Microoxic-Anoxic Niche of Beggiatoa spp.: Microelectrode Survey of Marine and Freshwater Strains

Beggiatoa spp. grow optimally in media containing opposed gradients of oxygen and soluble sulfide, although some strains also require an organic substrate. By using microelectrodes, we characterized oxygen and sulfide gradients during their initial development in uninoculated media and in cultures of marine and freshwater strains. In gradient media, Beggiatoa strains always grew some distance below the air/agar interface as a dense “plate” of constantly gliding filaments with sharply demarcated upper and lower boundaries. Within established plates, the maximum oxygen partial pressure was 0.6 to 6.0% of air saturation and not significantly lower if filaments were fixing nitrogen. Oxygen penetrated only 100 to 300 μm into the plate, and the anoxic fraction increased from less than 10% to approximately 90% during later stages of growth. For lithoautotrophically grown marine strains, the linearity of the oxygen profile above the plate plus its drop to zero therein indicated that oxygen uptake for the entire tube occurred only within the Beggiatoa plate. Consequently, oxygen consumption could be predicted solely from the distance between the air/agar interface and the top of a plate, given the diffusion coefficient for oxygen. By contrast, for freshwater strains grown heterotrophically (with sulfide also in the medium), oxygen profiles were frequently nonlinear because of nonbiological reaction with sulfide which had diffused past the aggregated filaments. For all strains tested, microoxic aggregation also occurred in the absence of sulfide, apparently reflecting a step-up phobic response to oxygen.     

Organic Carbon Utilization by Obligately and Facultatively Autotrophic Beggiatoa Strains in Homogeneous and Gradient Cultures

Marine Beggiatoa strains MS-81-6 and MS-81-1c are filamentous gliding bacteria that use hydrogen sulfide and thiosulfate as electron donors for chemolithotrophic energy generation. They are known to be capable of chemolithoautotrophic growth in sulfide gradient media; here we report the first successful bulk cultivation of these strains in a defined liquid medium. To investigate their nutritional versatilities, strains MS-81-6 and MS-81-1c were grown in sulfide-oxygen gradient media supplemented with single organic compounds. Respiration rates and biomass production relative to those of controls grown in unsupplemented sulfide-limited media were monitored to determine whether organic compounds were utilized as sources of energy and/or cell carbon. With cells grown in sulfide gradient and liquid media, we showed that strain MS-81-6 strongly regulates two enzymes, the tricarboxylic acid cycle enzyme 2-oxoglutarate dehydrogenase and the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, in response to the presence of organic carbon (acetate) in the growth medium. In contrast, strain MS-81-1c lacked 2-oxoglutarate dehydrogenase activity and regulated ribulose-1,5-bisphosphate carboxylase/oxygenase activity only slightly in response to organic substrates. Tracer experiments with radiolabeled acetate showed that strain MS-81-1c did not oxidize acetate to CO(inf2) but could synthesize approximately 20% of its cell carbon from acetate. On the basis of these results, we conclude that Beggiatoa strain MS-81-1c is an obligate chemolithoautotroph, while strain MS-81-6 is a versatile facultative chemolithoautotroph.     

Chemotaxis toward Nitrogenous Compounds by Swimming Strains of Marine Synechococcus spp.

Many of the open-ocean isolates of the marine unicellular cyanobacterium Synechococcus spp. are capable of swimming motility, whereas coastal isolates are nonmotile. Surprisingly, the motile strains do not display phototactic or photophobic responses to light, but they do demonstrate positive chemoresponses to several nitrogenous compounds. The chemotactic responses of Synechococcus strain WH8113 were investigated using blind-well chemotaxis chambers fitted with 3.0-μm-pore-size Nuclepore filters. One well of each chamber contained cells suspended in aged Sargasso Sea water, and the other well contained the potential chemoattractant in seawater. The number of cells that crossed the filter into the attractant-seawater mixture was measured by direct cell counts and compared with values obtained in chambers lacking gradients. Twenty-two compounds were tested, including sugars, amino acids, and simple nitrogenous substrates, at concentrations ranging from 10⁻⁵ to 10⁻¹⁰ M. Strain WH8113 responded positively only to ammonia, nitrate, β-alanine, glycine, and urea. Typically, there was a 1.5- to 2-fold increase in cell concentrations above control levels in chambers containing these compounds, which is comparable to results from similar experiments using enteric and photoheterotrophic bacteria. However, the threshold levels of 10⁻⁹ to 10⁻¹⁰ M found for Synechococcus spp. chemoresponses were lower by several orders of magnitude than those reported for other bacteria and fell within a range that could be ecologically significant in the oligotrophic oceans. The presence of chemotaxis in motile Synechococcus spp. supports the notion that regions of nutrient enrichment, such as the proposed microzones and patches, may play an important role in picoplankton nutrient dynamics.     

Chemotaxis toward Nitrogenous Compounds by Swimming Strains of Marine Synechococcus spp

Many of the open-ocean isolates of the marine unicellular cyanobacterium Synechococcus spp. are capable of swimming motility, whereas coastal isolates are nonmotile. Surprisingly, the motile strains do not display phototactic or photophobic responses to light, but they do demonstrate positive chemoresponses to several nitrogenous compounds. The chemotactic responses of Synechococcus strain WH8113 were investigated using blind-well chemotaxis chambers fitted with 3.0-mum-pore-size Nuclepore filters. One well of each chamber contained cells suspended in aged Sargasso Sea water, and the other well contained the potential chemoattractant in seawater. The number of cells that crossed the filter into the attractant-seawater mixture was measured by direct cell counts and compared with values obtained in chambers lacking gradients. Twenty-two compounds were tested, including sugars, amino acids, and simple nitrogenous substrates, at concentrations ranging from 10 to 10 M. Strain WH8113 responded positively only to ammonia, nitrate, beta-alanine, glycine, and urea. Typically, there was a 1.5- to 2-fold increase in cell concentrations above control levels in chambers containing these compounds, which is comparable to results from similar experiments using enteric and photoheterotrophic bacteria. However, the threshold levels of 10 to 10 M found for Synechococcus spp. chemoresponses were lower by several orders of magnitude than those reported for other bacteria and fell within a range that could be ecologically significant in the oligotrophic oceans. The presence of chemotaxis in motile Synechococcus spp. supports the notion that regions of nutrient enrichment, such as the proposed microzones and patches, may play an important role in picoplankton nutrient dynamics.     

Colorless Sulfur Bacteria, Beggiatoa spp. and Thiovulum spp., in O2 and H2S Microgradients

The interactions between colorless sulfur bacteria and the chemical microgradients at the oxygen-sulfide interface were studied in Beggiatoa mats from marine sediments and in Thiovulum veils developing above the sediments. The gradients of O₂, H₂S, and pH were measured by microelectrodes at depth increments of 50 μm. An unstirred boundary layer in the water surrounding the mats and veils prevented microturbulent or convective mixing of O₂ and H₂S. The two substrates reached the bacteria only by molecular diffusion through the boundary layer. The bacteria lived as microaerophiles or anaerobes even under stirred, oxic water. Oxygen and sulfide zones overlapped by 50 μm in the bacterial layers. Both compounds had concentrations in the range of 0 to 10 μmol liter⁻¹ and residence times of 0.1 to 0.6 s in the overlapping zone. The sulfide oxidation was purely biological. Diffusion calculations showed that formation of mats on solid substrates or of veils in the water represented optimal strategies for the bacteria to achieve a stable microenvironment, a high substrate supply, and an efficient competition with chemical sulfide oxidation. The continuous gliding movement of Beggiatoa cells in mats or the flickering motion of Thiovulum cells in veils were important for the availability of both O₂ and H₂S for the individual bacteria.     

Colorless Sulfur Bacteria, Beggiatoa spp. and Thiovulum spp., in O(2) and H(2)S Microgradients

The interactions between colorless sulfur bacteria and the chemical microgradients at the oxygen-sulfide interface were studied in Beggiatoa mats from marine sediments and in Thiovulum veils developing above the sediments. The gradients of O(2), H(2)S, and pH were measured by microelectrodes at depth increments of 50 mum. An unstirred boundary layer in the water surrounding the mats and veils prevented microturbulent or convective mixing of O(2) and H(2)S. The two substrates reached the bacteria only by molecular diffusion through the boundary layer. The bacteria lived as microaerophiles or anaerobes even under stirred, oxic water. Oxygen and sulfide zones overlapped by 50 mum in the bacterial layers. Both compounds had concentrations in the range of 0 to 10 mumol liter and residence times of 0.1 to 0.6 s in the overlapping zone. The sulfide oxidation was purely biological. Diffusion calculations showed that formation of mats on solid substrates or of veils in the water represented optimal strategies for the bacteria to achieve a stable microenvironment, a high substrate supply, and an efficient competition with chemical sulfide oxidation. The continuous gliding movement of Beggiatoa cells in mats or the flickering motion of Thiovulum cells in veils were important for the availability of both O(2) and H(2)S for the individual bacteria.     

Polysulfides as Intermediates in the Oxidation of Sulfide to Sulfate by Beggiatoa spp.

Zero-valent sulfur is a key intermediate in the microbial oxidation of sulfide to sulfate. Many sulfide-oxidizing bacteria produce and store large amounts of sulfur intra- or extracellularly. It is still not understood how the stored sulfur is metabolized, as the most stable form of S⁰ under standard biological conditions, orthorhombic α-sulfur, is most likely inaccessible to bacterial enzymes. Here we analyzed the speciation of sulfur in single cells of living sulfide-oxidizing bacteria via Raman spectroscopy. Our results showed that under various ecological and physiological conditions, all three investigated Beggiatoa strains stored sulfur as a combination of cyclooctasulfur (S₈) and inorganic polysulfides (S_n²⁻). Linear sulfur chains were detected during both the oxidation and reduction of stored sulfur, suggesting that S_n²⁻ species represent a universal pool of bioavailable sulfur. Formation of polysulfides due to the cleavage of sulfur rings could occur biologically by thiol-containing enzymes or chemically by the strong nucleophile HS⁻ as Beggiatoa migrates vertically between oxic and sulfidic zones in the environment. Most Beggiatoa spp. thus far studied can oxidize sulfur further to sulfate. Our results suggest that the ratio of produced sulfur and sulfate varies depending on the sulfide flux. Almost all of the sulfide was oxidized directly to sulfate under low-sulfide-flux conditions, whereas only 50% was oxidized to sulfate under high-sulfide-flux conditions leading to S⁰ deposition. With Raman spectroscopy we could show that sulfate accumulated in Beggiatoa filaments, reaching intracellular concentrations of 0.72 to 1.73 M.     

Phylogeny and distribution of nitrate-storing Beggiatoa spp. in coastal marine sediments

Filamentous sulphide-oxidizing Beggiatoa spp. often occur in large numbers in the coastal seabed without forming visible mats on the sediment surface. We studied the diversity, population structure and the nitrate-storing capability of such bacteria in the Danish Limfjorden and the German Wadden Sea. Their distribution was compared to the vertical gradients of O2, NO3- and H2S as measured by microsensors. The main Beggiatoa spp. populations occurred in a 0.5-3 cm thick intermediate zone, below the depth of oxygen and nitrate penetration but above the zone of free sulphide. The Beggiatoa spp. filaments were found to store nitrate, presumably in liquid vacuoles up to a concentration of 370 mM NO3-, similar to the related large marine sulphur bacteria, Thioploca and Thiomargarita. The observations indicate that marine Beggiatoa spp. can live anaerobically and conserve energy by coupling sulphide oxidation with the reduction of nitrate to dinitrogen and/or ammonia. Calculations of the diffusive nitrate flux and the potential sulphide oxidation by Beggiatoa spp. show that the bacteria may play a critical role for the sulphur cycling and the nitrogen balance in these coastal environments. 16S rDNA sequence analysis shows a large diversity of these uncultured, nitrate-storing Beggiatoa spp. Smaller (9-17 micro m wide) and larger (33-40 micro m wide) Beggiatoa spp. represent novel phylogenetic clusters distinct from previously sequenced, large marine Beggiatoa spp. and Thioploca spp. Fluorescence in situ hybridization (FISH) of the natural Beggiatoa spp. populations showed that filament width is a conservative character of each phylogenetic species but a given filament width may represent multiple phylogenetic species in a mixed population.     

Isolation of Methylophaga spp. from Marine Dimethylsulfide-Degrading Enrichment Cultures and Identification of Polypeptides Induced during Growth on Dimethylsulfide

Dimethylsulfide (DMS)-degrading enrichment cultures were established from samples of coastal seawater, nonaxenic Emiliania huxleyi cultures, and mixed marine methyl halide-degrading enrichment cultures. Bacterial populations from a broad phylogenetic range were identified in the mixed DMS-degrading enrichment cultures by denaturing gradient gel electrophoresis (DGGE). Sequences of dominant DGGE bands were similar to those of members of the genera Methylophaga and Alcanivorax. Several closely related Methylophaga strains were obtained that were able to grow on DMS as the carbon and energy source. Roseobacter-related populations were detected in some of the enrichment cultures; however, none of the Roseobacter group isolates that were tested were able to grow on DMS. Oxidation of DMS by Methylophaga sp. strain DMS010 was not affected by addition of the inhibitor chloroform or methyl tert-butyl ether, suggesting that DMS metabolism may occur by a route different from those described for Thiobacillus species and other unidentified marine isolates. Addition of DMS and methanethiol to whole-cell suspensions of strain DMS010 induced oxygen uptake when strain DMS010 was grown on DMS but not in cells grown on methanol. The apparent K_ms of strain DMS010 for DMS and for methanethiol were 2.1 and 4.6 μM, respectively, when grown on DMS. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the biomass of strain DMS010 and analysis of peptide bands by mass spectrometry techniques and N-terminal sequencing provided the first insight into the identity of polypeptides induced during growth on DMS. These included XoxF, a homolog of the large subunit of methanol dehydrogenase for which a biological role has not been identified previously.     

Degradation of Putrescine and Cadaverine in Seawater Cultures by Marine Bacteria

Marine bacteria removed two diamines, putrescine and cadaverine, from coastal seawater supplemented only with these compounds. Batch cultures of natural bacterial communities were grown in filtered seawater (0.05 μm) supplemented with 500 μg of putrescine or cadaverine per liter. Increases in bacterial cell number were counted with an epifluorescence microscope after acridine orange staining. Removal of diamines from seawater was monitored by high-performance liquid chromatography. Diamines were removed from the seawater cultures within 48 h with no corresponding increase in bacterial yield, growth rate, or viability relative to control (unsupplemented) cultures. Shipboard experiments with open-ocean deep water (1,500 m) showed similar, if slower, removal of putrescine from seawater. Unlike uptake experiments with amino acids, labeled putrescine experiments indicated that most putrescine carbon is mineralized to CO₂ rather than assimilated by the bacteria. After growth in unsupplemented control cultures, the bacteria showed a significant potential to mineralize putrescine, indicating a general degradation potential for this compound by marine bacteria even if the compound was not present during growth. Indicators of metabolic activity such as glucose and glutamic acid uptake and mineralization were not affected by the presence of putrescine. This shows that at the concentrations added, the diamines are not toxic, and therefore detoxification was not the reason for degradation of the diamines by the bacteria.     

Occurrence of Aflatoxins and Aflatoxin-Producing Strains of Aspergillus spp. in Soybeans

Above average rainfall in Maryland during August, September, and October 1971 resulted in heavy mold growth in soybeans while still in the field. Of 28 samples of soybean seed, aflatoxins were found in 14, 2 of which had been used in poultry feed. Aflatoxins were identified by thin-layer chromatography, spectrophotometry, and chicken embryo bioassay. Aspergillus spp. were isolated from 11 samples, and 5 of these isolates produced aflatoxins when grown in liquid culture.     

Novel Phycoerythrins in Marine Synechococcus spp. : Characterization and Evolutionary and Ecological Implications

Four clones of the marine, unicellular, cyanobacteria Synechococcus spp., were examined for the spectral and biochemical features of their phycoerythrins (PE) and their photosynthetic characteristics. Two spectral types of PE which are distinct from known PEs were found. One PE type possessed absorption maxima at 500 and 545 nm and a fluorescence emission at 560 nm. Upon denaturation in acid-urea, two chromophore absorption maxima were obtained, one corresponding to phycourobilin (A_max 500 nm) and one at 558 nm, ascribed to a phycoerythrobilin-like chromophore. The ratio of phycoerythrobilin-like to phycourobilin chromophores was 4.9:1.3. This PE possessed two subunits of M_rs of 17.0 and 19.5 kD for the α and β subunits, respectively. The other PE possessed a single symmetrical absorption at 551 nm and a fluorescence emission at 570 nm. This phycobiliprotein showed a single chromophore absorption band (A_max 558 nm) and yielded two polypeptides, an α of 17.5 kD and a β subunit of 20.8 kD. Both PEs showed a (α, β)_n structure. The presence of phycoerythrobilin-like chromophores (A_max 558 nm) appears to be diagnostic of this marine cyanobacterial group. The features of these PEs combined with additional biochemical data, suggest a possible evolutionary link between the PE-containing marine Synechococcus group and the red algal chloroplast. When the Synechococcus clones were grown under low light intensity the PE-containing clones showed higher photosynthetic performance, larger photosynthetic units sizes, reaction center I to II ratios near unity, and steeper initial slopes of photosynthesis versus irradiance curves than a non-PE-containing clone. These findings demonstrate the high photosynthetic efficiency of PE-containing clones in low light environments common to middepth neritic and oceanic habitats.     

Detection of Specific Strains and Variants of Streptococcus cremoris in Mixed Cultures by Immunofluorescence

Antisera against four different strains of Streptococcus cremoris were raised by injecting rabbits with washed suspensions of whole cells. These antisera interacted specifically with the corresponding strain in a mixture of up to nine different S. cremoris strains. The antisera could be used for analyzing the composition of mixed cultures containing these strains by immunofluorescence. Competition experiments were performed in batch and continuous cultures under amino acid limitation. A bacteriophage-sensitive variant of S. cremoris SK11 (SK1128) could be distinguished from a bacteriophage-resistant variant (SK1143) by the same immunofluorescence technique. The competition between the two variants and the stability of both variants in pure cultures were followed with the specific antibodies. Antibodies against the purified proteolytic system of S. cremoris Wg2 were used to determine the presence of proteases by immunofluorescence in several S. cremoris strains under different culture conditions. The described immunofluorescence methods can be used to analyze complex mixed starter cultures common in the dairy industry as the strains and variants present in these mixtures can be recognized microscopically.     

Distribution of Thermus spp. in Icelandic Hot Springs and a Thermal Gradient

The growth range in nature of bacteria belonging to the genus Thermus was investigated by sampling 55 different hot springs in Iceland. The springs ranged in temperature from 32 to 99°C, and in pH from 2.1 to 10.1. Viable counts of Thermus spp. ranging from 10 to 10⁴ CFU/100 ml of spring water were found in 27 of the springs sampled. The temperature range for these bacteria was found to be 55 to 85°C, and the pH range was from about 6.5 to above 10. Thermus spp. were found in springs containing up to 1 mM dissolved sulfide and having conductivity up to 2,000 μS/cm. The distribution of Thermus spp. in a hot spring thermal gradient was also investigated and found to agree well with the overall distribution in individual springs.     

Effect of Temperature on Photosynthesis and Growth in Marine Synechococcus spp.

In this study, we develop a mechanistic understanding of how temperature affects growth and photosynthesis in 10 geographically and physiologically diverse strains of Synechococcus spp. We found that Synechococcus spp. are able to regulate photochemistry over a range of temperatures by using state transitions and altering the abundance of photosynthetic proteins. These strategies minimize photosystem II (PSII) photodamage by keeping the photosynthetic electron transport chain (ETC), and hence PSII reaction centers, more oxidized. At temperatures that approach the optimal growth temperature of each strain when cellular demand for reduced nicotinamide adenine dinucleotide phosphate (NADPH) is greatest, the phycobilisome (PBS) antenna associates with PSII, increasing the flux of electrons into the ETC. By contrast, under low temperature, when slow growth lowers the demand for NADPH and linear ETC declines, the PBS associates with photosystem I. This favors oxidation of PSII and potential increase in cyclic electron flow. For Synechococcus sp. WH8102, growth at higher temperatures led to an increase in the abundance of PBS pigment proteins, as well as higher abundance of subunits of the PSII, photosystem I, and cytochrome b6f complexes. This would allow cells to increase photosynthetic electron flux to meet the metabolic requirement for NADPH during rapid growth. These PBS-based temperature acclimation strategies may underlie the larger geographic range of this group relative to Prochlorococcus spp., which lack a PBS.Marine picocyanobacteria are the most abundant phytoplankton, inhabiting nearly every area of the surface ocean and dominating in tropical and subtropical waters. The smallest and most abundant marine picocyanobacteria belong to the genera Synechococcus and Prochlorococcus, which together account for one-third of the total primary production on Earth (Partensky et al., 1999b). Marine Synechococcus spp. are genetically diverse (Scanlan et al., 2009; Mazard et al., 2012), play an important role in the biogeochemical cycling of carbon (Grob et al., 2007), and are found from the equator to the polar circle, though they are less abundant at higher latitudes (Agusti, 2004; Scanlan et al., 2009; Huang et al., 2012). Temperature is a major factor that controls photosynthetic rates, and the biogeography of Synechococcus spp. strains in the modern ocean has been linked to temperature (Zwirglmaier et al., 2008). In this study, we explore the effect of temperature on growth and photosynthesis in several Synechococcus spp. strains.Photosynthetic electron transport in cyanobacteria, including Synechococcus spp., shares similarities with that of plants and green algae (Fig. 1). Photosynthetic organisms are commonly able to perform photosynthesis efficiently over a range of temperatures bracketing the optimal growth temperature (T_opt). However, decreased metabolic rates at temperatures too far below T_opt can cause an imbalance between photochemistry and metabolism, leading to photodamage (Huner et al., 1996). By contrast, elevated temperatures may affect membrane fluidity and denature proteins, which can also lead to a decline in photosynthetic efficiency (Falk et al., 1996). A range of diverse acclimation strategies have evolved among algae and plants to balance electron flow through the electron transport chain (ETC) during temperature fluctuations (Maxwell et al., 1994; Krol et al., 1997; Gray et al., 1998; Miśkiewicz et al., 2000).Open in a separate windowFigure 1.PBS structure and linear photosynthetic electron flow in cyanobacteria. In this schematic, the PBS is in “state 1,” indicating it is associated with a PSII dimer. Photosynthetic electron flow pathways are indicated by black arrows, and chemical reactions are indicated by blue arrows. Major ETC components include PSII, PSI, PQ/plastoquinol (PQH₂), cytochrome b6f (Cyt b6f), plastocyanin (PLC), ferredoxin (FX), flavodoxin (FL), and ferredoxin/flavodoxin NADP reductase (FNR). Other proteins depicted include the phycobiliproteins APC, PC, two forms of PE (PE I and PE II), PSII chlorophyll-binding proteins CP47 and CP43, the PSII core polypeptides D1 and D2, the PSI chlorophyll-binding core proteins PsaA and PsaB, and the PSI reaction center subunit PsaD. [See online article for color version of this figure.]Less is known about mechanisms marine cyanobacteria use to acclimate to temperature. Cyanobacteria differ from plants and green algae in that photosynthesis and respiration occur in the same membrane. In addition, the ratios of PSII:PSI are more variable in cyanobacteria (Campbell et al., 1998; Bailey et al., 2008), which can impact the flow of electrons through the ETC. Cells must prevent overreduction of the ETC because this can lead to damage of the D1 polypeptide of PSII in a process called photoinhibition; to sustain PSII activity, replacement of the damaged D1 by de novo protein synthesis is required (Aro et al., 1993). Cyanobacteria have evolved a suite of strategies to balance electron flow in the thylakoid membrane when the cells are exposed to high light; important strategies include nonphotochemical quenching (El Bissati et al., 2000; Bailey and Grossman, 2008) and alternative electron flow pathways (Asada, 1999; Bailey et al., 2008; Mackey et al., 2008). Cyanobacteria may also selectively funnel light energy to PSII or PSI to regulate the amount of electrons entering and exiting the ETC (Campbell et al., 1998).In cyanobacteria, including Synechococcus spp., the main light-harvesting antennae are water-soluble pigment-protein complexes called phycobilisomes (PBSs; Grossman et al., 1993; Six et al., 2007). Unlike the antenna of plants and algae that are embedded within the thylakoid membrane, PBSs are located on the cytoplasmic surface of the membrane (Fig. 1). Structurally, the PBS consists of phycobiliproteins, including the PBS core allophycocyanin (APC) and lateral rods of phycocyanin (PC) and phycoerythrin (PE; Fig. 1). The PBS core has evolved together with the core genome of Synechococcus spp., whereas the rod components appear to have evolved separately through gene duplication, DNA exchange between cells, and possibly virally mediated lateral gene transfer (Six et al., 2007). Each phycobiliprotein binds chromophores called phycobilins (linear tetrapyrroles) that selectively absorb different wavelengths of green-red light, thereby extending the range of photosynthetically active radiation the cell can use beyond that of chlorophyll (Campbell et al., 1998). The PBS is capable of rapid diffusion over the thylakoid membrane surface (Mullineaux et al., 1997), where it can associate with either PSI or PSII. The PBS is a mobile antenna element that does not bind chlorophyll and that likely associates with reaction centers by weak interactions with lipid head groups (Sarcina et al., 2001).State transitions, the movements of PBS or other antenna pigments between reaction centers, allow the cells to avoid PSII photodamage by balancing electron flow such that electrons do not accumulate within the ETC. Whether the PBS associates with PSI or PSII is determined by the redox poise of the plastoquinone (PQ) pool (Fig. 1), which serves as an indicator of electron flow through the ETC. When the PQ pool is oxidized, the PBS becomes associated with PSII (state 1) such that the rate of linear electron flow increases. By contrast, a reduced PQ pool elicits affiliation of the PBS with PSI (state 2), which could increase the withdrawal of electrons from the ETC. In the dark, the PQ pool tends to be reduced due to respiratory electron flow, and the PBS affiliates primarily with PSI.Recent ocean basin scale research has shed light on the role of temperature on the global distributions of Synechococcus spp. in the ocean. Collectively, these studies have shown that marine Synechococcus spp. tolerate a broad range of temperatures, likely due to high genetic diversity among strains. For example, of the four clades that dominate in natural communities, clades I and IV typically inhabit cooler waters north of 30°N and south of 30°S (Brown et al., 2005; Zwirglmaier et al., 2007, 2008), while clades II and III generally inhabit warmer tropical and subtropical waters (Fuller et al., 2006; Zwirglmaier et al., 2008). Other Synechococcus spp. sequences have been recently identified from colder waters in the northern Bering Sea and Chukchi Sea, suggesting that a possible cold adaptation could exist in some strains present at high latitudes (Huang et al., 2012). Still, other studies have found no relationship between Synechococcus spp. abundance and temperature (Zinser et al., 2007), suggesting that additional factors (e.g. nutrient availability) may be responsible for shaping Synechococcus spp. community structure (Palenik et al., 2003, 2006; Scanlan et al., 2009).While field surveys have made great strides in understanding the role of temperature in controlling picocyanobacteria distributions, much remains to be learned about the range of growth responses to temperature that can occur in marine Synechococcus spp. To date, characterization of individual Synechococcus spp. strains includes work with two isolates from the Sargasso Sea, showing variable responses to temperature (Moore et al., 1995; Fu et al., 2007). These studies demonstrate the potential for changing sea surface temperature (SST) to influence the biogeochemical role of Synechococcus spp. in the Sargasso Sea; however, little is known about whether these responses can be generalized to other strains or environments. Changes in growth rate and photosynthetic efficiency, if they occur, could alter global Synechococcus spp. distributions, affect ecosystem structure, and ultimately impact marine biogeochemical cycles and Earth’s climate, and thus could have important implications for the earth system.A mechanistic understanding of how temperature affects growth and photosynthesis in geographically and physiologically diverse strains of Synechococcus spp. is needed to clarify how temperature influences Synechococcus spp. biogeography, as well as to provide insights into how populations are likely to respond to increased SST in the future. The goal of this study is to characterize the growth, photosynthetic efficiency, and light-harvesting characteristics of 10 diverse Synechococcus spp. isolates over a range of temperatures. Using chlorophyll fluorescence analysis, we show that regulation of light harvesting via state transitions is an important acclimation process that allows cells to increase photosynthetic electron flow under high temperature conditions. This effect is enhanced for strains with higher proportions of phycoerythrobilin and phycouribilin. We use global proteome data from Synechococcus sp. WH8102 to show that this temperature-dependent enhancement is brought about in part by an increase in the abundance of PBS proteins, as well as proteins from PSII, PSI, and other ETC components. The results are discussed in the context of Synechococcus spp. biogeography in the modern ocean, and potential implications for how cells could respond to future increases in SST are considered.     

Ecology of Thioploca spp.: Nitrate and Sulfur Storage in Relation to Chemical Microgradients and Influence of Thioploca spp. on the Sedimentary Nitrogen Cycle

Microsensors, including a recently developed NO₃⁻ biosensor, were applied to measure O₂ and NO₃⁻ profiles in marine sediments from the upwelling area off central Chile and to investigate the influence of Thioploca spp. on the sedimentary nitrogen metabolism. The studies were performed in undisturbed sediment cores incubated in a small laboratory flume to simulate the environmental conditions of low O₂, high NO₃⁻, and bottom water current. On addition of NO₃⁻ and NO₂⁻, Thioploca spp. exhibited positive chemotaxis and stretched out of the sediment into the flume water. In a core densely populated with Thioploca, the penetration depth of NO₃⁻ was only 0.5 mm and a sharp maximum of NO₃⁻ uptake was observed 0.5 mm above the sediment surface. In sediments with only few Thioploca spp., NO₃⁻ was detectable down to a depth of 2 mm and the maximum consumption rates were observed within the sediment. No chemotaxis toward nitrous oxide (N₂O) was observed, which is consistent with the observation that Thioploca does not denitrify but reduces intracellular NO₃⁻ to NH₄⁺. Measurements of the intracellular NO₃⁻ and S⁰ pools in Thioploca filaments from various depths in the sediment gave insights into possible differences in the migration behavior between the different species. Living filaments containing significant amounts of intracellular NO₃⁻ were found to a depth of at least 13 cm, providing final proof for the vertical shuttling of Thioploca spp. and nitrate transport into the sediment.     

Microscale Distribution of Populations and Activities of Nitrosospira and Nitrospira spp. along a Macroscale Gradient in a Nitrifying Bioreactor: Quantification by In Situ Hybridization and the Use of Microsensors

The change of activity and abundance of Nitrosospira and Nitrospira spp. along a bulk water gradient in a nitrifying fluidized bed reactor was analyzed by a combination of microsensor measurements and fluorescence in situ hybridization. Nitrifying bacteria were immobilized in bacterial aggregates that remained in fixed positions within the reactor column due to the flow regimen. Nitrification occurred in a narrow zone of 100 to 150 μm on the surface of these aggregates, the same layer that contained an extremely dense community of nitrifying bacteria. The central part of the aggregates was inactive, and significantly fewer nitrifiers were found there. Under conditions prevailing in the reactor, i.e., when ammonium was limiting, ammonium was completely oxidized to nitrate within the active layer of the aggregates, the rates decreasing with increasing reactor height. To analyze the nitrification potential, profiles were also recorded in aggregates subjected to a short-term incubation under elevated substrate concentrations. This led to a shift in activity from ammonium to nitrite oxidation along the reactor and correlated well with the distribution of the nitrifying population. Along the whole reactor, the numbers of ammonia-oxidizing bacteria decreased, while the numbers of nitrite-oxidizing bacteria increased. Finally, volumetric reaction rates were calculated from microprofiles and related to cell numbers of nitrifying bacteria in the active shell. Therefore, it was possible for the first time to estimate the cell-specific activity of Nitrosospira spp. and hitherto-uncultured Nitrospira-like bacteria in situ.