Multimodal Optical Microscopy Methods Reveal Polyp Tissue Morphology and Structure in Caribbean Reef Building Corals
Summary
An integrated suite of imaging techniques has been applied to determine polyp morphology and tissue structure in the Caribbean corals Montastraeaannularis and M. faveolata. Fluorescence, serial block face, and two-photon confocal laser scanning microscopy have identified lobate structure, polyp walls, and estimated chromatophore and zooxanthellae densities and distributions.
Abstract
An integrated suite of imaging techniques has been applied to determine the three-dimensional (3D) morphology and cellular structure of polyp tissues comprising the Caribbean reef building corals Montastraeaannularis and M. faveolata. These approaches include fluorescence microscopy (FM), serial block face imaging (SBFI), and two-photon confocal laser scanning microscopy (TPLSM). SBFI provides deep tissue imaging after physical sectioning; it details the tissue surface texture and 3D visualization to tissue depths of more than 2 mm. Complementary FM and TPLSM yield ultra-high resolution images of tissue cellular structure. Results have: (1) identified previously unreported lobate tissue morphologies on the outer wall of individual coral polyps and (2) created the first surface maps of the 3D distribution and tissue density of chromatophores and algae-like dinoflagellate zooxanthellae endosymbionts. Spectral absorption peaks of 500 nm and 675 nm, respectively, suggest that M. annularis and M. faveolata contain similar types of chlorophyll and chromatophores. However, M. annularis and M. faveolata exhibit significant differences in the tissue density and 3D distribution of these key cellular components. This study focusing on imaging methods indicates that SBFI is extremely useful for analysis of large mm-scale samples of decalcified coral tissues. Complimentary FM and TPLSM reveal subtle submillimeter scale changes in cellular distribution and density in nondecalcified coral tissue samples. The TPLSM technique affords: (1) minimally invasive sample preparation, (2) superior optical sectioning ability, and (3) minimal light absorption and scattering, while still permitting deep tissue imaging.
Introduction
Global warming and accompanying environmental change are directly affecting the health and distribution of tropical marine corals1-4. Multiple impacts are being observed, including coral bleaching and the emergence of infectious diseases5-6. However, more accurate prediction of future coral response to these environmental threats will require that a histological “baseline” be established, which defines tissue morphology and cell composition and distribution for “apparently healthy” corals. In turn, “impacted” corals can then be quantitatively compared. Furthermore, this baseline should be established for apparently healthy corals under a variety of environmental conditions, so that “healthy response” can also be gauged across environmental gradients. As an initial step toward establishing this baseline, a high-resolution 3D study has been undertaken of how apparently healthy coral polyp tissue morphology and cellular composition responds to increases in water depth (WD) and accompanying decreases in sunlight irradiance. Results can then be used to establish a more comprehensive mechanistic understanding of coral adaptation, as well as to gain insight into coral-symbiont evolution and the enhancement of light harvesting.
Stony corals (Scleractinia) are colonial marine invertebrate animals that play host to a complex assemblage of other microorganisms, collectively referred to as the coral holobiont7-10. The research undertaken in the present study seeks to use a suite of cutting-edge imaging technologies to simultaneously track changes with increasing water depth in the tissue pigments and symbiotic zooxanthellae of apparently healthy host corals. This will establish the required comparative tissue cell “baseline” across a bathymetric gradient for apparently healthy corals and act as indicators of coral health10. Coral pigments, called chromatophores, act to absorb, reflect, scatter, refract, diffract, or otherwise interfere with incident solar radiation11. The zooxanthellae-chromatophore endosymbiotic relationship has enabled the coevolution of strategically advantageous light-harvesting optimization and skeletal growth strategies, as well as trophic plasticity (shifting feeding strategies back-and-forth from autotrophy to heterotrophy) for the coral animal12.
The southern Caribbean island nation of Curaçao (formerly part of the Netherlands Antilles) lies approximately 65 km north of Venezuela within the east-west trending Aruba-La Blanquilla archipelago (Figure 1A). The 70 km long southern coast of Curaçao contains a continuous modern and Miocene-Pliocene-Pleistocene-Holocene ancient fringing coral reef tract13,14. Mean annual SST on Curaçao varies approximately 3 °C annually, ranging from a minimum of 26 °C in late January to a maximum of 29 °C in early September, with a mean annual temperature of 27.5±0.5 ºC (NOAA SST Data Sets, 2000-2010). The coral reef at Playa Kalki (12°22’31.63”N, 69°09’29.62”W), lying near the northwestern tip of Curaçao (Figure 1A), was chosen for sampling because it has been previously well-studied and the marine ecosystem at this location is bathed in fresh nonpolluted seawater7,15-19. Two closely related scleractinian coral species, M. annularis and M. faveolata, were chosen for experimentation and analysis in this study because each species: (1) exhibits distinctly different and nonoverlapping bathymetric distributions on the reef tract with respect to the shelf break and the associated carbonate sedimentary depositional environments (M. annularis range = 0-10 m WD; M. faveolata range = 10-20 m WD20; Figures 1B, 2A, and 2B); (2) is a common coral reef framework builder throughout the Caribbean Sea21; and (3) has well-studied ecological, physiological, and evolutionary relationships22.
Field sampling for the present study was conducted using standard SCUBA diving techniques offshore of Playa Kalki on Curaçao. A shallow-to-deep water bathymetric transect was established that ran across the shelf, over the shelf break, and into the deep water fore reef environments. Apparently healthy coral heads were then identified for sampling along this bathymetric transect, including: (1) three individual ~ 1 m diameter coral heads of M. annularis, all of which were at 5 m water depth (WD); and (2) three individual ~ 1 m diameter coral heads of M. faveolata, all of which were at 12 m WD. Photosynthetically active radiation (PAR) was measured as 33-36% PAR at 5 m WD and 18-22% PAR at 10 m WD. Sampling was conducted in January when the SST was 26 °C at the water depths of both the 5 m and 12 m. Each of these six coral heads was sampled in triplicate at equivalent spatial positions (i.e., approximately 45° N latitude on each of the six hemispherical coral heads). Each individual sample consisted of a 2.5 cm diameter coral tissue-skeleton core biopsy that was collected with a cleaned arch punch. Three coral tissue-skeleton biopsies were sampled on standard SCUBA with gloved hands from each of the coral heads (9 from M. annularis colonies at 5 m WD and 9 from M. faveolata at 12 m WD). Immediately upon collection at depth, each biopsy core sample was placed in a sterile 50 ml polypropylene centrifuge tube, screw-top sealed, and returned to the surface. The seawater was decanted from each centrifuge tube and each core biopsy was then immersed, stored, and transported in 4% paraformaldehyde.
SBFI imaging has previously been performed on a wide range of biological samples, including whole-brain and whole-heart human tissues, intact mouse embryos, zebra fish embryos, and multiple types of animal samples with intact bones23-30. Most of these studies utilized optical/light microscopy with either fluorescence or bright field techniques. However, studies have been conducted at ultra-high magnifications using scanning electron serial block face imaging in the past31. In the present study, a modified SBFI protocol has been developed for and applied to corals for the first time. Because M. annularis and M. faveolata coral polyps are 1-2 mm in thickness, none of the routine light microscopy techniques would be capable of penetrating the entire thickness of coral polyp tissue. Therefore, we have SBFI sample preparation protocol specifically designed for coral samples. In addition, we have custom designed a stereomicroscope holder, which is motorized to move in both x and y directions. This apparatus takes images of the block face of the sample rather than collecting the sections using a regular microtome in front of the microscope. We also introduced another nonlinear optical two-photon microscopic technique to image the same coral polyps across the entire thickness of the coral tissues. This overcomes the limitations imposed by SBFI in terms of decalcification and the possibility of changes in tissue morphology and volume (shrinking) that may be induced by sample preparation (dehydration) and processing protocols. Furthermore, the emission profiles from the corals were spectrally resolved to identify their peak emissions and variations between the chromatophores and the photosynthetic zooxanthellae. These results were evaluated in the context of the method used and their individual advantages regarding acquisition time, analysis time, and the ability to resolve fine structural details without compromising structural integrity of the coral tissue.
Protocol
Representative Results
Discussion
Coral reef research is a highly interdisciplinary research effort, involving analysis of the simultaneous physical, chemical, and biological phenomena that operate in the marine environment. The study of complex coral reef ecosystems is therefore best completed within a ‘Powers of Ten’ contextual framework (Figure 10). This graphic compilation illustrates that the coral ecosystem covers a wide range of spatial dimensions (10-9 to 105 m). Furthermore, this exercise i…
Disclosures
The authors have nothing to disclose.
Acknowledgements
We thank Donna Epps, histologist at Institute for Genomic Biology, University of Illinois Urbana-Champaign (UIUC), for her capable technical assistance in sample preparation and sectioning. This work was supported by a research grant to B.W. Fouke from the Office of Naval Research (N00014-00-1-0609). In addition, C.A.H. Miller received grants from the UIUC Department of Geology Wanless Fellowship, UIUC Department of Geology Leighton fund and UIUC Department of Geology Roscoe Jackson fieldwork fund. Interpretations presented in this manuscript are those of the authors and may not necessarily represent those of the granting institutions. We also thank the Caribbean Research and Management of Biodiversity (Carmabi) laboratory on Curaçao for their support and collaboration in collecting the coral tissue biopsy samples. We thank Claudia Lutz, IGB Media Communication Specialist for her able language correction.
Materials
| Coral Tissue Skeleton | None | None | 2.5 cm Biopsy from natural habitat |
| Arch Punch Coring Device | C.S. Osborne and Company | No. 149 | For Coral biopsy collection |
| Paraformaldehyde | Electron Microscopy Sciences | RT 15700 | 16% Pre-diluted |
| Histoclear/Safeclear II | Electron Microscopy Sciences | RT 64111-04 | Non-Toxic alternate to Xylene, Dehydration and Deparafinization |
| Xylene and Ethanol | Fisher Scientific | Fisher Scientific | Dehydration |
| Paraffin Wax | Richard Allen Scientific | Type H REF 8338 | Infiltration solution |
| Vybar | The Candle Maker | None | Component of Red Wax |
| Stearin | The Candle Maker | None | Component of Red Wax |
| Sudan IV | Fisher Chemical | S667-25 | Red Wax-Opaque background |
| Wheat Germ Agglutinin (WGA) | Life Technologies | W32466 | For labeling Coral Mucus |
| Prolong Gold | Life Technologies | P36095 | Anti-fade mounting media |
| Fluoro Dish | World Precision Instruments | FD-35-100 | For two-photon imaging |
| XY Motor, Driver and Controller | Lin Engineering | 211-13-01R0, R325, R256-RO | XY Translational Movement |
| Hot Plate | Corning | DC-220 | Melting all wax |
| Convection Oven | Yamato | DX-600 | Infiltration and Embedding |
| Tissue Processor | Leica | ASP 300 | Dehydration, Infiltration |
| Microtome | Leica | RM2055 | Disposable knifes |
| Stereo Microscope | Carl Zeiss | Stereolumar V 12 | 1.5x (30 mm WD) Objective |
| Fluorescence Microscope with ApoTome | Carl Zeiss | Axiovert M 200, ApoTome I System | Imaging thin section of a polyp: Zooxanthellae |
| Axiocam camera | Carl Zeiss | MRm | Monochrome camera 1388×1040 pixels |
| Axiovision Software | Carl Zeiss | Version 4.8 | Image acquisition program |
| Two-Photon Laser | Spectraphysics | Maitai eHP, pulsed laser (70 fs) | With DeepSee module |
| Laser Scanning Microscope | Carl Zeiss | LSM 710 with Spectral Detector | 34 channel PMT detection |
| Zen Software | Carl Zeiss | 2010 or above | for two-photon and spectral image acquisition |
| Imaris Suite Software | Bitplane, Inc., | Version 7.0 or above | 3D Volume, Iso-surface Rendering, Visualization |
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