Importance of canopy structure on photosynthesis in single- and multi-species assemblages of marine macroalgae

Plant communities utilize available irradiance with different efficiency depending not only on their photosynthetic characteristics but also on the canopy structure and density. The importance of canopy structure are well studied in terrestrial plant communities but poorly studied in aquatic plant communities. The objective of this study was to evaluate macroalgal community photosynthesis in artificial constructed communities of one to four species with different morphologies along a range of leaf (i.e.=thallus) area densities. In a laboratory set-up we measured net photosynthesis and dark respiration in constructed assemblages of macroalgae, excluding effects other than photosynthesis of individual tissue and distribution of photons in the canopy from influencing metabolism. We hypothezised that 1) canopy structure determines the actual rates of photosynthesis relative to the optimal rates and 2) multi-species communities attain higher maximum photosynthetic rates than single species communities. We found that differences in canopy structure outweighed large differences in tissue photosynthesis resulting in relatively similar maximum community photosynthetic rates among the different single and multi-species assemblages (20.1–40.5 μmol O₂ m⁻² s⁻¹). Canopy structure influenced community photosynthesis both at low and high leaf area densities because it determines the ability of macroalgae to use the photosynthetic potential of their individual tissues. Due to an averaging effect the photosynthetic rate at high leaf area density was more similar among multi-species community than among single-species communities. Multi-species communities had, on average, a slightly higher photosynthetic production than expected from photosynthesis of single species communities. Moreover multi-species communities were capable of exposing new tissue to irradiance up to high densities thereby avoiding a decrease in net photosynthesis. This finding suggests that multi-species communities may be able to maintain higher biomass per unit ground area than single-species communities.     

Scaling of photosynthetic production of aquatic macrophytes – a review

Most studies on photosynthetic production of aquatic macrophytes have been made on detached leaves and algal thalli. This may have given the false impression that production is often saturated by light and that inorganic carbon and nutrients are more important limiting factors. However, studies on the more relevant ecological scale of macrophyte communities lead to a completely different perception because community production is light limited due to intense self‐shading. Relatively high irradiances are needed for photosynthesis to balance respiratory costs and not even maximum irradiances at noon during summer saturate photosynthetic production. The fundamental importance of light is confirmed by the close coupling to community light absorptance of both maximum production in high‐light environments and efficiency of light use in low‐light environments. The upper boundaries of light‐limited and light‐saturated production are distinctly and linearly related to community absorptance. Moreover, higher diversity in the community has a positive and stabilizing influence on light absorptance and production because different species supplement each other temporally and spatially. Close predictions of actual production rates in macroalgal communities throughout the year are possible solely from determinations of incident and absorbed irradiance. Along with the increasing regulating role of light from leaves or thalli to entire communities the importance of temperature, inorganic carbon and other resources decreases. Thus, ten‐fold rise of CO₂ relative to atmospheric saturation does not enhance maximum production in dense communities of efficient HCO₃^?‐users and only doubles production of pure CO₂‐users. A challenge therefore exists establishing the importance of light for photosynthetic production of macrophytes from individuals to communities and re‐evaluating the postulated main importance of inorganic carbon and temperature in the scenarios of globally rising CO₂ and temperature.     

Contrasting oxygen dynamics in the freshwater isoetid Lobelia dortmanna and the marine seagrass Zostera marina

BACKGROUND: and Aims Submerged plants possess well-developed aerenchyma facilitating intra-plant gas-phase diffusion of O2 to below-ground tissues, which are usually buried in anoxic sediments. However, aquatic habitats differ in terms of O2 fluctuations in the water column and in O2 consumption of the sediment, and aquatic plants differ in aerenchymal volume and resistance to O2 diffusion through the plant and across leaf and root surfaces. The hypothesis that the freshwater isoetid Lobelia dortmanna and the marine seagrass Zostera marina should display pronounced contrasts in intra-plant O2 dynamics because of differences in morphology/anatomy, physiology and growth habitat was tested. METHODS: In order to determine the O2 dynamics and relate this to the anatomy and morphology of the two species, O2 microelectrodes were inserted in the aerenchyma of leaves and roots, the sediment pore-water, and the water column in the field. Manipulation of water column O2 in the laboratory was also carried out. KEY RESULTS: It was found that intra-plant transport of O2 between leaf and root tips takes place more readily in L. dortmanna than in Z. marina due to shorter distances and greater cross-sections of the aerenchyma. The major exchange of O2 across roots of L. dortmanna can be accounted for by small intra-plant resistances to diffusion, larger root than leaf surfaces, and greater radial diffusive resistance of leaves than roots. In contrast, the major O2 exchange across leaves than roots of Z. marina can be accounted for by the opposite anatomical-morphological features. The larger aerenchymal volume and the smaller metabolic rates of L. dortmanna compared to Z. marina imply that turnover of O2 is slower in the aerenchyma of L. dortmanna and O2 fluctuations are more dampened following changes in irradiance. Also, O2 accumulated in the aerenchyma can theoretically support dark respiration for a few hours in L. dortmanna but for only a few minutes in Z. marina. CONCLUSIONS: The build-up of O2 in the pore-water of L. dortmanna sediments during the day as a result of high release of photosynthetic O2 from roots and low O2 consumption of sediments means that sediment, aerenchyma and water are important O2 sources for respiration during the following night, while Z. marina relies on the water column as the sole source of O2 because its sediments are anoxic. These differences between L. dortmanna and Z. marina appear to represent a general difference between the isoetid species mainly inhabiting sediments of low reducing capacity of oligotrophic lakes and the elodeid freshwater species and marine seagrasses mainly inhabiting sediments of higher reducing capacity in more nutrient-rich habitats.     

The dynamics of food chains under climate change and nutrient enrichment

Warming has profound effects on biological rates such as metabolism, growth, feeding and death of organisms, eventually affecting their ability to survive. Using a nonlinear bioenergetic population-dynamic model that accounts for temperature and body-mass dependencies of biological rates, we analysed the individual and interactive effects of increasing temperature and nutrient enrichment on the dynamics of a three-species food chain. At low temperatures, warming counteracts the destabilizing effects of enrichment by both bottom-up (via the carrying capacity) and top-down (via biological rates) mechanisms. Together with increasing consumer body masses, warming increases the system tolerance to fertilization. Simultaneously, warming increases the risk of starvation for large species in low-fertility systems. This effect can be counteracted by increased fertilization. In combination, therefore, two main drivers of global change and biodiversity loss can have positive and negative effects on food chain stability. Our model incorporates the most recent empirical data and may thus be used as the basis for more complex forecasting models incorporating food-web structure.     

Spectral shifting of UV-A wavelengths to blue light for enhancing growth rate of cyanobacteria

This study investigates the modification of UV-A radiation for enhancing the growth rate of Synechococcus species in a simulated photobioreactor made of UV-stabilized polycarbonate (PC) plates. Uvitex OB as was dissolved in a poly(methyl methacrylate) (PMMA)-based resin and the mixture was applied as a coating to shift the UV-A radiation to photosynthetically active radiation (PAR). The feasibility of applying the Uvitex OB in a PMMA-based resin, which was used as fluorescent layer to shift UV-A to PAR range, was explored. For this purpose, a variety of concentrations of Uvitex OB in resin were prepared and coated on PC sheets which were placed between the radiation source and the culture flask. Compared to the uncoated PC sheet, the PC panels coated with a few number of the wavelength shifter layer caused an approximately 4-fold increase in cell number during the same culture period.     

Influence of fluorescent coating at rear and front side of a flat panel photobioreactor on algal growth

The focus of this study is the enhancement of microalgae growth rate using spectral conversion of green light. For this purpose, three reactors were considered and fluorescent pigment Rhodamine 6G was dissolved in a thermoplastic acrylic resin, the mixture was then applied on the front side of the first reactor, and on a mirror located at the rear side of the second one. Comparing their maximum specific growth rate (μ _max) of Chlorella sp. to that in the third (uncoated) reactor, the former resulted in an increase up to 15% while the latter in decrease to at least 30%. Also, the rear side coated reactor showed up to 50% increase in biomass productivity rate (P) in early 4 days of experiment. However, this value decreased over time and the uncoated reactor in 12 days exhibited higher biomass productivity rate.     

Interactive effects of warming,eutrophication and size structure: impacts on biodiversity and food‐web structure

Warming and eutrophication are two of the most important global change stressors for natural ecosystems, but their interaction is poorly understood. We used a dynamic model of complex, size‐structured food webs to assess interactive effects on diversity and network structure. We found antagonistic impacts: Warming increases diversity in eutrophic systems and decreases it in oligotrophic systems. These effects interact with the community size structure: Communities of similarly sized species such as parasitoid–host systems are stabilized by warming and destabilized by eutrophication, whereas the diversity of size‐structured predator–prey networks decreases strongly with warming, but decreases only weakly with eutrophication. Nonrandom extinction risks for generalists and specialists lead to higher connectance in networks without size structure and lower connectance in size‐structured communities. Overall, our results unravel interactive impacts of warming and eutrophication and suggest that size structure may serve as an important proxy for predicting the community sensitivity to these global change stressors.     

SIFamide in the brain of the sphinx moth, Manduca sexta

SIFamides form a group of highly conserved neuropeptides in insects, crustaceans, and chelicerates. Beyond their biochemical commonalities, the neuroanatomical distribution of SIFamide in the insect nervous system also shows a remarkable degree of conservation. Thus, expression of SIFamide has been found to be restricted to four neurons of the pars intercerebralis in different holometabolous species. By means of immunohistological stainings, we here show that in Manduca sexta, those four cells are complemented by additional immunoreactive cells located in the vicinity of the mushroom body calyx. Immunopositive processes form arborizations throughout the brain, innervating major neuropils like the antennal lobes, the central complex, and the optic neuropils.