Mads Lichtenberg

Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals.

The light field on coral reefs varies in intensity and spectral composition, and is the key regulating factor for phototrophic reef organisms, for example scleractinian corals harbouring microalgal symbionts. However, the actual efficiency of light utilization in corals and the mechanisms affecting the radiative energy budget of corals are underexplored. We present the first balanced light energy budget for a symbiont-bearing coral based on a fine-scale study of the microenvironmental photobiology of the massive coral Montastrea curta. The majority (more than 96%) of the absorbed light energy was dissipated as heat, whereas the proportion of the absorbed light energy used in photosynthesis was approximately 4.0% under an irradiance of 640 µmol photons m−2 s−1. With increasing irradiance, the proportion of heat dissipation increased at the expense of photosynthesis. Despite such low energy efficiency, we found a high photosynthetic efficiency of the microalgal symbionts showing high gross photosynthesis rates and quantum efficiencies (QEs) of approximately 0.1 O2 photon−1 approaching theoretical limits under moderate irradiance levels. Corals thus appear as highly efficient light collectors with optical properties enabling light distribution over the corallite/tissue microstructural canopy that enables a high photosynthetic QE of their photosynthetic microalgae in hospite.

Epiphyte-cover on seagrass (Zostera marina L.) leaves impedes plant performance and radial O2 loss from the below-ground tissue

The O2 budget of seagrasses is a complex interaction between several sources and sinks, which is strongly regulated by light availability and mass transfer over the diffusive boundary layer (DBL) surrounding the plant. Epiphyte growth on leaves may thus strongly affect the O2 availability of the seagrass plant and its capability to aerate its rhizosphere as a defence against plant toxins. We used electrochemical and fiber-optic microsensors to quantify the O2 flux, DBL and light microclimate around leaves with and without filamentous algal epiphytes. We also quantified the below-ground radial O2 loss from roots (~1 mm from the root-apex) to elucidate how this below-ground oxic microzone was affected by the presence of epiphytes. Epiphyte-cover on seagrass leaves (~21% areal cover) resulted in reduced light quality and quantity for photosynthesis, thus leading to reduced plant fitness. A ~4 times thicker diffusive boundary layer around leaves with epiphyte-cover impeded gas (and nutrient) exchange with the surrounding water-column and thus the amount of O2 passively diffusing into the leaves in darkness. During light exposure of the leaves, radial oxygen loss from the below-ground tissue was ~2 times higher from plants without epiphyte-cover. In contrast, no O2 was detectable at the surface of the root-cap tissue of plants with epiphyte-cover during darkness, leaving the plants more susceptible to sulphide intrusion. Epiphyte growth on seagrass leaves thus negatively affects the light climate and O2 uptake in darkness, hampering the plants performance and thereby reducing the oxidation capability of its below-ground tissue.

Nanoparticle‐based measurements of pH and O2 dynamics in the rhizosphere of Zostera marina L.: effects of temperature elevation and light‐dark transitions

Seagrasses can modulate the geochemical conditions in their immediate rhizosphere through the release of chemical compounds from their below-ground tissue. This is a vital chemical defence mechanism, whereby the plants detoxify the surrounding sediment. Using novel nanoparticle-based optical O2 and pH sensors incorporated in reduced and transparent artificial sediment, we investigated the spatio-temporal dynamics of pH and O2 within the entire rhizosphere of Zostera marina L. during experimental manipulations of light and temperature. We combined such measurements with O2 microsensor measurements of the photosynthetic productivity and respiration of seagrass leaves. We found pronounced pH and O2 microheterogeneity within the immediate rhizosphere of Z. marina, with higher below-ground tissue oxidation capability and rhizoplane pH levels during both light exposure of the leaf canopy and elevated temperature, where the temperature-mediated stimuli of biogeochemical processes seemed to predominate. Low rhizosphere pH microenvironments appeared to correlate with plant-derived oxic microzones stimulating local sulphide oxidation and thus driving local proton generation, although the rhizoplane pH levels generally where much higher than the bulk sediment pH. Our data show that Z. marina can actively alter its rhizosphere pH microenvironment alleviating the local H2 S toxicity and enhancing nutrient availability in the adjacent sediment via geochemical speciation shift.

Fiber-Optic Probes for Small-Scale Measurements of Scalar Irradiance

A new method for producing fiber‐optic microprobes for scalar irradiance (=fluence rate) measurements is described. Such fine‐scale measurements are important in many photobiological disciplines. With the new method, it is possible to cast spherical 30–600 μm wide light integrating sensor tips onto tapered or untapered optical fibers. The sensor tip is constructed by first casting a clear polymethyl methacrylate (PMMA) sphere (~80% of the size of the final probe tip diameter) onto the optical fiber via dip‐coating. Subsequently, the clear sphere is covered with light diffusing layers of PMMA mixed with TiO2 until the fiber probe exhibits a satisfactory isotropic response (typically ±5–10%). We also present an experimental setup for measuring the isotropic response of fiber‐optic scalar irradiance probes in air and water. The fiber probes can be mounted in a syringe equipped with a needle, facilitating retraction of the spherical fiber tip. This makes it, e.g. possible to cut a hole in cohesive tissue with the needle before inserting the probe. The light‐collecting properties of differently sized scalar irradiance probes (30, 40, 100, 300 and 470 μm) produced by this new method were compared to probes produced with previously published methods. The new scalar irradiance probes showed both higher throughput of light, especially for blue light, as well as a better isotropic light collection over a wide spectral range. The new method also allowed manufacturing of significantly smaller scalar irradiance microprobes (down to 30 μm tip diameter) than hitherto possible, and such sensors allow minimally invasive high‐resolution scalar irradiance measurements in thin biofilms, leaves and animal tissues.

Photosynthetic Acclimation of Symbiodinium in hospite Depends on Vertical Position in the Tissue of the Scleractinian Coral Montastrea curta

Coral photophysiology has been studied intensively from the colony scale down to the scale of single fluorescent pigment granules as light is one of the key determinants for coral health. We studied the photophysiology of the oral and aboral symbiont band of scleractinian coral Montastrea curta to investigate if different acclimation to light exist in hospite on a polyp scale. By combined use of electrochemical and fiber-optic microsensors for O2, scalar irradiance and variable chlorophyll fluorescence, we could characterize the physical and chemical microenvironment experienced by the symbionts and, for the first time, estimate effective quantum yields of PSII photochemistry and rates of electron transport at the position of the zooxanthellae corrected for the in-tissue gradient of scalar irradiance. The oral- and aboral Symbiodinium layers received ∼71% and ∼33% of surface scalar irradiance, respectively, and the two symbiont layers experience considerable differences in light exposure. Rates of gross photosynthesis did not differ markedly between the oral- and aboral layer and curves of PSII electron transport rates corrected for scalar irradiance in hospite, showed that the light use efficiency under sub-saturating light conditions were similar between the two layers. However, the aboral Symbiodinium band did not experience photosynthetic saturation, even at the highest investigated irradiance where the oral layer was clearly saturated. We thus found a different light acclimation response for the oral and aboral symbiont bands in hospite, and discuss whether such response could be shaped by spectral shifts caused by tissue gradients of scalar irradiance. Based on our experimental finding, combined with previous knowledge, we present a conceptual model on the photophysiology of Symbiodinium residing inside living coral tissue under natural gradients of light and chemical parameters.

Pronounced gradients of light, photosynthesis and O2 consumption in the tissue of the brown alga Fucus serratus

Macroalgae live in an ever-changing light environment affected by wave motion, self-shading and light-scattering effects, and on the thallus scale, gradients of light and chemical parameters influence algal photosynthesis. However, the thallus microenvironment and internal gradients remain underexplored. In this study, microsensors were used to quantify gradients of light, O2 concentration, variable chlorophyll fluorescence, photosynthesis and O2 consumption as a function of irradiance in the cortex and medulla layers of Fucus serratus. The two cortex layers showed more efficient light utilization compared to the medulla, calculated both from electron transport rates through photosystem II and from photosynthesis-irradiance curves. At moderate irradiance, the upper cortex exhibited onset of photosynthetic saturation, whereas lower thallus layers exhibited net O2 consumption. O2 consumption rates in light varied with depth and irradiance and were more than two-fold higher than dark respiration. We show that the thallus microenvironment of F. serratus exhibits a highly stratified balance of production and consumption of O2 , and when the frond was held in a fixed position, high incident irradiance levels on the upper cortex did not saturate photosynthesis in the lower thallus layers. We discuss possible photoadaptive responses and consequences for optimizing photosynthetic activity on the basis of vertical differences in light attenuation coefficients.

In Situ Hydrogen Dynamics in a Hot Spring Microbial Mat during a Diel Cycle

ABSTRACT Microbes can produce molecular hydrogen (H2) via fermentation, dinitrogen fixation, or direct photolysis, yet the H2 dynamics in cyanobacterial communities has only been explored in a few natural systems and mostly in the laboratory. In this study, we investigated the diel in situ H2 dynamics in a hot spring microbial mat, where various ecotypes of unicellular cyanobacteria (Synechococcus sp.) are the only oxygenic phototrophs. In the evening, H2 accumulated rapidly after the onset of darkness, reaching peak values of up to 30 μmol H2 liter−1 at about 1-mm depth below the mat surface, slowly decreasing to about 11 μmol H2 liter−1 just before sunrise. Another pulse of H2 production, reaching a peak concentration of 46 μmol H2 liter−1, was found in the early morning under dim light conditions too low to induce accumulation of O2 in the mat. The light stimulation of H2 accumulation indicated that nitrogenase activity was an important source of H2 during the morning. This is in accordance with earlier findings of a distinct early morning peak in N2 fixation and expression of Synechococcus nitrogenase genes in mat samples from the same location. Fermentation might have contributed to the formation of H2 during the night, where accumulation of other fermentation products lowered the pH in the mat to less than pH 6 compared to a spring source pH of 8.3. IMPORTANCE Hydrogen is a key intermediate in anaerobic metabolism, and with the development of a sulfide-insensitive microsensor for H2, it is now possible to study the microdistribution of H2 in stratified microbial communities such as the photosynthetic microbial mat investigated here. The ability to measure H2 profiles within the mat compared to previous measurements of H2 emission gives much more detailed information about the sources and sinks of H2 in such communities, and it was demonstrated that the high rates of H2 formation in the early morning when the mat was exposed to low light intensities might be explained by nitrogen fixation, where H2 is formed as a by-product.

Light Sheet Microscopy Imaging of Light Absorption and Photosynthesis Distribution in Plant Tissue

Fine-scale characterization of light absorption and photosynthesis across plant tissue sections shows that quantum yields of PSII are highly affected by tissue light gradients. In vivo variable chlorophyll fluorescence measurements of photosystem II (PSII) quantum yields in optically dense systems are complicated by steep tissue light gradients due to scattering and absorption. Consequently, externally measured effective PSII quantum yields may be composed of signals derived from cells differentially exposed to actinic light, where cells located deeper inside tissues receive lower irradiance than cells closer to the surface and can display distinct photophysiological status. We demonstrate how measured distributions of PSII quantum yields in plant tissue change under natural tissue light gradients as compared with conventionally measured quantum yields with even exposure to actinic light. This was achieved by applying actinic irradiance perpendicular to one side of thallus cross sections of the aquatic macrophyte Fucus vesiculosus with laser light sheets of defined spectral composition, while imaging variable chlorophyll fluorescence from cross sections with a microscope-mounted pulse amplitude-modulated imaging system. We show that quantum yields are highly affected by light gradients and that traditional surface-based variable chlorophyll fluorescence measurements result in substantial underestimations and/or overestimations, depending on incident actinic irradiance. We present a method for using chlorophyll fluorescence profiles in combination with integrating sphere measurements of reflectance and transmittance to calculate depth-resolved photon absorption profiles, which can be used to correct apparent PSII electron transport rates to photons absorbed by PSII. Absorption profiles of the investigated aquatic macrophyte were different in shape from what is typically observed in terrestrial leaves, and based on this finding, we discuss strategies for optimizing photon absorption via modulation of the structural organization of phytoelements according to in situ light environments.

Diffusion or advection? Mass transfer and complex boundary layer landscapes of the brown alga Fucus vesiculosus.

The role of hyaline hairs on the thallus of brown algae in the genus Fucus is long debated and several functions have been proposed. We used a novel motorized set-up for two-dimensional and three-dimensional mapping with O2 microsensors to investigate the spatial heterogeneity of the diffusive boundary layer (DBL) and O2 flux around single and multiple tufts of hyaline hairs on the thallus of Fucus vesiculosus. Flow was a major determinant of DBL thickness, where higher flow decreased DBL thickness and increased O2 flux between the algal thallus and the surrounding seawater. However, the topography of the DBL varied and did not directly follow the contour of the underlying thallus. Areas around single tufts of hyaline hairs exhibited a more complex mass-transfer boundary layer, showing both increased and decreased thickness when compared with areas over smooth thallus surfaces. Over thallus areas with several hyaline hair tufts, the overall effect was an apparent increase in the boundary layer thickness. We also found indications for advective O2 transport driven by pressure gradients or vortex shedding downstream from dense tufts of hyaline hairs that could alleviate local mass-transfer resistances. Mass-transfer dynamics around hyaline hair tufts are thus more complex than hitherto assumed and may have important implications for algal physiology and plant–microbe interactions.

Photosynthesis and Metabolism of Seagrasses

Seagrasses have a unique leaf morphology where the major site for chloroplasts is in the epidermal cells, stomata are absent and aerenchyma is present inside the epidermis. This means that the major site for photosynthesis is in the epidermis. Furthermore the lack of stomata means that the route for carbon uptake is via inorganic carbon (Ci) uptake across the vestigial cuticle and through the outer plasma membranes. Since the leaf may at times be in an unstirred situation diffusion through an unstirred layer outside the leaf may be a large obstacle to carbon uptake. The existence of a carbon concentrating mechanism is discussed, but its existence to date is not proven. Active bicarbonate uptake across the plasmalemma does not seem to operate; an external carbonic anhydrase and an extrusion of protons seem to play a role in enhancing CO2 uptake. There is some evidence that a C4 mechanism plays a role in carbon fixation but more evidence from “omics” is required. Photorespiration certainly occurs in seagrasses and an active xanthophyll cycle is present to cope with damaging high light, but both these biochemical mechanisms need further work. Finally, epiphytes pose a problem which impedes the uptake of Ci and modifies the light environment inside the leaves.