The profiles of O,, H,S, and pH within a microbial mat of the hypcrsaline pond Solar Lake, Sinai, were measured by 2-208pm-thick microelectrodes during diurnal and artificial light cycles. The oxygen concentration in the photic layer varied from a maximum of 1,400 PM during the day to 0 during the night. The pH in the same layer varied between 9.6 in the early afternoon and 7.7 in the early morning. Sulfide was not present in the photic zone during the day, but built up to about 50 /IM during the night. The diffusion gradients of sulfide and oxygen were very steep and the two compounds coexisted in a layer only 0.25 mm thick during the day. Diffusion flux calculations showed that the average turnover time of sulfide within this layer was 21 s. The rapid turnover indicated that the oxidation of sulfide must be biologically mediated. Oxygenic photosynthesis was measured by a new oxygen microprofile method which accurately determines the vertical distribution of photosynthetic activity. There was no difference in the efficiency of photosynthesis between morning and afternoon. The photosynthetic efficiency of the whole mat was about fourfold higher at low light intensities, ~120 PEinst. m-z.s-1, than at high light intensities, 120-1,600 pEinst*m-"es-'. Anoxygenic photosynthesis within the mat was not quantitatively important.

Variations in sediment N:C ratios were correlated with water depth and season. 15NH,+ was used to measure the rates of NH,+ production (d) and incorporation into bacterial cells (i) in sediments from different stations, at different seasons. The validity of the rates d and i was indicated by the predicted correlation of cl:i ratios with N:C ratios of the sediment, and the predicted N:C ratio at which net NH4+ uptake occurred. There was also a correlation between rate cl and product (total NH,+). In the O-2-cm stratum correlations were also established between d, exchangeable NH4+ pool, ratio exchangeable NH?+ : porewater NH4+, flux of NH,+ from sediment, and flux of NH4+ into exchangeable pool. The NO:,-flux from sediment was correlated with nitrification rate and with season. Benthic infauna increased the flux of NHI' from the sediment by 50%. The rates of transfer of nitrogen (NO,-, NH?+, N,) from sediment to water were 44-66% of the net rates of organic nitrogen mineralization (d -i). Flux of NO,-+ NII,+ from the sediment could supply 30-82% of the nitrogen requirement of the planktonic primary producers.Available nitrogen occurs in sediment in the following major pools: organic N, porewater ammonium (NH,+pw), exchangeable ammonium ( NH4+ex), dissolved nitrate, and nitrogen gas. Our objective here was to measure these pools and the rates that connect them to each other and to the overlying water.The rate of organic-N mineralization, equivalent to the rate of NH,+ production (d), and the rate of NH,+ incorporation into cells (i), was measured by lsNH4+ dilution. Factors affecting the sediment organic N:C ratio, and the effect of the N: C ratio on the ratio d:i were investigated.The NH,+ that was not incorporated into cells (net ammonium production, di) had three possible fates. Some passed from the sediment to the overlying water, some was oxidized to nitrate, and the rest entered the sediment NH4+ pool. We examined the factors, mainly the ratio of NH4+ex : NH,+pw in the 0-2-cm stratum, which regulated the fate of NH,+. We also examined the factors, mainly seasonal changes, which regulated the loss of NO,-

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.Abstract Membrane-covered platinum electrodes with a tip diameter of 2-8 pm were used for an amperometric assay of dissolved oxygen in marine sediments. The oxygen profile extended to 3-5mm depth in nonilluminated sediment; even at high light intensities and at low temperatures it did not extend below lo-mm depth in a homogeneous sandy sediment. Oxygen profiles recorded during light-dark cycles were used to estimate the rates of oxygen production and consumption and also to calculate the apparent diffusion coefficient for oxygen in the sediment. Apparently macrofaunal activity, rather than molecular diffusion and water turbulence, was important for the occasional transport of oxygen into deeper layers and thus for the provision of oxidized conditions (positive redox potential) down to 5-10 cm below the sediment surface.

The decay rate of particulate organic carbon (PaC) and nitrogen (PON) was followed during 94 days in three homogenized sediment microcosms: I. With a natural density of the polychaete Nereis virens (NOx-cores); 2. Defaunated, with an aerobic water phase (Ox-cores); and 3. Defaunated, with an anaerobic water phase (An-cores). In all cores there was a marked preferential mineralization of paN compared to pac. The presence of Nereis increased the net decomposition of pac and paN 2.6 and 1.6 times relative to Ox-cores. Ventilation of burrow structures by the worms increased the flux of O2, TC02 and DIN across the sediment-water interface 2.5-3.5 times. This significantly decreased the pore water concentrations of TC02 and DIN. Similarly, nitrification and denitrification were stimulated 2.3-2.4 times due to nereid activity. Oxygen did not increase organic degradation: in fact, the decay of pac and PON was faster in An-than in Ox-cores, 1.5-1.6 and 1.2 times, respectively. Sulfate reduction, measured at the end of experiment, was surprisingly low in the aerobic NOx-and Ox-cores relative to An-cores. Net ammonium production measured at the end of the experiment agreed with the mean loss of paN for Ox-and An-cores, but was low for NOx-cores, suggesting that a high C:N substrate was being degraded in these cores at the end. An empirical model describing the temporal decay pattern of pac and PON is presented: the detritus in all cores were initially composed of two fractions (similar C:N); a readily degradable (-43%) and a low degradable (-57%) fraction. A substantial part ofthe degradable fraction in NOx-cores was used during the experiment, with nitrogen being mineralized preferentially. The mean C:N molar ratio of detritus used was 5.9, compared to a value of 15.5 determined at the end. The Ox-and An-cores, however, showed similar C:N ratios for the detritus used during the experiment (3.7 and 4.8) and that measured at the end (4.2 and 4.6). Presumably not all the low C:N detritus had yet been mineralized in these cores at the end of experiment.

The depth of penetration of oxygen into coastal marine sediments (water depth, 4 to 44 meters; 6° to 10°C) varied from 1 to 5.5 millimeters, as measured with membrane-covered oxygen microelectrodes. Below these upper few millimeters, oxygen was present only in the immediate vicinity of animal burrows. The depth of oxygen penetration is related to the rate at which oxygen is consumed in the sediment.

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The diurnal variation in the microgradients of 02, H2S, and Eh were studied in the benthic cyanobacterial mats of a hypersaline desert lake (Solar Lake, Sinai). The results were related to light intensity, light penetration into the mat, temperature, pH, NH4', photosynthetic activity, pigments, and the zonation of the microbial community. Extreme diurnal variation was found, with an 02 peak of 0.5 mM at 1 to 2 mm of depth below the mat surface during day and a H2S peak of 2.5 mM at 2 to 3 mm of depth at night. At the 02-H2S interface, the two compounds coexisted over a depth interval of 0.2 to 1 mm and with a turnover time of a few minutes. The photic zone reached 2.5 mm into the mat in summer, and the main 14CO2 light fixation took place at 1 to 2 mm of depth. During winter, light and photosynthesis were restricted to the uppermost 1 mm. The quantitative dynamics of 02 and H2S were calculated from the chemical gradients and from the measured diffusion coefficients.