黄河滩地和稻田土中铁还原菌、不产氧光合细菌分布机制

【目的】探讨沿黄流域土壤中铁还原菌(ferric reducing bacteria, Fe RB)、不产氧光合细菌(anoxygenic phototrophic bacteria, An PB)的分布机制。【方法】以沿黄流域(原阳段)为研究对象,采集黄河滩地和稻田土样,利用16Sr RNA基因高通量测序和实时荧光定量分析技术,结合统计学分析,揭示Fe RB、An PB菌群结构、丰度和主要环境影响因子。【结果】二者中的优势Fe RB在科(属)水平为Hydrogenophilaceae(Thiobacillus)、 Bacillaceae(Bacillus)、 Clostridiaceae、Rhodobactereace(Rhodobacter)、 Geobacteraceae(Geobacter),优势An PB为Rhodobactereace(Rhodobacter)、 Chloroflexaceae(Chloronema)、 Acetobacteraceae(Roseomonas)。An PB中Rhodobacteraceae与Fe RB中Bacillaceae、 Clostri...     

Dynamics of aerobic anoxygenic phototrophic bacteria in the East China Sea

Aerobic anoxygenic phototrophic bacteria (AAPB) are a group of heterotrophic bacteria capable of photosynthesis. The dynamics of AAPB in the East China Sea, a typical marginal sea characterized by diverse physical-chemical and ecological conditions, were investigated from April 2002 to September 2003. The results showed that the abundance of AAPB varied from 0.16 to 7.9 x 10(4) cells mL(-1) and the percentage of AAPB (AAPB%) in the total heterotrophic bacterial abundance varied from 0.5% to 11.6% over a gradient of environmental conditions. The abundance of AAPB and AAPB% was higher in coastal and continental shelf waters than in oceanic waters. An interesting seasonal pattern was observed in the Yangtze River estuary: the abundance of AAPB was highest in summer and lowest in winter; however, AAPB% was higher in winter than in the other seasons. Throughout the investigation period, variation of AAPB abundance with temperature was much less than that of nonAAPB abundance, suggesting that low temperature was not a limiting factor for AAPB in this case. Close correlation between AAPB and chlorophyll a was observed in each season, suggesting that dependence of AAPB on dissolved organic carbon produced by phytoplankton (PDOC) may be one key factor controlling AAPB distribution.     

Diverse arrangement of photosynthetic gene clusters in aerobic anoxygenic phototrophic bacteria

Background

Aerobic anoxygenic photototrophic (AAP) bacteria represent an important group of marine microorganisms inhabiting the euphotic zone of the ocean. They harvest light using bacteriochlorophyll (BChl) a and are thought to be important players in carbon cycling in the ocean.

Methodology/Principal Findings

Aerobic anoxygenic phototrophic (AAP) bacteria represent an important part of marine microbial communities. Their photosynthetic apparatus is encoded by a number of genes organized in a so-called photosynthetic gene cluster (PGC). In this study, the organization of PGCs was analyzed in ten AAP species belonging to the orders Rhodobacterales, Sphingomonadales and the NOR5/OM60 clade. Sphingomonadales contained comparatively smaller PGCs with an approximately size of 39 kb whereas the average size of PGCs in Rhodobacterales and NOR5/OM60 clade was about 45 kb. The distribution of four arrangements, based on the permutation and combination of the two conserved regions bchFNBHLM-LhaA-puhABC and crtF-bchCXYZ, does not correspond to the phylogenetic affiliation of individual AAP bacterial species. While PGCs of all analyzed species contained the same set of genes for bacteriochlorophyll synthesis and assembly of photosynthetic centers, they differed largely in the carotenoid biosynthetic genes. Spheroidenone, spirilloxanthin, and zeaxanthin biosynthetic pathways were found in each clade respectively. All of the carotenoid biosynthetic genes were found in the PGCs of Rhodobacterales, however Sphingomonadales and NOR5/OM60 strains contained some of the carotenoid biosynthetic pathway genes outside of the PGC.

Conclusions/Significance

Our investigations shed light on the evolution and functional implications in PGCs of marine aerobic anoxygenic phototrophs, and support the notion that AAP are a heterogenous physiological group phylogenetically scattered among Proteobacteria.     

Leucine incorporation by aerobic anoxygenic phototrophic bacteria in the Delaware estuary

Aerobic anoxygenic phototrophic (AAP) bacteria are well known to be abundant in estuaries, coastal regions and in the open ocean, but little is known about their activity in any aquatic ecosystem. To explore the activity of AAP bacteria in the Delaware estuary and coastal waters, single-cell ³H-leucine incorporation by these bacteria was examined with a new approach that combines infrared epifluorescence microscopy and microautoradiography. The approach was used on samples from the Delaware coast from August through December and on transects through the Delaware estuary in August and November 2011. The percent of active AAP bacteria was up to twofold higher than the percentage of active cells in the rest of the bacterial community in the estuary. Likewise, the silver grain area around active AAP bacteria in microautoradiography preparations was larger than the area around cells in the rest of the bacterial community, indicating higher rates of leucine consumption by AAP bacteria. The cell size of AAP bacteria was 50% bigger than the size of other bacteria, about the same difference on average as measured for activity. The abundance of AAP bacteria was negatively correlated and their activity positively correlated with light availability in the water column, although light did not affect ³H-leucine incorporation in light–dark experiments. Our results suggest that AAP bacteria are bigger and more active than other bacteria, and likely contribute more to organic carbon fluxes than indicated by their abundance.     

浑善达克沙地生物土壤结皮及其下层土壤中好氧不产氧光营养细菌群落结构及多样性

【目的】揭示浑善达克沙地不同类型生物土壤结皮(Biological soil crusts,BSCs)及其下层土壤好氧不产氧光营养细菌(Aerobic anoxygenic phototrophic bacteria,AAPB)群落结构及多样性。【方法】利用Illumina Mi Seq二代高通量测序平台对puf M基因进行测序,使用生物信息学分析方法对序列进行比对分析AAPB的群落结构和多样性。【结果】生物土壤结皮及其下层土壤中,Proteobacteria和Alpha-Proteobacteria是优势门和纲,主要有6个属Bradyrhizobium(9.69%–90.02%)、Brevundimonas(0.83%–16.04%)、Methylobacterium(1.74%–12.56%)、Rhodospirillum(0.91%–32.87%)、Roseiflexus(0.02%–1.79%)和Sphingomonas(0.13%–11.23%);结皮层样品间及下层土壤样品间AAPB种类相似,但丰度有差异;随结皮的发育,结皮层及其下层土壤中AAPB群落多样性升高。【结论】浑善达克沙地BSCs中AAPB群落结构相对复杂,与水体和一般土壤环境中的组成区别明显;AAPB多样性高,且多样性随发育阶段升高而升高,预示着AAPB在荒漠生态系统稳定中有重要的作用。     

Responses of aerobic anoxygenic phototrophic bacteria to algal blooms in the East China Sea

Aerobic anoxygenic phototrophic bacteria (AAPB) are a new functional group of heterotrophic bacteria capable of phototrophy, and are suggested to be closely related with phytoplankton. However, less known is the relationship between AAPB and dominant phytoplankton populations. In this tudy, the responses of AAPB to algal blooms (ABs) in the AB-frequent-occurrence area of the East China Sea were investigated during four cruises from March to June 2005, using an advanced ‘Time-series observation-based cyanobacteria-calibrated InfraRed Epifluorescence Microscopy (TIREM)’ approach. Generally, total bacterial abundances at the bloom stations were higher than or similar to those at the non-bloom stations during the same time period. Interestingly, the responses of AAPB to ABs seemed to be more diverse and complex. AAPB abundance was higher at the stations with ABs where Thalassiosira curviseriata Takano and Skeletonema costatum (Greville) Cleve, Noctiluca scintillans (Macartney) Kofoid et Swezy, or Prorocentrum donghaiense Lu and Karenia mikimotoi Hansen co-dominated than those at the non-bloom stations during the same time period. However at the stations with a bloom of Akashiwo sanguinea Hansen, AAPB abundance only accounted for ~20% of the average abundance of AAPB at the non-bloom stations. In addition, variations of AAPB’s proportion to total bacterial abundance (AAPB%) in response to ABs basically followed AAPB abundance. Overall, our results suggest that the responses of AAPB to ABs are AB-species specific and somewhat independent of chlorophyll a concentration.     

Spatial and temporal distribution of aerobic anoxygenic phototrophic bacteria: key functional groups in biological soil crusts

Several significant ecosystem services are performed by biological soil crusts (BSCs) in drylands, wherein photoautotrophic microorganisms are commonly critical contributors. However, aerobic anoxygenic phototrophic bacteria (AAnPB) are rarely reported in BSCs, despite being the second major branch of Earth's phototrophic microbes. Here, we collected different types of BSCs and their subsoils from temperate deserts, investigated distributions of AAnPB communities among BSCs using cultivation and high-throughput sequencing approaches, predicted keystone species by co-occurrence network analysis, and verified their effects on BSCs formation through microcosm experiments. The absolute abundances and diversity of AAnPB were higher in BSCs and were closely related with BSCs successional stages, as well as soil organic carbon contents. AAnPB communities in both BSCs and their subsoils were dominated by Proteobacteria and Alphaproteobacteria, specifically Acetobacteraceae, Rhodospirillaceae, Roseiflexaceae, Sphingomonadaceae and Caulobacteraceae families. Mean annual precipitation, pH and available nutrients were the primary factors that shaped AAnPB community structures. The predicted keystone species belonged to the families Acetobacteraceae, Rhodospirillaceae and Sphingomonadanceae. Microcosm experiments demonstrated that inoculation with strains from the three families greatly accelerated the formation and development of BSCs. These observations suggest that AAnPB are likely important functional groups in BSCs that significantly contribute to their formation and important ecosystem services.     

Effects of elevated atmospheric CO2 on paddy soil nitrogen content during rice season

Taking the China rice/wheat FACE (Free-Air Carbon Dioxide Enrichment) as a platform, this paper studied the effects of elevated CO2 on the NH4(+)-N and NO3(-)-N contents at different depths of paddy soil in rice season. Under elevated CO2, the NH4(+)-N content in plough layer increased at early growth stage but decreased at late growth stage, and the soil NO3(-)-N content at the depths 5, 15, 30, 60, and 90 cm increased by 46.5%, 36.8%, 23.3%, 103.7%, and 42.7%, respectively, with a significant increase occurred at the depths 60 cm (P < 0.01) and 90 cm (P < 0.05), compared with the control.     

Formation of polyhydroxyalkanoate in aerobic anoxygenic phototrophic bacteria and its relationship to carbon source and light availability

Aerobic anoxygenic phototrophic bacteria (AAPB) are unique players in carbon cycling in the ocean. Cellular carbon storage is an important mechanism regulating the nutrition status of AAPB but is not yet well understood. In this paper, six AAPB species (Dinoroseobacter sp. JL1447, Roseobacter denitrificans OCh 114, Roseobacter litoralis OCh 149, Dinoroseobacter shibae DFL 12(T), Labrenzia alexandrii DFL 11(T), and Erythrobacter longus DSMZ 6997) were examined, and all of them demonstrated the ability to form the carbon polymer polyhydroxyalkanoate (PHA) in the cell. The PHA in Dinoroseobacter sp. JL1447 was identified as poly-beta-hydroxybutyrate (PHB) according to evidence from Fourier transform infrared spectroscopy, differential scanning calorimetry, and (1)H nuclear magnetic resonance spectroscopy examinations. Carbon sources turned out to be critical for PHA production in AAPB. Among the eight media tested with Dinoroseobacter sp. JL1447, sodium acetate, giving a PHA production rate of 72%, was the most productive carbon source, followed by glucose, with a 68% PHA production rate. Such PHA production rates are among the highest recorded for all bacteria. The C/N ratio of substrates was verified by the experiments as another key factor in PHA production. In the case of R. denitrificans OCh 114, PHA was not detected when the organism was cultured at C/N ratios of <2 but became apparent at C/N ratios of >3. Light is also important for the formation of PHA in AAPB. In the case of Dinoroseobacter sp. JL1447, up to a one-quarter increase in PHB production was observed when the culture underwent growth in a light-dark cycle compared to growth completely in the dark.     

大气C02浓度升高对稻田土壤溶液中阴离子浓度的影响

利用中国稻/麦轮作FACE(Free-Air Carbon Dioxide Enrichment)试验平台,研究了 大气CO2浓度升高对稻季各生育期耕层土壤溶液中Cl-、CO2-3、HCO-3、SO2-4、NO-3和溶解无机磷(DIP)等阴离子浓度的影响,探讨了稻田生态系统土壤元素地球化学循环对大气CO2浓度升高的响应.结果表明:大气CO2浓度升高降低了土壤溶液SO2-4浓度,提高了HCO-3浓度,与5 cm处相比,其在15 cm处的影响程度更大;大气CO2浓度升高有增加土壤溶液NO-3和溶解无机磷(DIP)的趋势;对CO2-3和Cl-未表现出明显的规律性影响.文章还分析了大气CO2浓度升高对土壤溶液阴离子浓度产生影响的可能机理.     

Abundance and diversity of aerobic anoxygenic phototrophic bacteria in saline lakes on the Tibetan plateau

Aerobic anoxygenic phototrophic (AAP) bacteria are heterotrophic prokaryotes that are capable of utilizing light as an energy source but are not capable of producing molecular oxygen. Recently, multiple studies have found that AAP bacteria are widely distributed in oceans and estuaries and may play an important role in carbon cycling. However, AAP bacteria in inland lake ecosystems have not been investigated in depth. In this study, the abundance and diversity of the pufL-M genes, encoding photosynthetic reaction centers of AAP bacteria, were determined in the oxic water column and anoxic sediments of saline lakes (Qinghai, Erhai, and Gahai Lakes) on the Tibetan Plateau, China. Our results indicated that AAP bacteria were abundant in inland lakes, with the proportion of AAP bacteria (in total bacteria) comparable to those in the oceans, but with a lower diversity. Salinity and pH were found to be potential factors controlling the AAP bacterial diversity and community composition. Our data have implications for a better understanding of the potential role of AAP bacteria in carbon cycling in inland lake ecosystems.     

Distinct distribution pattern of abundance and diversity of aerobic anoxygenic phototrophic bacteria in the global ocean

Aerobic anoxygenic phototrophic bacteria (AAPB) are an important bacterial group with capability of harvesting light energy, and appear to have a particular role in the ocean's carbon cycling. Yet the significance of AAPB relative to total bacteria (AAPB%) in different marine regimes are still controversial, and variation trend of genetic diversity of AAPB along environmental gradients remains unclear. Here we present the first comprehensive observation of the global distribution of AAPB in the Pacific, Atlantic and Indian oceans, revealing a general pattern of high abundance of AAPB and AAPB% in coastal waters than oceanic waters. The Indian Ocean contained relatively high AAPB% compared with the other two oceans, corresponding to the high primary production in this region. Both abundance of AAPB and AAPB% were positively correlated with the concentration of chlorophyll a, while the diversity of AAPB decreased with increasing chlorophyll a values. Our results suggest that AAPB abundance and diversity follow opposite trends from oligotrophic to eutrophic regimes in the ocean.     

Summer distribution and diversity of aerobic anoxygenic phototrophic bacteria in the Mediterranean Sea in relation to environmental variables

Aerobic anoxygenic phototrophic bacteria (AAP) represent an important fraction of bacterioplankton assemblages in various oceanic regimes. Although their abundance and distribution have been explored recently in diverse oceanic regions, the environmental factors controlling the population structure and diversity of these photoheterotrophic bacteria remain poorly understood. Here, we investigate the horizontal and vertical distributions and the genetic diversity of AAP populations collected in late summer throughout the Mediterranean Sea using pufM-temporal temperature gel gradient electrophoresis (TTGE) and clone library analyses. The TTGE profiles and clone libraries analyzed using multivariate statistical methods demonstrated a horizontal and vertical zonation of AAP assemblages. Physicochemical parameters such as pH, inorganic nitrogen compounds, photosynthetically active radiation, total organic carbon and to a lesser extent particulate organic nitrogen and phosphorus, and biogenic activities (e.g. bacterial production, cell densities), acted in synergy to explain the population changes with depth. About half of the pufM sequences were <94% identical to known sequences. The AAP populations were predominantly (~80%) composed of Gammaproteobacteria, unlike most previously explored marine systems. Our results suggest that genetically distinct ecotypes inhabiting different niches may exist in natural AAP populations of the Mediterranean Sea whose genetic diversity is typical of oligotrophic environments.     

Above- and belowground response of Populus grandidentata to elevated atmospheric CO2 and soil N availability

Soil N availability may play an important role in regulating the long-term responses of plants to rising atmospheric CO₂ partial pressure. To further examine the linkage between above- and belowground C and N cycles at elevated CO₂, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO₂ in open bottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralization in local dry oak forests. Nitrogen added during August was enriched with ¹⁵N to trace the flux of N within the plant-soil system. Above-and belowground growth, CO₂ assimilation, and leaf N content were measured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and ¹⁵N. There was no CO₂ treatment effect on leaf area, root length, or net assimilation prior to the completion of N addition. Following the N addition, leaf N content increased in both CO₂ treatments, but net assimilation showed a sustained increase only in elevated CO₂ grown plants. Root relative extension rate was greater at elevated CO₂, both before and after the N addition. Although final root biomass was greater at elevated CO₂, there was no CO₂ effect on plant N uptake or allocation. While low soil N availability severely inhibited CO₂ responses, high CO₂ grown plants were more responsive to N. This differential behavior must be considered in light of the temporal and spatial heterogeneity of soil resources, particularly N which often limits plant growth in temperate forests.     

Comparison of growth rates of aerobic anoxygenic phototrophic bacteria and other bacterioplankton groups in coastal Mediterranean waters

Growth is one of the basic attributes of any living organism. Surprisingly, the growth rates of marine bacterioplankton are only poorly known. Current data suggest that marine bacteria grow relatively slowly, having generation times of several days. However, some bacterial groups, such as the aerobic anoxygenic phototrophic (AAP) bacteria, have been shown to grow much faster. Two manipulation experiments, in which grazing, viruses, and resource competition were reduced, were conducted in the coastal Mediterranean Sea (Blanes Bay Microbial Observatory). The growth rates of AAP bacteria and of several important phylogenetic groups (the Bacteroidetes, the alphaproteobacterial groups Roseobacter and SAR11, and the Gammaproteobacteria group and its subgroups the Alteromonadaceae and the NOR5/OM60 clade) were calculated from changes in cell numbers in the manipulation treatments. In addition, we examined the role that top-down (mortality due to grazers and viruses) and bottom-up (resource availability) factors play in determining the growth rates of these groups. Manipulations resulted in an increase of the growth rates of all groups studied, but its extent differed largely among the individual treatments and among the different groups. Interestingly, higher growth rates were found for the AAP bacteria (up to 3.71 day⁻¹) and for the Alteromonadaceae (up to 5.44 day⁻¹), in spite of the fact that these bacterial groups represented only a very low percentage of the total prokaryotic community. In contrast, the SAR11 clade, which was the most abundant group, was the slower grower in all treatments. Our results show that, in general, the least abundant groups exhibited the highest rates, whereas the most abundant groups were those growing more slowly, indicating that some minor groups, such the AAP bacteria, very likely contribute much more to the recycling of organic matter in the ocean than what their abundances alone would predict.     

Abundance and genetic diversity of aerobic anoxygenic phototrophic bacteria of coastal regions of the pacific ocean

Aerobic anoxygenic phototrophic (AAP) bacteria are photoheterotrophic microbes that are found in a broad range of aquatic environments. Although potentially significant to the microbial ecology and biogeochemistry of marine ecosystems, their abundance and genetic diversity and the environmental variables that regulate these properties are poorly understood. Using samples along nearshore/offshore transects from five disparate islands in the Pacific Ocean (Oahu, Molokai, Futuna, Aniwa, and Lord Howe) and off California, we show that AAP bacteria, as quantified by the pufM gene biomarker, are most abundant near shore and in areas with high chlorophyll or Synechococcus abundance. These AAP bacterial populations are genetically diverse, with most members belonging to the alpha- or gammaproteobacterial groups and with subclades that are associated with specific environmental variables. The genetic diversity of AAP bacteria is structured along the nearshore/offshore transects in relation to environmental variables, and uncultured pufM gene libraries suggest that nearshore communities are distinct from those offshore. AAP bacterial communities are also genetically distinct between islands, such that the stations that are most distantly separated are the most genetically distinct. Together, these results demonstrate that environmental variables regulate both the abundance and diversity of AAP bacteria but that endemism may also be a contributing factor in structuring these communities.     

大气CO2浓度升高对稻季土壤中麦秸降解及氮素分趋的影响

利用中国唯一的稻麦轮作FACE(free-air carbon dioxide enrichment,开放式空气CO2浓度增高)试验平台,研究大气CO2浓度升高对稻季土壤中小麦秸秆降解速率及其氮素分趋的影响.试验设置Ambient(目前空气对照)和FACE(Ambient+200 μmol·mol-1)两个CO2浓度以及低氮处理(LN,150 kg·hm-2)和高氮处理(HN,250 kg·hm-2)两个氮肥水平,在稻季之初按标记麦秸/土壤重量比0.3%添加15N标记小麦秸秆,根据水稻生长时期依次采样测定秸秆降解速率,并通过分析土壤全氮、植株全氮及其15N丰度来观察已降解秸秆的氮素分趋情况.结果发现,大气CO2浓度升高对高氮处理土壤中小麦秸秆降解速率没有显著影响,但显著促进了低氮处理土壤中小麦秸秆的降解(p < 0.05),使其提高到与高氮处理土壤相当水平;大气CO2浓度升高显著增加了已降解秸秆中氮素的流失,在高氮处理土壤中尤为严重,而对植物吸收已降解秸秆中的氮素没有显著影响.结果表明,大气CO2浓度升高在土壤氮素相对不足时会加速土壤中小麦秸秆的降解,而在土壤氮素相对充足时又会加大降解秸秆中氮素的流失.     

Response of soil biota to elevated atmospheric CO2 in poplar model systems

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO₂ would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO₂ in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO₂. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO₂. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO₂. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO₂ treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO₂, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO₂ was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose. Received: 18 March 1997 / Accepted: 5 August 1997     

Effects of elevated atmospheric CO2 concentrations on soil microorganisms

Effects of elevated CO(2) on soil microorganisms are known to be mediated by various interactions with plants, for which such effects are relatively poorly documented. In this review, we summarize and synthesize results from studies assessing impacts of elevated CO(2) on soil ecosystems, focusing primarily on plants and a variety the of microbial processes. The processes considered include changes in microbial biomass of C and N, microbial number, respiration rates, organic matter decomposition, soil enzyme activities, microbial community composition, and functional groups of bacteria mediating trace gas emission such as methane and nitrous oxide. Elevated CO(2) in atmosphere may enhance certain microbial processes such as CH(4) emission from wetlands due to enhanced carbon supply from plants. However, responses of extracellular enzyme activities and microbial community structure are still controversy, because interferences with other factors such as the types of plants, nutrient availabilitial in soil, soil types, analysis methods, and types of CO(2) fumigation systems are not fully understood.     

Competition between anoxygenic phototrophic bacteria and colorless sulfur bacteria in a microbial mat

Abstract The populations of chemolithoautotrophic (colorless) sulfur bacteria and anoxygenic phototrophic bacteria were enumerated in a marine microbial mat. The highest population densities were found in the 0–5 mm layer of the mat: 2.0 × 10⁹ cells cm⁻³ sediment, and 4.0 × 10⁷ cells cm⁻³ sediment for the colorless sulfur bacteria and phototrophs, respectively. Kinetic parameters for thiosulfate-limited growth were assessed for Thiobacillus thioparus T5 and Thiocapsa roseopersicina M1, both isolated from microbial mats. For Thiobacillus T5, growing at a constant oxygen concentration of 43 μmol l⁻¹, μ_max was 0.336 h⁻¹ and K _s 0.8 μmol l⁻¹. Phototrophically grown Thiocapsa strain M1 displayed a μ_max of 0.080 h⁻¹ and a K _s of 8 μmol l⁻¹ when anoxically grown under thiosulfate limitation. In a competition experiment with thiosulfate as electron donor, Thiocapsa became dominant during a 10-h oxic/14-h anoxic regimen at continuous illumination, despite the higher affinity for thiosulfate of Thiobacillus .