Evidence for suboxic nitrification in recent marine sediments

The classical scheme of biogeochemical zones (BGZ) is known to be an oversimplification of the microbial processes that occur in organic-rich marine sediments. Results from a coupled deployment of pore-water gel probes in Loch Duich, Scotland, provide direct evidence for rapid recycling within the iron reduction (FeR) and sulphate reduction (SR) zones. High resolution porewater profiles obtained using diffusive equilibrium in thin films (DET) gel probes found a nitrate peak at the boundary between the FeR and SR zones. This non-steady state feature is consistent with recycling of reduced N occurring throughout the FeR zone. Both conventional pore-water iron profiles and results from diffusive gradient in thin films (DGT) probes indicate that iron is solubilised and precipitated in rapid Fe/S recycling reactions throughout the SR zone. The presence of such complex recycling reactions confirms the oversimplification of the classical BGZ scheme

High-resolution pore-water sampling with a gel sampler

Sediment pore-water profiles were sampled at high resolution (millimeter scale) with a polyacrylamide gel probe. This simple procedure involves inserting a 1-mm-thick gel held in a plastic probe into sediment. The gel reaches diffusive equilibrium in <1-2 h. For anions, the gel was sectioned, back-equilibrated into distilled-deionized water, and anions determined by high-performance liquid chromatography. Laboratory trials showed recovery of 104+/-4% Cl, 102+/-2% NO3, 101+/-1% SO4, and 102+/-2% NH4. For Fe and Mn, the gel was fixed in 0.01 M NaOH for similar to 3 h, subsectioned, extracted with 1 M HNO3, and analyzed by atomic absorption spectrometry. Field trials were undertaken in Esthwaite Water, a seasonally anoxic lake in the English Lake District. Gel probe data compared well with conventional pore-water extractions

Use of a high-resolution pore-water gel profiler to measure groundwater fluxes at an underwater saline seepage site in Lake Kinneret, Israel

We have used new gel pore-water profilers and conventional seepage meters to determine the advective flux of water from an underwater saline seep into Lake Kinneret. The gel probes sampled pore waters from medium to coarse sands that could not be sampled by conventional coring methods. The anions Cl, Br, and SO4 were constant at levels just above those for the lake for 3-5 cm into the sediment due to wave action or other turbulent mixing processes. There was then a sharp increase in concentration to values of approximately 8,000 mg Cl liter(-1), 370 mg SO4 liter(-1), and 120 mg Br liter(-1) at a depth of similar to 8.5 cm. Using an advection-diffusion model, the linear interstitial advection velocity (LIV) of the groundwater into the lake was calculated to vary between 140 and 275 cm yr(-1). The LN values from conventional seepage flux meters at the same site were 30 and 164 cm yr(-1). Differences between the LIV measurements of these two methods may be due to a number of possible factors, including groundwater flux heterogeneity

Phosphorus limitation of primary productivity in the eastern Mediterranean Sea

Although NO3- is generally considered to limit primary productivity in most of the world’s oceans, previous studies have suggested the Mediterranean Sea may be an exception. In this study of the southeastern Mediterranean, we found that all the PO43- was removed from the upper water column during the winter phytoplankton bloom in the core and boundary of a warm-core eddy, while measurable (0.3-0.6 µM) NO3- remained. The N:P (NO3-: PO43-) ratio in the core and boundary of the Cyprus eddy was 27.4 and the slope of the linear portion of the N vs. P scattergram with 25.5 with a positive intercept of 0.5 µM on the NO3- axis. A similar N:P ratio (28-29), slope (21-23), and intercept (0.9-1.1) was found for the water column across much of the southern Levantine basin. These data, taken together with the results of incubation experiments, lead us to conclude that the southeastern Mediterranean is strongly P limited. The degree of P limitation increases from west to east across the entire basin. We suggest that removal of PO43 by adsorbtion on Fe- rich dust particles may be an important process controlling the concentration of P in the water column

Atmospheric input of nitrogen and phosphorus to the Southeast Mediterranean: Sources, fluxes, and possible impact

Estimates of the sources and wet deposition fluxes of inorganic nutrients (PO43-, NO3-, NO2-, NH4+) have been made using a long-term wet atmospheric deposition measurement at three sites along the Mediterranean coast of Israel. The nutrient composition in rainwater indicated a dominant anthropogenic source for NO, and NH: and a continental, natural, and anthropogenic, rock/soil source for PO43-. The calculated long-term dissolved inorganic N (IN) and inorganic P (IP) fluxes were 0.28 and 0.009 g m(-2) yr(-1) to the coastal zone and estimated as 0.24 and 0.008 g m(-2) yr(-1) to the Southeast (SE) Mediterranean, with a possible increasing pattern of the annual dissolved IN fluxes. Concentration of total and seawater leachable LP (LIP) from dust was examined on 20 Whatman 41 filters collected during 1996. The mean total IP concentration in dust was 0.13 +/- 0.11% (geomean = 0.09%), with a mean of 387 +/- 205 mu g IP per g of dust leached by seawater. LIP from dust varies between 6 and 85% (mean of 38%) of the dry total IF. Dust of desert-type (Saharan) events exhibited lower LIP solubility in seawater (similar to 25%, median) than air masses of European origin (similar to 45%, median). The calculated ratio of wet deposition to total (wet and dry) deposition here of 0.2 showed the importance of dry deposition of P in the SE Mediterranean basin compared to atmospheric inputs into the northwestern basin. The total IP and seawater LIP fluxes from dry deposition were estimated as 0.04 and 0.01 g m(-2) yr(-1), respectively. Atmospheric inputs of bioavailable N and P represent an imbalanced contribution to the new production of 8-20 and 4-11%, respectively, and reinforce the unusual N: P ratios (similar to 27) and possible P limitation in the SE Mediterranean

Anoxic nitrification in marine sediments

Nitrate peaks are found in pore-water profiles in marine sediments at depths considerably below the conventional zone of oxic nitrification. These have been interpreted to represent nonsteady- state effects produced by the activity of nitrifying bacteria, and suggest that nitrification occurs throughout the anoxic sediment region. In this study, ΣNO3 peaks and molecular analysis of DNA and RNA extracted from anoxic sediments of Loch Duich, an organic-rich marine fjord, are consistent with nitrification occurring in the anoxic zone. Analysis of ammonia oxidiser 16S rRNA gene fragments amplified from sediment DNA indicated the abundance of autotrophic ammonia-oxidising bacteria throughout the sediment depth sampled (40 cm), while RT-PCR analysis indicated their potential activity throughout this region. A large non-steady-state pore-water ΣNO3 peak at ~21 cm correlated with discontinuities in this ammonia-oxidiser community. In addition, a subsurface nitrate peak at ~8 cm below the oxygen penetration depth, correlated with the depth of a peak in nitrification rate, assessed by transformation of 15N-labelled ammonia. The source of the oxidant required to support nitrification within the anoxic region is uncertain. It is suggested that rapid recycling of N is occurring, based on a coupled reaction involving Mn oxides (or possibly highly labile Fe oxides) buried during small-scale slumping events. However, to fully investigate this coupling, advances in the capability of high-resolution pore-water techniques are required

Bacterially mediated removal of phosphorus and cycling of nitrate and sulfate in the waste stream of a "zero-discharge" recirculating mariculture system

Simultaneous removal of nitrogen and phosphorus by microbial biofilters has been used in a variety of water treatment systems including treatment systems in aquaculture. In this study, phosphorus, nitrate and sulfate cycling in the anaerobic loop of a zero-discharge, recirculating mariculture system was investigated using detailed geochemical measurements in the sludge layer of the digestion basin. High concentrations of nitrate and sulfate, circulating in the overlying water (~15 mM), were removed by microbial respiration in the sludge resulting in a sulfide accumulation of up to 3 mM. Modelling of the observed S and O isotopic ratios in the surface sludge suggested that, with time, major respiration processes shifted from heterotrophic nitrate and sulfate reduction to autotrophic nitrate reduction. The much higher inorganic P content of the sludge relative to the fish feces is attributed to conversion of organic P to authigenic apatite. This conclusion is supported by: (a) X-ray diffraction analyses, which pointed to an accumulation of a calcium phosphate mineral phase that was different from P phases found in the feces, (b) the calculation that the pore waters of the sludge were highly oversaturated with respect to hydroxyapatite (saturation index = 4.87) and (c) there was a decrease in phosphate (and in the Ca/Na molar ratio) in the pore waters simultaneous with an increase in ammonia showing there had to be an additional P removal process at the same time as the heterotrophic breakdown of organic matter

Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation

The eastern Mediterranean has a high nitrate to phosphate (N : P) ratio (~28 : 1) in the deep water and a highly unusual P limitation of the primary productivity. We present a detailed nutrient budget of inputs to the basin, which shows that there is a high N: P ratio (>16 : 1) in all the input sources, particularly from the atmospheric source, where the N: P ratio was 117 : 1. The high N: P ratio is retained within the system because there is no significant denitrification in either the sediments or intermediate water. This is because of the extreme oligotrophic nature of the system, which is caused by the unusual anti-estuarine flow at the Straits of Sicily. Support for this conclusion is provided by the observation that the only area of the eastern Mediterranean where the N: P ratio in deeper water is ~16 : 1 is the northern Adriatic, which is also the only area with significant denitrification. The N budget (total input to basin vs. net output at the straits of Sicily) balances closely. This N balance suggests that N fixation is an insignificant process in this P-limited system. The unusually light 15N values in the deep water nitrate and particulate organic nitrogen can be explained by processes other than nitrogen fixation. These processes include a lack of significant denitrification in the basin and by particulate organic matter exported from surface waters during the Plimited winter plankton bloom