRP) concentrations measured in rivers as well as the in-shore waters of Lake Victoria, it was possible to determine the potential release of SRP into the lake by sediment from the two catchments. For the 2000 water-year, it was estimated that about 28.65 ± 0.89 (mean ± SD) and 66 ± 6.76 tons of SRP were released into Lake Victoria by sediment deposited by the Simiyu and Kagera rivers, respectively. The implications of these results to future management of cultural eutrophication in Lake Victoria are also discussed.     

Benthic sediment influence on dissolved phosphorus concentrations in a headwater stream

Phosphate interacts with inorganic sediment particles through sorption reactions in streams. Collectively, this phosphorus (P) buffering mechanism can be an important determinant of soluble reactive P (SRP) concentrations. If sorption reactions control SRP concentrations in a stream, then differences in sediment characteristics may cause spatial differences in SRP concentrations. This prediction was tested by examining sediment-buffering characteristics and spatial variation in SRP among reaches with distinct sediment composition (i.e., fine versus coarse particles) in two tributaries of Boulder Creek, a headwater stream in central Wisconsin. SRP concentrations were significantly lower and algal available P and P sorption capacity were significantly higher in the reach dominated by fine sediments. Although fine particles such as sand had the greatest P sorption capacity, no retention could be attributed to biotic processes, whereas over 50% of P retention in coarse particles such as gravel could be linked to biotic uptake. Equilibrium P concentration (EPC₀) assays from different sediment fractions also indicate that biotic uptake is relatively unimportant in sand particles (EPC_live 10 μg/L: EPC_killed 10 μg/L) but very important in gravel or larger particles (EPC_live 10 μg/L: EPC_killed 80 μg/L). Thus, sediment influence on stream water P concentrations can shift predictably from abiotic sorption in reaches with fine particles to biotic retention in areas dominated by coarse sediments. Consequently, changes in sediment composition due to natural or anthropogenic disturbance have the potential to alter the type and strength of sediment-associated processes determining ambient stream P concentrations.     

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia .
2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m⁻² of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m⁻², respectively.
3. With the resuspended sediment, 39.4 mg P  m⁻² day⁻¹ was brought to the water column outside the stand, 22.4 mg P  m⁻² day⁻¹ at the edge and 13.4 mg P  m⁻² day⁻¹ within the stand.
4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.     

Hyporheic community composition in a gravel-bed stream: influence of vertical hydrological exchange, sediment structure and physicochemistry

Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel‐bed stream in New Zealand. 2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini‐piezometers, each installed in a different upwelling or downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates. 3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L^?1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than downwelling areas. 4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter. 5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations. 6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.     

Phosphorus budgetary analysis of sediment–water interface in a short-term anoxic condition in shallow Lake Kasumigaura,Japan

To investigate the effective depth from the surface sediment, and phosphorus fractions related to phosphorus release under short-term anoxic conditions, varying lengths of sediment cores taken from Lake Kasumigaura, a large shallow polymictic lake in Japan, were incubated for a few weeks and then analyzed. Results showed few differences in total phosphorus (TP) amount per unit area in overlying waters irrespective of the core thickness, and sums of TP in both overlying water and 0- to 2-cm sediment layers were nearly equal before and after the experiment, indicating that phosphorus was released mainly from the 0- to 2-cm layer by dissolution. In contrast, phosphorus was decreased in pore water below a 2-cm depth, probably through sorption to sediment solids. The citrate-dithionite-bicarbonate total phosphorus (CBD-TP) and non-reactive phosphorus extracted by NaOH (NaOH-NRP) in sediment solids in the 0- to 2-cm layer decreased during the experiment. The decreases of CBD-TP were 10 times higher than those of NaOH-NRP, suggesting that the released phosphorus came mainly from the fraction bound to iron in Lake Kasumigaura.     

A study of absorption characteristics of chromophoric dissolved organic matter and particles in Lake Taihu,China

Absorptions by non-phytoplankton particles and phytoplankton, and chromophoric dissolved organic matter (CDOM) were measured at 50 sites in large, shallow, Lake Taihu in winter and summer 2006 to study their seasonal and spatial variations, and their relative contributions to total absorption. The CDOM absorption was significantly higher in winter than in summer, due to degradation and release of fixed carbon in phytoplankton and submerged aquatic vegetation (SAV). The hyperbolic model was used to model the spectral absorption of CDOM, and the mean spectral slope of 6.38 nm⁻¹ was obtained. At most sites, the spectral absorption of non-phytoplankton particles was similar to that of the total particles, demonstrating that the absorption of the total particles is dominated by the absorption of non-phytoplankton particles. In summer, phytoplankton absorption increased markedly, due to frequent algal blooms especially in Meiliang Bay. In winter, the significant increase in non-phytoplankton particle absorption resulted from the increase of inorganic particulate matter caused by sediment resuspension. Strong linear relationships were found between a _d(440) and total suspended matter (TSM), organic suspended matter (OSM), and inorganic suspended matter (ISM). Strong linear relationships were also found between a _ph(440), a _ph(675) and chlorophyll a (Chl-a) concentration. The total relative contributions of non-phytoplankton particles over the range of photosynthetically active radiation (PAR) (400–700 nm) were 48.4 and 79.9% in summer and winter respectively. Non-phytoplankton particle absorption dominated the total absorption, especially in winter, in Lake Taihu, due to frequent sediment resuspension in the large shallow lake as a result of strong windy conditions. The results indicate that strong absorption by CDOM and non-phytoplankton particles at the blue wavelength has an impact on the spectral availability, and acts as a selection factor for the composition of the phytoplankton community, with cyanobacteria being the dominate species in Lake Taihu. Handling editor: L. Naselli-Flores     

Turnover of DNA-P and phospholipid-P in lake sediments

Identifying and quantifying the forms of phosphorus (P) in lake sediments is a prerequisite for understanding lake trophic status and possible exports of P downstream. Organic P is one of the most important P forms found in the sediment, where orthophosphate diesters, including DNA and phospholipids, represent a degradable P pool that can support primary production and eutrophication. In this study, sediment cores from the eutrophic Lake Erken and the oligotrophic Lake Ånnsjön, both in steady state regarding long-term P input revealed trends in the degradation of DNA-P and PL-P with sediment depth. Comparisons were performed based on the differentiation of essentially permanent or recalcitrant P and temporary, potentially mobile P for the respective fractions. The temporary P pool was defined as the part of the total P pool calculated for values higher than the level at which the measured P concentration converged to a constant value and the recalcitrant pool was defined as the difference between the total and the temporary. The temporary diester-P pool comprised over 20 % of the total temporary P in Lake Erken and around 4 % in Lake Ånnsjön. The decrease in P concentrations with depth was more rapid for DNA-P compared to PL-P in both lakes, suggesting that DNA-P has a more prominent role in internal loading. The study shows that P mobilization potential can be different for different P fractions, which is important when assessing their contribution to internal loading of P within an aquatic system.     

Orthophosphate turnover in East African lakes

Summary Turnover rates of ³²P–PO₄ and concentrations of orthophosphate as soluble reactive phosphorus (SRP) were measured in five East African waters. Rapid incorporation of ³²P–PO₄ by the seston and orthophosphate concentrations below the limit of detectibility were found in Lakes Elmenteita, Naivasha, and Naivasha Crater Lake. Turnover was slow and orthophosphate concentration high in both Lake Nakuru and the Crescent Island Crater basin of Lake Naivasha. Further experiments in Lake Nakuru indicated that colloidal binding of orthophosphate was limited and that particles retained by an 8.0 filter incorporated 66% as much tracer as particles retained by a 0.1 filter. These experiments strengthen our conclusion that a large quantity of orthophosphate is available for algal use in Lake Nakuru.     

Influence of <Emphasis Type="Italic">Potamogeton crispus</Emphasis> growth on nutrients in the sediment and water of Lake Tangxunhu

An incubation experiment was performed on Potamogeton crispus (P. crispus) using sediment collected from Lake Tangxunhu in the center of China, in order to determine the effects of plant growth on Fe, Si, Cu, Zn, Mn, Mg, P, and Ca concentrations in the sediments and overlying waters. After 3 months of incubation, Ca, Mg, and Si concentrations in the water column were significantly lower, and P and Cu concentrations were significantly higher than in unplanted controls. The effect of P. crispus growth on sediment pore waters and water-extractable elements varied. Concentrations of Ca, Mg, Si, Fe, Cu, and Zn were significantly higher, and P was significantly lower, than in pore waters of the control. Water-extracted concentrations of Fe, Mg, and Si in the sediments were lower, and P was higher, than in the control. Presence of P. crispus generally enhanced concentration gradients of elements between pore waters and overlying waters but not for P. The growth of P. crispus was associated with an increase in water pH and formation of root plaques, resulting in complex effects on the sediment nutritional status. Handling editor: S. M. Thomaz     

Temporal and spatial variations of aquatic environmental characteristics and sediment bacterial community in five regions of Lake Taihu

Sediment bacterial community and their relation with environmental factors were investigated in the five different trophic status lake regions sediment, Meiliang Bay, Wuli Lake, Gonghu Bay, Western Lake Taihu and Xukou Bay in a large, shallow, eutrophic freshwater lake (Lake Taihu, China). Water and surface sediment samples were collected at 35 sampling sites in January 2014 (winter) and July 2015 (summer). The physicochemical characterization showed that there were obvious changes in the trophic status and eutrophic index of five lake regions, which was mainly due to the difference of organic matter source. Based on the analysis of aquatic environmental characteristics, the organic nitrogen or nitrate nitrogen was the main storing form in the overlying water of five lake regions. In addition, nitrate nitrogen in pore water was lower than in overlying water, while ammonia nitrogen in pore water was higher than in overlying water. According to the DGGE profiles, temporal and spatial variations of bacterial community were apparent. Bacterial diversity was higher in summer than in winter and increased with the decrease in the lake region trophic status. The dendrogram of the bacterial community similarities revealed that samples were almost all grouped into two defined clusters (summer and winter), which indicated that season rather than region was the dominant factor. Canonical correspondence analysis demonstrated that ammonia nitrogen and nitrate–nitrite nitrogen in the sediment and pore water, organic matter and temperature significantly influenced the sediment bacterial community in the five lake regions.     

Pore water dynamics in the sediment of a shallow and hypertrophic lake

Seasonal variations in pore water with main stress on pH and phosphate were investigated in the sediment of the shallow and hypertrophic Lake Søbygaard, Denmark. The purpose was to evaluate factors affecting the internal phosphorus loading. Pore water was obtained by in situ incubation of ceramic cups, sampled anaerobicaly from a fixed position in the sediment. The method is evaluated. During summer, pH and phosphate concentrations increased in the upper 8–10 cm of the sediment. Increased pH was most pronounced in the upper 5 cm, where pH increased to between 9 and 10. This is believed to be caused by the photosynthetically elevated pH in the above lake water. Phosphate concentrations increased with depth, from 0–2 mg P 1^–1 in the upper 5 cm to 3–6 mg P 1^–1 in 6–10 cm depth. Average phosphate gradient in the upper 6–8 cm was 1.0 mg P 1^–1 cm ^–1 in the summer decreasing to 0.2 mg P 1^–1 cm ^t1 in the autumn/winter. In spite of low redox potential, Fe(II) was not present in the upper 20 cm. The seasonal variation in pore water phosphate is believed mainly to be due to the variations in pore water pH inducing a substitution of phosphate ions with hydroxyl ions on ironhydroxides during summer. A considerable sedimentation of organic bound phosphorus and decomposition in the sediment is also considered important. Phosphorus release from the sediment is facilitated by bio- and gas turbation and by the frequent occurrence of resuspension caused by windaction. Net release rate is highly variable during the season. The summer average is 40 mg P m^–2 d^–1.     

Dimethylsulfide and methane thiol in sediment porewater of a Danish estuary

Seasonal variation of dimethylsulfide (DMS) and methane thiol (MSH) concentrations in sediment porewater was determined in a Danish estuary. Dimethylsulfide (DMDS) was never found. Detectable DMS levels of up to 0.1 M were found only in the summer and only within the upper 5 cm of the sediment. The DMS accumulation was probably associated with decomposing fragments of macro-algae in the surface layer. Significant MSH accumulation of up to 1 M was found only in the deep, CH₄-rich sediment below the SO₄ ^2- zone. With depth, a detectable MSH level could thus be observed below the 1 mM SO₄ ^2--isopleth which also marked the SO₄ ^2--CH₄ transition. The transition zone was located deeper in the sediment in winter (20–25 cm depth) than in summer (5–10 cm depth). The absence of MSH in the SO₄ ^2- zone could be due to rapid utilization of the compound by SO₄ ^2--reducing bacteria. A possible involvement of MSH in anaerobic CH₄ oxidation at the transition zone is discussed; CH₄ and sulfide (HS^- form, pH 7) are proposed to form MSH and H₂ which in turn may be metabolized by, e.g. SO₄ ^2--reducing bacteria.