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Microalgal photosynthesis & ecology

The image shows a diver using microsensors to study oxygen evolution and the physico-chemical microenvironment of corals in situ.

 

Photosynthetic micro-organisms (microalgae and  cyanobacteria) account for about 45% of global carbon fixation and are the world's most important primary producers. Microalgae can occur in a range of aquatic environments including marine, fresh and estuarine environments, where they can populate the open ocean or live in a benthic (e.g. sediment) environment. Some species of microalgae live in an animal host, such as corals sea anemones and sea slugs. Previous research has shown that microalgae that live in coral animals have outstanding photosynthetic performance with photosynthetic quantum efficiencies approaching the theoretical maximum (Brodersen et al. 2014). As part of biomicfuel, we are studying the photosynthetic performance of microalgae in corals as well as in our 3D bioprtined algal hydrogels. By studying the photosynthetic performance of microalgae for a range of physico-chemical microenvironments in corals, we hope to understand the interplay of key parameters in optimizing microalgal photosynthesis for biotechnological applications. 

Characterisation of aquatic photosynthesis in benthic habitats can be studied with experimental techniques quantifying photosynthetic oxygen production, carbon fixation and chlorophyll a fluorescence measurements. Oxygen evolution can be directly quantified using oxygen microsensors.

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Microsensors

 

Microsensors are powerful tools to study the physiology of microorganisms in their natural habitat because of their small sensing tips (usually <100 micrometer) that faciliatate measurements at high spatial and temporal resolution. There is a wide range of microsensors that can measure various chemical species and physical parameters (e.g. light and temperature). The image to the left shows combined measurements of light and oxygen on a coral reef during low tide.

In contrast to gas exchange or C-fixation measurements that require significant sample handling and/or incubation, variable chlorophyll fluorescence measurements rely on optical light pulsing schemes that are applied externally with minimal sample manipulation or directly in the natural habitat, and a large variety of commercial instruments for cuvette-based, fiber-optic or imaging measurements are available (e.g. Schreiber,2004). In coral reef science, pulse amplitude modulated (PAM) chlorophyll a fluorimeters are by far the most commonly used instrument to probe photosynthesis.

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PAM fluorimetry

Pulse-amplitude modulated fluorimetry allows for a rapid determination of photosynthetic parameters, non-invasively, in situ and during ambient fluctuating light. PAM measurements rely on chl a fluorescence (i.e. 660-760 nm) of photosystem II. PAM allows for calculating the proportion of photochemistry and heat dissipation through quenching analysis of a sample in dark and light adapted states. The image on the left shows the distribution of the maximum quantum yield of PSII over a coral surface.

References and further reading:

Brodersen, K.E., Lichtenberg, M., Ralph, P.J., Kühl, M. and Wangpraseurt, D., 2014. Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals. Journal of the Royal Society Interface11(93), p.20130997.

Revsbech, N.P., 1989. An oxygen microsensor with a guard cathode. Limnology and Oceanography34(2), pp.474-478.

Kühl, M., Cohen, Y., Dalsgaard, T., Jørgensen, and B.B. Revsbech, N.P., , 1995. Microenvironment and photosynthesis of zooxanthellae in scleractinian corals studied with microsensors for 02, pH and light. Mar. Ecol. Prog. Ser117, pp.159-172.

Schreiber, U., 2004. Pulse-amplitude-modulation (PAM) fluorometry and saturation pulse method: an overview. Chlorophyll a Fluorescence, pp.279-319.

Suggett, D.J., Prášil, O. and Borowitzka, M.A., 2010. Chlorophyll a fluorescence in aquatic sciences: methods and applications (pp. 427-433). Dordrecht, The Netherlands: Springer.

Wangpraseurt, D., Polerecky, L., Larkum, A.W., Ralph, P.J., Nielsen, D.A., Pernice, M. and Kühl, M., 2014. The in situ light microenvironment of corals. Limnology and Oceanography59(3), pp.917-926.

Wangpraseurt, D., Tamburic, B., Szabó, M., Suggett, D., Ralph, P.J. and Kühl, M., 2014. Spectral effects on Symbiodinium photobiology studied with a programmable light engine. PloS one9(11), p.e112809.