The responses of photosynthesis and oxygen consumption to short-term changes in temperature and irradiance in a cyanobacterial mat (Ebro Delta, Spain)

Evaporative silicification in floating microbial mats: patterns of oxygen production and preservation potential in silica-undersaturated streams, El Tatio, Chile

Microbial mats floating within multiple hydrothermally sourced streams in El Tatio, Chile, frequently exhibit brittle siliceous crusts (~1 mm thick) above the air-water interface. The partially silicified mats contain a diverse assemblage of microbial clades and metabolisms, including cyanobacteria performing oxygenic photosynthesis. Surficial crusts are composed of several amorphous silica layers containing well-preserved filaments (most likely cyanobacteria) and other cellular textures overlying EPS-rich unsilicified mats. Environmental logs, silica crust distribution, and microbial preservation patterns provide evidence for crust formation via repeated cycles of evaporation and silica precipitation. Within the mats, in situ microelectrode profiling reveals that daytime oxygen concentrations and pH values are diminished beneath silica crusts compared with adjacent unencrusted communities, indicating localized inhibition of oxygenic photosynthesis due to light attenuation. As a result, aqueous conditions under encrusted mats have a higher saturation state with regard to amorphous silica compared with adjacent, more active mats where high pH increases silica solubility, likely forming a modest feedback loop between diminished photosynthesis and crust precipitation. However, no fully lithified sinters are associated with floating encrusted mats in El Tatio streams, as both subaqueous and subaerial silica precipitation are limited by undersaturated, low-SiO₂ (<150 ppm) stream waters. By contrast, well-cemented sinters can form by evaporation in silica-undersaturated solutions above 200 ppm SiO₂ . Floating mats in El Tatio therefore represent a specific sinter preservation window, where evaporation in silica-undersaturated microbial mats produces crusts, which preserve cells and affect mat chemistry, but low-silica concentrations prevent the formation of lasting sinter deposits. Patterns of silica precipitation in El Tatio microbial communities show that the preservation potential of silicifying mats in the rock record is strongly dependent on aqueous silica concentrations.

Biophysical properties at patch scale shape the metabolism of biofilm landscapes

Phototrophic biofilms form complex spatial patterns in streams and rivers, yet, how community patchiness, structure and function are coupled and contribute to larger-scale metabolism remains unkown. Here, we combined optical coherence tomography with automated O₂ microprofiling and amplicon sequencing in a flume experiment to show how distinct community patches interact with the hydraulic environment and how this affects the internal distribution of oxygen. We used numerical simulations to derive rates of community photosynthetic activity and respiration at the patch scale and use the obtained parameter to upscale from individual patches to the larger biofilm landscape. Our biofilm landscape approach revealed evidence of parallels in the structure-function coupling between phototrophic biofilms and their streambed habitat.

Influence of Physical Perturbation on Fe(II) Supply in Coastal Marine Sediments

Iron (Fe) biogeochemistry in marine sediments is driven by redox transformations creating Fe(II) and Fe(III) gradients. As sediments are physically mixed by wave action or bioturbation, Fe gradients re-establish regularly. In order to identify the response of dissolved Fe(II) (Fe²⁺) and Fe mineral phases toward mixing processes, we performed voltammetric microsensor measurements, sequential Fe extractions, and Mössbauer spectroscopy of 12 h light-dark cycle incubated marine coastal sediment. Fe²⁺ decreased during 7 days of undisturbed incubation from approximately 400 to 60 μM. In the first 2-4 days of incubation, Fe²⁺ accumulated up to 100 μM in the top 2 mm due to Fe(III) photoreduction. After physical perturbation at day 7, Fe²⁺ was re-mobilized reaching concentrations of 320 μM in 30 mm depth, which decreased to below detection limit within 2 days afterward. Mössbauer spectroscopy showed that the relative abundance of metastable iron-sulfur mineral phases (FeS_x) increased during initial incubation and decreased together with pyrite (FeS₂) after perturbation. We show that Fe²⁺ mobilization in marine sediments is stimulated by chemical changes caused by physical disturbances impacting the Fe redox distribution. Our study suggests that, in addition to microbial and abiotic Fe(III) reduction, including Fe(III) photoreduction, physical mixing processes induce chemical changes providing sediments and the inhabiting microbial community with Fe²⁺.

Microsensors and microscale gradients in biofilms

Understanding the limiting factors and mechanisms of biofilm processes requires the direct measurement of microscale gradients using the appropriate tools. Microscale measurements can provide mechanistic information that cannot be obtained from bulk-scale measurements. Among the most used and trusted tools in microscale biofilm research are microsensors. The goal of this chapter is to introduce microsensor technology along with several examples to illustrate microscale processes in biofilms that are usually absent in bulk. We define a microsensor for biofilm research as a needle-type sensor with tip diameter of a few microns and a length up to several hundred microns. Microsensors can be used noninvasively to monitor in situ biofilm processes. Both optical and electrochemical microsensors can be used for biofilm applications. Because of newly discovered biofilm processes, the design and use of microsensors require customization and carefully designed experiments. In this chapter we present several examples describing the use of microsensors (1) in environmental biofilms, (2) in medical biofilms, and (3) in biofilms for energy and bioproducts. Microsensors can be the most useful if the measured profiles are integrated into the study of overall biofilm processes.

Photosynthetic activity and community shifts of microphytobenthos covered by green macroalgae

Macroalgae blooms, a frequent consequence of eutrophication in coastal areas, affect the photosynthetic activity of sediments dominated by microphytobenthos (MPB). Light spectra, steady-state (after 1 h) microprofiles of O2 , gross photosynthesis (Pg ), community respiration in light (RL ) and net community photosynthesis (Pn ) were measured in diatom- and cyanobacteria-dominated communities below increasing layers of Ulva. Photosynthetic photon flux (PPF) decreased exponentially with increasing layers of algae and the light spectrum was increasingly enriched in the green and deprived in blue and red regions. Sediment Pg , Pn and RL decreased as the number of Ulva layers increased; however, 1.6 times higher macroalgal density was necessary to fully inhibit cyanobacteria Pg compared with diatoms, indicating that cyanobacteria were better adapted to this light environment. Long-term (3 weeks) incubations of diatom-dominated sediments below increasing layers of Ulva resulted in a shift in the taxonomic composition of the MPB towards cyanobacteria. Hence, changes in the light climate below macroalgal accumulations can negatively affect the photosynthetic activity of sediments. However, spectral niche differentiation of MPB taxonomic groups and concurrent changes in the MPB community may provide sediments with increased resilience to the detrimental effects of eutrophication.

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.

Species-specific interactions between algal endosymbionts and coral hosts define their bleaching response to heat and light stress

The impacts of warming seas on the frequency and severity of bleaching events are well documented, but the potential for different Symbiodinium types to enhance the physiological tolerance of reef corals is not well understood. Here we compare the functionality and physiological properties of juvenile corals when experimentally infected with one of two homologous Symbiodinium types and exposed to combined heat and light stress. A suite of physiological indicators including chlorophyll a fluorescence, oxygen production and respiration, as well as pigment concentration consistently demonstrated lower metabolic costs and enhanced physiological tolerance of Acropora tenuis juveniles when hosting Symbiodinium type C1 compared with type D. In other studies, the same D-type has been shown to confer higher thermal tolerance than both C2 in adults and C1 in juveniles of the closely related species Acropora millepora. Our results challenge speculations that associations with type D are universally most robust to thermal stress. Although the heat tolerance of corals may be contingent on the Symbiodinium strain in hospite, our results highlight the complexity of interactions between symbiotic partners and a potential role for host factors in determining the physiological performance of reef corals.

Phototrophic biofilms and their potential applications

Phototrophic biofilms occur on surfaces exposed to light in a range of terrestrial and aquatic environments. Oxygenic phototrophs like diatoms, green algae, and cyanobacteria are the major primary producers that generate energy and reduce carbon dioxide, providing the system with organic substrates and oxygen. Photosynthesis fuels processes and conversions in the total biofilm community, including the metabolism of heterotrophic organisms. A matrix of polymeric substances secreted by phototrophs and heterotrophs enhances the attachment of the biofilm community. This review discusses the actual and potential applications of phototrophic biofilms in wastewater treatment, bioremediation, fish-feed production, biohydrogen production, and soil improvement.

Temporal variations of microbial activity and diversity in marine tropical sediments (New Caledonia lagoon)

Temporal variations of oxygen consumption, sensitivity to metal spiking, and microbial diversity were investigated during a one-year survey at the sediment-water interface in the tropical lagoon of New Caledonia. Sediment oxygen consumption (SOC) exhibited strong variations with time with maximum rates during February (Austral summer) and minimum values during July (cold period). SOC was strongly positively correlated with temperature, with an apparent activation energy (E (a)) of 41 kJ mol(-1), corresponding to an apparent Q (10)(20-30 degrees C) of 1.75. Strong short-term variations of SOC were also observed with ratios between two consecutive samplings reaching up to twofold of magnitude within one week, whereas the maximum/minimum ratio over the whole year was equal to 2.73. In most cases, metal spiking led to a strong decrease of SOC; however, in a third of sampling dates, spiking did not significantly decrease activity. These periods of apparent metal tolerance were not characterized by a particular bacterial community structure. Bacterial community structure estimated from terminal restriction fragment length polymorphism (T-RFLP) analysis exhibited strong variations over the one-year survey, and no seasonality was observed for bacterial richness. However, on average, the Whittaker similarity index between two consecutive T-RFLP profiles was above 60% suggesting a relative stability of the bacterial community structure on the short timescale with prominent T-RFs representing on average more than 67% of relative abundance occurring over most of the year, whereas other T-RFs only occurred during some periods.

Regulation of photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat (Camargue, France) by irradiance, temperature and salinity

Short-term effects of irradiance (0-1560 micromol photons m(-2) s(-1)), temperature (10-25 degrees C), and salinity (40-160) on oxygenic photosynthesis and oxygen consumption in a hypersaline mat (Salin-de-Giraud, France) were investigated with microsensors under controlled laboratory conditions. Dark O(2) consumption rates were mainly regulated by the mass transfer limitations imposed by the diffusive boundary layer. Areal rates of net photosynthesis increased with irradiance and saturated at irradiances >400 micromol photons m(-2) s(-1). At low irradiances, oxygen consumption increased more strongly with temperature than photosynthesis, whereas the opposite was observed at saturating irradiances. Net photosynthesis vs. irradiance curves were almost unaffected by decreasing salinity (100 to 40), whereas increasing salinities (100 to 160) led to a decrease of net photosynthesis at each irradiance. Dark O(2) consumption rates, maximal gross and net photosynthesis at light saturation were relatively constant over a broad salinity range (60-100) and decreased at salinities above the in situ salinity of 100. Within the range of natural variation, temperature was more important than salinity in regulating photosynthesis and oxygen consumption. At higher salinities the inhibitory impact of salinity on these processes and therefore the importance of salinity as a regulating environmental parameter increased, indicating that in more hypersaline systems, salinity has a stronger limiting effect on microbial activity.

Spatial dominance and inorganic carbon assimilation by conspicuous autotrophic biofilms in a physical and chemical gradient of a cold sulfurous spring: the role of differential ecological strategies

The community composition and ecophysiological features of microbial autotrophic biofilms were studied in Fuente Podrida, a cold sulfur spring located in East Spain. We demonstrated how different ecophysiological strategies, such as resistance and/or utilization of sulfide and oxygen, light adaptation, or resistance to high water flow, allow each of the microorganisms described to efficiently colonize several areas within the environmental gradient. In the zone of the spring constantly influenced by sulfide-rich waters, biofilms were formed by purple bacteria, cyanobacteria, and filamentous colorless sulfur bacteria. Purple bacteria showed higher photosynthetic efficiency per pigment unit than cyanobacteria, although they were dominant only in anoxic areas. Two filamentous cyanobacteria, strain UVFP1 and strain UVFP2, were also abundant in the sulfide-rich area. Whereas the cyanobacterial strain UVFP2 shows a strategy based on the resistance to sulfide of oxygenic photosynthesis, strain UVFP1, additionally, has the capacity for sulfide-driven anoxygenic photosynthesis. Molecular phylogenetic analyses cluster the benthic strain UVFP1 with genus Planktothrix, but with no particular species, whereas UVFP2 does not closely cluster with any known cyanobacterial species. The colorless sulfur bacterium Thiothrix sp. extended throughout the zone in which both sulfide and oxygen were present, exhibiting its capacity for chemolithoautotrophic dark carbon fixation. Downstream from the source, where springwater mixes with well-oxygenated stream water and sulfide disappears, autotrophic biofilms were dominated by diatoms showing higher photosynthetic rates than cyanobacteria and, by a lesser extent, by a sulfide-sensitive cyanobacterium (strain UVFP3) well adapted to low light availability, although in the areas of higher water velocity far from the river shore, the dominance shifted to crust-forming cyanobacteria. Both types of microorganisms were highly sensitive to sulfide impeding them from occupying sulfide-rich areas of the spring. Sulfide, oxygen, light availability, and water velocity appear as the main factors structuring the autotrophic community of Fuente Podrida spring.

Irradiance regulation of photosynthesis and respiration in modern marine microbialites built by benthic cyanobacteria in a tropical lagoon (New Caledonia)

Microbialites are organosedimentary deposits that have built up as a result of the growth and binding of detrital sediment by a benthic microbial community. This study focuses on microbialites built by monospecific populations of cyanobacteria in the south-west lagoon of New Caledonia, where they have been observed down to 20-25 m depth. The aim was to study their photosynthetic and respiratory responses to various light intensities. The Phormidium sp. TK1 microbialite was collected at 19 m depth and the P. crosbyanum (Tilden) microbialite was collected at 0.5 and 13 m depth. Phormidium sp. TK1 showed all the characteristic features of a low-light adapted species. The initial slope of the Photosynthesis versus Irradiance curve for this microbialite was close to the maximum quantum yield indicating an efficient light absorption and utilization at low light. The photosynthesis maximum was located 0.2-0.4 mm below the surface and did not shift with changing light intensity. Respiration rates were low and not enhanced by light; photoinhibition was observed at higher light intensities. In Phormidium crosbyanum (Tilden) microbialites, the photosynthesis maximum shifted downward to lower depths with increasing light, probably as a result of phototactic migration of cyanobacterial filaments, and light-enhanced respiration was observed at light intensities above light saturation. The photosynthetic para- meters measured in P. crosbyanum indicate that P. crosbyanum is capable of photo-acclimation at high light intensities. The gross productivity of the different microbialites was comparable to values measured in cyanobacterial stromatolites observed in other shallow environments. However, the microbialites studied here were characterized by a lower respiration / production ratio which indicates a higher growth efficiency.

Combined phospholipid biomarker-16S rRNA gene denaturing gradient gel electrophoresis analysis of bacterial diversity and physiological status in an intertidal microbial mat

A combined lipid biomarker-16S rRNA gene denaturing gradient gel electrophoresis analysis was used to monitor changes in the physiological status, biomass, and microbial composition of a microbial mat. In the morning hours, an increase in the biomass of layers containing a high density of phototrophs and a decrease in the growth rate in the deep layers were observed. The combined approach also revealed differences in major groups of microorganisms, including green nonsulfur, gram-positive, and heterotrophic bacteria.