Concurrent imaging of chlorophyll fluorescence, Chlorophyll a content and green fluorescent proteins‐like proteins of symbiotic cnidarians

Abstract

Research on photosynthetic cnidarians has been mainly focused on the symbiosis established between the cnidarian host and its dinoflagellates endosymbionts from genus Symbiodinium. Despite the potential of imaging techniques for assessing the spatial distribution of key parameters of cnidarian photobiology, such as photochemical activity, chlorophyll a content or green fluorescent proteins (GFPs), to our best knowledge, no study has ever attempted to simultaneous map these three features. In this study, we developed a modified imaging pulse amplitude fluorometer by applying excitation light of different wavelengths and selectively detecting short spectral bands through bandpass filters. The imaging system was used to sequentially excite and quantify chlorophyll variable fluorescence (maximum quantum yield of photosystem II, F v /F m ), Chl a content (normalized difference vegetation index) and relative content of GFPs. The spatial distribution of these photophysiological parameters was mapped both horizontally, across the surface of the soft corals Sarcophyton cf. glaucum and Sinularia flexibilis and the zoanthid Protopalythoa sp., and vertically, throughout a vertical section of S. cf. glaucum. Results showed bleached areas within each individual coral colony and registered photophysiological changes with S. cf. glaucum tissue depth. Analysis of Protopalythoa sp. polyps' expansion revealed differential surface patterns of NDVI and GFP concentration, and a negative relation between these latter parameters within each polyp. This novel non-invasive approach allowed a high-resolution characterization of the spatial relationship between these key parameters through the analysis of image information on a pixel-by-pixel basis, which has great potential for investigating the physiological state of symbiotic associations.

White but not bleached: photophysiological evidence from white Montastraea cavernosa reveals potential overestimation of coral bleaching

“…4c). While NDVI has been used as a proxy of Chl a content (Rocha et al 2013a;Wijgerde et al 2014;Leal et al 2014a), the present study demonstrates that, at least for M. cavernosa, NDVI may not be a reliable proxy for Chl a content; white colonies displayed a relatively higher Chl a than bleached specimens, but NDVI results do not show this pattern ( Fig. 4c; Table 1).…”

“…The Normalized Difference Vegetation Index (NDVI) (Rouse et al 1973) was calculated as a proxy of Chl a content (Leal et al 2014a) as follows:…”

Photobiology of the zoanthid Zoanthus sociatus in intertidal and subtidal habitats

“…F v /F m was higher in ST specimens and lower in IT zoanthids (Fig. 2a), but still within the levels observed for photophysiologically healthy symbiotic zoantharians and corals Warner et al 1999;Fitt et al 2001;Wijgerde et al 2014;Leal et al 2015a). Reduced photochemical efficiency in IT zoanthids is likely associated with stress from exposure conditions.…”

“…In contrast, the contents of b-Car and Phe-a were higher in ST specimens. Quantitative differences in photosynthetic pigments are usually associated with irradiance or the balance between the rates of light collection and light use (Venn et al 2006;Kuguru et al 2010;Leal et al 2015a). However, it is unlikely that large irradiance differences are observed among the surveyed habitats and that irradiance is a limiting factor in any of the habitats here analysed.…”

“…However, photobiological adaptations of symbiotic zoantharians differently exposed to tides remain poorly explored. Few studies addressing the photobiological processes of zoantharians are available (Karlson 1988;Kemp et al 2006;Hibino et al 2013;Leal et al 2015a), and, to our best knowledge, the photosynthetic pigments of zoantharians have never been investigated.…”