Hyperspectral Distinction of Two Caribbean Shallow-Water Corals Based on Their Pigments and Corresponding Reflectance

Abstract

Abstract:The coloration of tropical reef corals is mainly due to their association with photosynthetic dinoflagellates commonly known as zooxanthellae. Combining High Performance Liquid Chromatography (HPLC), spectroscopy and derivative analysis we provide a novel approach to discriminate between the Caribbean shallow-water corals Acropora cervicornis and Porites porites based on their associated pigments. To the best of our knowledge, this is the first time that the total array of pigments found within the coral holobiont is reported. A total of 20 different pigments were identified including chlorophylls, carotenes and xanthophylls. Of these, eleven pigments were common to both species, eight were present only in A. cervicornis, and three were present only in P. porites. Given that these corals are living in similar physical conditions, we hypothesize that this pigment composition difference is likely a consequence of harboring different zooxanthellae clades with a possible influence of endolithic green or brown algae. We tested the effect of this difference in pigments on the reflectance spectra of both species. An important outcome was the correlation of total pigment concentration with coral reflectance spectra up to a 97% confidence level. Derivative analysis of the reflectance curves showed particular differences between species at wavelengths where several chlorophylls, carotenes and xanthophylls absorb. Within species variability of spectral features was not significant while interspecies variability was highly significant. We recognize that the OPEN ACCESS Remote Sens. 2012, 4 3814 detection of such differences with actual airborne or satellite remote sensors is extremely difficult. Nonetheless, based on our results, the combination of these techniques (HPLC, spectroscopy and derivative analysis) can be used as a robust approach for the development of a site specific spectral library for the identification of shallow-water coral species. Studies (Torres-Pérez, NASA Postdoctoral Program) are currently underway to further apply this approach to other Caribbean benthic coral reef features. The data will be used with planned and future airborne and satellite studies of the site and for algorithm development to advance the use of future airborne and satellite instrument capabilities (NASA PRISM and HyspIRI) for discrimination of coral reef benthic composition.

Spectral Reflectance of Palauan Reef-Building Coral with Different Symbionts in Response to Elevated Temperature

“…The relatively low correlation between ΣR and symbiont density in Inshore (S. trenchii) P. rugosa may be the result of significantly different photoacclimation and pigment status of the symbiont cells. Previous research found a strong inverse relation with high predictability (R 2 ě 0.80) for ΣR and total tissue pigment concentration [12,60]. Integrated reflectance shows potential as a remote sensing tool for rough estimation of symbiont cell densities, however preceding parameterizations will be required for symbiont and coral species, as illustrated here by the higher overall reflectance of P. rugosa compared to C. serailia and corresponding distinct relationships between ΣR and symbiont density.…”

“…Torres-Pérez et al [12] were able to discern variability in the magnitude of near infrared reflectance related to different species of coral. Our analysis however was not able to reliably estimate the reflectance beyond 700 nm because of the experimental setup where the near infrared was highly absorbed due to water molecules.…”

“…The triple peaked patterns were conserved regardless of symbiont genetic identity or treatment and are the result of absorption dips in neighboring wavebands due common pigments of Symbiodinium. In particular, Torres-Pérez et al [12,60] found that common pigments in two species of Caribbean corals included chlorophyll a with peaks at 427, 665, and 685 nm, chlorophyll c 2 at 634 nm, and diadino/diatoxanthin at 442 nm. Peridinin, a diagnostic pigment for many dinoflagellates [12] has an absorption maximum at 475 nm that creates an upward slope in reflectance between 500 and 550 nm [24], which is evident in our measurements though partially masked in Inshore C. serailia (Figure 4) by a large fluorescence feature.…”

“…The location of peaks in the 2nd derivative of reflectance has been linked to the absorption spectra of specific endosymbiont and host pigments and can be utilized to locate important reflectance features [12,[24][25][26]30,36]. The 2nd derivative of reflectance was calculated for each fragment at Day 0 and peaks located during processing.…”

“…To remove noise due to environmental factors including capillary waves due to non-flat water surface and low signal in some wavelengths, spectra were smoothed 3 times with a Savitsky-Golay filter of order 3 and 15 nm width [5,12,25,26]. Standard deviation of R crl was also generated for each fragment from the imagery.…”