Limitation of oxygenic photosynthesis and oxygen consumption by phosphate and organic nitrogen in a hypersaline microbial mat: a microsensor study

Short-term efficiency of epibenthic microbial mat components on phosphorus sorption

Microbial mats may be an alternative tool for phosphorus (P) remediation of eutrophic coastal waters. The main objective of this work was to determine the importance that the living and non-living components of the mats have on P short-term sorption. Microbial mats were collected in the Paso Seco coastal flat, Argentina (40°38'3.32″S; 62°12'24.85″W), and incubated under controlled conditions in the lab. An adsorption curve was performed with the microbial mats. Active mats had a Freundlich constant 8.9-fold higher than underlying sandy sediments. Collected samples were then treated as follows: maintaining and disturbing their structural integrity (natural and autoclaved, respectively), and both conditions were incubated with filtered seawater, without and with phosphate addition (0 and 5 mg P L^-1, respectively). Natural mats had a significantly-higher phosphate removal percentage than autoclaved ones, suggesting that living microorganisms increase P short-term sorption efficiency by ~25%, while non-living matter may account for the rest.

The Effect Of microbial Mats In The Decay Of Anurans With Implications For Understanding Taphonomic Processes In The Fossil Record

The pattern and sequence of the decomposition of the Pipidae African dwarf frog (Hymenochirus boettgeri) is tracked in an experiment with microbial mats in order to explore soft tissue preservation over three years. Frog decay in microbial mats is preceded by rapid entombment (25–30 days) and mediated by the formation of a sarcophagus, which is built by a complex microbial community. The frog carcasses maintained a variety of soft tissues for years. Labile organic structures show greater durability within the mat, cells maintain their general shape (bone marrow cells and adipocytes), and muscles and connective tissues (adipose and fibrous tendons) exhibit their original organic structures. In addition, other soft tissues are promptly mineralized (day 540) in a Ca-rich carbonate phase (encephalic tectum) or enriched in sulphur residues (integumentary system). The result is coherent with a bias in soft-tissue preservation, as some tissues are more likely to be conserved than others. The outcomes support observations of exceptionally preserved fossil anurans (adults and tadpoles). Decomposition in mats shows singular conditions of pH and dissolved oxygen. Mineralization processes could be more diverse than in simple heterotrophic biofilms, opening new taphonomic processes that have yet to be explored.

Interaction between lithification and resource availability in the microbialites of Río Mesquites, Cuatro Ciénegas, México

Lithified microbial structures (microbialites) have been present on Earth for billions of years. Lithification may impose unique constraints on microbes. For instance, when CaCO3 forms, phosphate may be captured via coprecipitation and/or adsorption and potentially rendered unavailable for biological uptake. Therefore, the growth of microbes associated with CaCO3 may be phosphorus-limited. In this study, we compared the effects of resource addition on biogeochemical functions of microbial communities associated with microbialites and photoautotrophic microbial communities not associated with CaCO3 deposition in Río Mesquites, Cuatro Ciénegas, México. We also manipulated rates of CaCO3 deposition in microbialites to determine whether lithification reduces the bioavailability of phosphorus (P). We found that P additions significantly increased rates of gross primary production (F2,13 = 103.9, P < 0.001), net primary production (F2,13 = 129.6, P < 0.0001) and ecosystem respiration (F2,13 = 6.44, P < 0.05) in the microbialites, while P addition had no effect on photoautotrophic production in the non-CaCO3 -associated microbial communities. Growth of the non-CaCO3-associated phototrophs was only marginally stimulated when nitrogen and P were added simultaneously (F1,36 = 3.98, P = 0.053). In the microbialites, resource additions led to some shifts in the abundance of Proteobacteria, Bacteroidetes and Cyanobacteria but mostly had little effect on bacterial community composition. Ca(2+) uptake rates increased significantly with organic carbon additions (F1,13 = 8.02, P < 0.05). Lowering of CaCO3 deposition by decreasing calcium concentrations in the water led to increased microbial biomass accumulation rates in terms of both organic carbon (F4,48 = 5.23, P < 0.01) and P (F6,48 = 13.91, P < 0.001). These results provide strong evidence in support of a role of lithification in controlling P limitation of microbialite communities.

Nitrogen mineralization and geochemical characteristics of amino acids in surface sediments of a typical polluted area in the Haihe River Basin, China

Studies of nitrogen mineralization and diagenetic status of organic matter evaluated by total hydrolysable amino acids (THAAs) were designed to test the hypothesis that nitrogen mineralization in sediments was a potential source of ammonium in strongly artificially disturbed rivers such as the Ziya River watershed. Ammonium and organic nitrogen in both water and sediment samples were the major forms of nitrogen in the watershed. NH3-N was significantly correlated with organic nitrogen in both water (R = 0.823, P < 0.01) and sediments (R = 0.787, P < 0.01). Organic nitrogen with an average content of 3,275.21 ± 1,476.10 mg · kg(-1), accounted for 82.73 % of total nitrogen (TN) in sediments. Organic nitrogen was a potential source of ammonia release into overlying water. Nitrogen mineralization experiments showed that accumulated dissolved inorganic nitrogen ranged from 326.15 to 545.72 mg · kg(-1) and accumulated NH3-N ranged from 320.95 to 533.93 mg · kg(-1). Most of the mineralized nitrogen was NH3-N ( approximately 98.17%) and mineralized nitrogen in sediments ranged from 6.20 to 22.10% of TN. Twenty amino acids were detected, accounting for 45.70 % of organic nitrogen. Protein amino acids, accounting for 89.22% of THAAs, were the dominant THAAs in sediments. The ratio of L-glutamic acid to γ-aminobutyric acid and degradation index showed that the organic matter was poorly degraded and presented a high potential risk of ammonium mineralization.

Contrasted effects of natural complex mixtures of PAHs and metals on oxygen cycle in a microbial mat

The contamination of polluted environments is often due to a complex mixture of pollutants sometimes at trace levels which nevertheless may have significant effects on the diversity and functioning of organisms. The aim of this study was to assess the functional responses of a microbial mat exposed to a natural complex mixture of PAHs and metals as a function of the maturation stage of the biofilm. Microbial mats sampled in a slightly polluted environment were exposed to contaminated water of a retention basin of an oil refinery. The responses of the microbial mats differed according to season. In spring 2012, strong inhibition of both oxygen production and respiration was observed relative to the control, with rates representing less than 5% of the control after 72 h of incubation. A decrease of microbial activities was followed by a decrease of the coupling between autotrophs and heterotrophs. In contrast, in autumn 2012, no significant changes for oxygen production and respiration were observed and the coupling between autotrophs and heterotrophs was not altered. The differences observed between the spring and autumn mats might be explained by the maturity of the microbial mat with dominance of heterotrophic bacteria in spring, and diatoms and cyanobacteria in autumn, as well as by the differences in the chemical composition of the complex mixture of PAHs and metals.

Community structure and activity of a highly dynamic and nutrient-limited hypersaline microbial mat in Um Alhool Sabkha, Qatar

The Um Alhool area in Qatar is a dynamic evaporative ecosystem that receives seawater from below as it is surrounded by sand dunes. We investigated the chemical composition, the microbial activity and biodiversity of the four main layers (L1–L4) in the photosynthetic mats. Chlorophyll a (Chl a) concentration and distribution (measured by HPLC and hyperspectral imaging, respectively), the phycocyanin distribution (scanned with hyperspectral imaging), oxygenic photosynthesis (determined by microsensor), and the abundance of photosynthetic microorganisms (from 16S and 18S rRNA sequencing) decreased with depth in the euphotic layer (L1). Incident irradiance exponentially attenuated in the same zone reaching 1% at 1.7-mm depth. Proteobacteria dominated all layers of the mat (24%–42% of the identified bacteria). Anoxygenic photosynthetic bacteria (dominated by Chloroflexus) were most abundant in the third red layer of the mat (L3), evidenced by the spectral signature of Bacteriochlorophyll as well as by sequencing. The deep, black layer (L4) was dominated by sulfate reducing bacteria belonging to the Deltaproteobacteria, which were responsible for high sulfate reduction rates (measured using ³⁵S tracer). Members of Halobacteria were the dominant Archaea in all layers of the mat (92%–97%), whereas Nematodes were the main Eukaryotes (up to 87%). Primary productivity rates of Um Alhool mat were similar to those of other hypersaline microbial mats. However, sulfate reduction rates were relatively low, indicating that oxygenic respiration contributes more to organic material degradation than sulfate reduction, because of bioturbation. Although Um Alhool hypersaline mat is a nutrient-limited ecosystem, it is interestingly dynamic and phylogenetically highly diverse. All its components work in a highly efficient and synchronized way to compensate for the lack of nutrient supply provided during regular inundation periods.

Extracellular DNA metabolism in Haloferax volcanii

Extracellular DNA is found in all environments and is a dynamic component of the microbial ecosystem. Microbial cells produce and interact with extracellular DNA through many endogenous mechanisms. Extracellular DNA is processed and internalized for use as genetic information and as a major source of macronutrients, and plays several key roles within prokaryotic biofilms. Hypersaline sites contain some of the highest extracellular DNA concentrations measured in nature–a potential rich source of carbon, nitrogen, and phosphorus for halophilic microorganisms. We conducted DNA growth studies for the halophilic archaeon Haloferax volcanii DS2 and show that this model Halobacteriales strain is capable of using exogenous double-stranded DNA as a nutrient. Further experiments with varying medium composition, DNA concentration, and DNA types revealed that DNA is utilized primarily as a phosphorus source, that growth on DNA is concentration-dependent, and that DNA isolated from different sources is metabolized selectively, with a bias against highly divergent methylated DNA. Additionally, fluorescence microscopy showed that labeled DNA co-localized with H. volcanii cells. The gene Hvo_1477 was also identified using a comparative genomic approach as a factor likely to be involved in DNA processing at the cell surface, and deletion of Hvo_1477 created a strain deficient in the ability to grow on extracellular DNA. Widespread distribution of Hvo_1477 homologs in archaea suggests metabolism of extracellular DNA may be of broad ecological and physiological relevance in this domain of life.

Competition for inorganic carbon between oxygenic and anoxygenic phototrophs in a hypersaline microbial mat, Guerrero Negro, Mexico

While most oxygenic phototrophs harvest light only in the visible range (400-700 nm, VIS), anoxygenic phototrophs can harvest near infrared light (> 700 nm, NIR). To study interactions between the photosynthetic guilds we used microsensors to measure oxygen and gross oxygenic photosynthesis (gOP) in a hypersaline microbial mat under full (VIS + NIR) and VIS illumination. Under normal dissolved inorganic carbon (DIC) concentrations (2 mM), volumetric rates of gOP were reduced up to 65% and areal rates by 16-31% at full compared with VIS illumination. This effect was enhanced (reduction up to 100% in volumetric, 50% in areal rates of gOP) when DIC was lowered to 1 mM, but diminished at 10 mM DIC or lowered pH. In conclusion, under full-light illumination anoxygenic phototrophs are able to reduce the activity of oxygenic phototrophs by efficiently competing for inorganic carbon within the highly oxygenated layer. Anoxygenic photosynthesis, calculated from the difference in gOP under full and VIS illumination, represented between 10% and 40% of the C-fixation. The DIC depletion in the euphotic zone as well as the significant C-fixation by anoxygenic phototrophs in the oxic layer influences the carbon isotopic composition of the mat, which needs to be taken into account when interpreting isotopic biosignals in geological records.

Electron paramagnetic resonance study of a photosynthetic microbial mat and comparison with Archean cherts

Organic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts.

Light utilization efficiency in photosynthetic microbial mats

Based on combined microsensor measurements of irradiance, temperature and O(2) , we compared light energy budgets in photosynthetic microbial mats, with a special focus on the efficiency of light energy conservation by photosynthesis. The euphotic zones in the three studied mats differed in their phototrophic community structure, pigment concentrations and thickness. In all mats, < 1% of the absorbed light energy was conserved via photosynthesis at high incident irradiance, while the rest was dissipated as heat. Under light-limiting conditions, the photosynthetic efficiency reached a maximum, which varied among the studied mats between 4.5% and 16.2% and was significantly lower than the theoretical maximum of 27.7%. The maximum efficiency correlated linearly with the light attenuation coefficient and photopigment concentration in the euphotic zone. Higher photosynthetic efficiency was found in mats with a thinner and more densely populated euphotic zone. Microbial mats exhibit a lower photosynthetic efficiency compared with ecosystems with a more open canopy-like organization of photosynthetic elements, where light propagation is not hindered to the same extent by photosynthetically inactive components; such components contributed about 40-80% to light absorption in the investigated microbial mats, which is in a similar range as in oceanic planktonic systems.

Diversity and function of Chloroflexus-like bacteria in a hypersaline microbial mat: phylogenetic characterization and impact on aerobic respiration

We studied the diversity of Chloroflexus-like bacteria (CLB) in a hypersaline phototrophic microbial mat and assayed their near-infrared (NIR) light-dependent oxygen respiration rates. PCR with primers that were reported to specifically target the 16S rRNA gene from members of the phylum Chloroflexi resulted in the recovery of 49 sequences and 16 phylotypes (sequences of the same phylotype share more than 96% similarity), and 10 of the sequences (four phylotypes) appeared to be related to filamentous anoxygenic phototrophic members of the family Chloroflexaceae. Photopigment analysis revealed the presence of bacteriochlorophyll c (BChlc), BChld, and gamma-carotene, pigments known to be produced by phototrophic CLB. Oxygen microsensor measurements for intact mats revealed a NIR (710 to 770 nm) light-dependent decrease in aerobic respiration, a phenomenon that we also observed in an axenic culture of Chloroflexus aurantiacus. The metabolic ability of phototrophic CLB to switch from anoxygenic photosynthesis under NIR illumination to aerobic respiration under non-NIR illumination was further used to estimate the contribution of these organisms to mat community respiration. Steady-state oxygen profiles under dark conditions and in the presence of visible (VIS) light (400 to 700 nm), NIR light (710 to 770 nm), and VIS light plus NIR light were compared. NIR light illumination led to a substantial increase in the oxygen concentration in the mat. The observed impact on oxygen dynamics shows that CLB play a significant role in the cycling of carbon in this hypersaline microbial mat ecosystem. This study further demonstrates that the method applied, a combination of microsensor techniques and VIS and NIR illumination, allows rapid establishment of the presence and significance of CLB in environmental samples.