Review: Hochberg EJ, Apprill AM, Atkinson MJ, Bidigare RR (2006) Bio-optical modeling of photosynthetic pigments in corals. Coral Reefs, 25:99-109.

Feature Paper: Hochberg EJ, Apprill AM, Atkinson MJ, Bidigare RR (2006) Bio-optical modeling of photosynthetic pigments in corals. Coral Reefs, 25:99-109.

Author Abstract: The spectral reflectance of coral is inherently related to the amounts of photosynthetic pigments present in the zooxanthellae. There are no studies, however, showing that the suite of major photosynthetic pigments can be predicted from optical reflectance spectra. In this study, we measured cm-scale in vivo and in situ spectral reflectance for several colonies of the massive corals Porites lobata and Porites lutea, two colonies of the branching coral Porites compressa, and one colony of the encrusting coral Montipora flabellata in Kaneohe Bay, Oahu, Hawaii. For each reflectance spectrum, we collected a tissue sample and utilized high-performance liquid chromatography to quantify six major photosynthetic pigments, located in the zooxanthellae. We used multivariate multiple regression analysis with cross-validation to build and test an empirical linear model for predicting pigment concentrations from optical reflectance spectra. The model accurately predicted concentrations of chlorophyll a, chlorophyll c2, peridinin, diadinoxanthin, diatoxanthin and beta-carotene, with correlation coefficients of 0.997, 0.941, 0.995, 0.996, 0.980 and 0.984, respectively. The relationship between predicted and actual concentrations was 1:1 for each pigment, except chlorophyll c2. This simple empirical model demonstrates the potential for routine, rapid, non-invasive monitoring of coral-zooxanthellae status, and ultimately for remote sensing of reef biogeochemical processes.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This paper is important because as reefs are more threatened every year through direct human impacts (e.g., dynamite fishing, dredging of coral for building materials and lime, overfishing, etc.) and global climate change (e.g., increased El Niño Southern Oscillation events that contribute heavily to bleaching of coral reefs), scientists struggle to map coral reefs now. Eric Hochberg, Marlin Atkinson, Serge Andréfouët, and a handful of other coral reef scientists are using satellite imagery of the oceans to map coral reef communities. This greatly speeds up the process of mapping in situ (on location) but isn't nearly as accurate. Therefore, "ground truthing" is always required but remote sensing scientists hope that in the future, once all the obstacles are overcome, that all coral reef communities can be mapped accurately. This would allow frequent monitoring of all reefs around the world (theoretically though cloud cover would be an issue), almost in real time, which would be a critical tool to coral reef managers.

So, before I get into the review of this week's paper I'll give a very brief overview of remote sensing (mapping from remote locations, as opposed to mapping while observing an ecosystem directly).

How is it possible for a satellite to see a coral reef? Just check out Google Earth and use the satellite function, such as this image of the upper Caribbean. Coral reefs are high-contrast light blue (because many reefs are shallow and often because of sand within the reef environment) and easy to see below.

The challenge is not seeing where shallow-water tropical potential reef boundaries are. The challenge is being able to accurately map coral reef communities. To do this you have to first obtain cloud-free and calm seas pictures of the area you want to survey (because without radio waves, you can't map corals through clouds). NASA has been building a "coral library" for some time to build the best pictures of reefs around the world, which they've mapped (in red below).

Once you have the kind of images you'd like, there are still A LOT of problems. Eric Hochberg has done a lot of work in correcting for distortions in wavelength due to seawater, but the short story is that it is quite difficult to map coral reefs. However, once the obstacles are overcome, you get an image like that below (from Eric Hochberg's research website).

I've left Eric's explanatory figure notes above, but because of his image resolution, it may be difficult to read for some. The main point is to look at the left image (satellite) of a coral reef in an area of Hawaii and see that through his computer processing of the image using various algorithms to correct for water column effects, he generates a false-color map (right) with different reef communities discriminated (corals are in red).

In Eric's image, corals are able to be distinguished from algae, bare rock, and sand because living corals reflect light differently than those other categories. This is because corals have color, which is a reflection of pigments inside their tissue. Some pigments are sunscreens that shallow-water corals develop to protect them from harmful UV radiation (just like humans can tan) and other pigments come from symbiotic algae that live in their tissues (called zooxanthellae), which are responsible for photosynthesis inside corals (after all, corals are animals and only plants can photosynthesize, so it is a parnership or symbiosis between some corals and algae). Pigments within corals are there to absorb certain wavelengths of light (some that are harmful, thereby protecting coral tissues, and some that are needed in the photosynthetic process). Because corals have a unique combination of pigments compared to algae and other substrates, they can be discriminated once pigment peaks are determined.

With that background in mind, we'll get on to the review of this week's paper, which deals with attempting to discriminate photosynthetic pigments within living corals in the field (i.e., underwater) instead of having to collect coral samples, kill them, and process them in a laboratory for pigment analysis.

The authors point out a lot of the above in their paper's introduction, but when dealing with coral pigments, the important point to keep in mind is that there are three sources of pigments for corals: 1) pigments produced by corals; 2) pigments produced by zooxanthellae living inside coral tissues; 3) pigments by invasive organisms (some algae and sponges bore into living coral skeletons) and diseases. The authors concentrate on zooxanthellar pigments and state that the "major photosynthetic pigments" for this group are "chlorophyll a, chlorophyll c2, peridinin, diadinoxanthin, diatoxanthin, and beta-carotene." From past work in the pigment community, peridinin was shown to be unique to dinoflagelates (the group of algae that include zooxanthellae). For a more technical review of the photo system responsible for photosynthesis in zooxanthellae, look to the full paper.

This paper is unique in that the authors look to the complete suite of coral pigments, which has promise for determining how ratios vary according to environmental conditions (thus potentially helping determine coral reef health, turbidity, or nutrient stress through pigment ratios) or how ratios of pigments vary according to coral species (though no one has yet been able to, or may be able to, determine different species of corals through remote sensing technology). Pigment concentrations are also critical for determining bleaching status of corals and may allow researchers fearing a bleaching event (through tracking satellite images of elevated sea surface temperatures) to track the response of corals in situ to temperature stressors. Not only is this of practical management value, but this would also allow tracking of variable coral species response to heat stress in the field or laboratory.

The researchers studied corals in Hawaii that belonged to 3 growth morphologies: massive, branching, and encrusting. The researchers measured coral reflectance (in the visible wavelengths of 400-700nm) in the field using a spectrometer. They then took micro-samples of coral tissue (6mm diameter using a cork borer) from the area of spectral measurement and processed the samples in the laboratory through techniques shown to release more pigments over time from sample tissues (freezing within 3 hours of sampling at -50ºC for 2 weeks, followed by liquid nitrogen immersion for 2 months). Samples were then prepared for high-performance liquid chromatography (HPLC) analysis of pigment concentrations. Again, for full methods, see the paper.

The authors then used multivariate multiple regression analyses "to determine the ability of spectral data to predict pigment concentration." Basically, the authors were taking their observed measurements of various pigment concentrations and trying to predict coral reflectance at different wavelengths. This means that given a certain combination of pigments (therefore, a pigment-concentration matrix), what will the color reflectance be? If corals have a distinct ratio between their pigments it will be possible (if the above analyses proved significant) to determine that a given sample does or does not come from coral. This is necessary to make the maps shown earlier where corals are shown as distinct from other benthic categories.

In short, the authors demonstrated that all corals (tested) showed 3 statistically-significant pigment reflectance peaks at 575, 605, and 650nm, though strength of peaks varied with coral species, form, and whether bleaching or normal. In other words, corals can be predicted by their pigment ratios (which affect reflectance peaks). The authors also showed that an entire system could be built for pigment analyses of corals for less than US$5,000 total, thereby opening the door for "simple and low-cost spectral measurements to provide for routine in situ monitoring of coral-zooxanthellae status, and eventually for remote sensing of coral reef color." This is great news for beginning researchers and small university laboratories or private field stations looking to contribute to ground-breaking science without breaking the bank.

I hope everyone enjoyed this review and that it provides a motivation to learn more about coral pigments, coral reflectance, and remote sensing of coral reefs. Stay tuned for next week when I'll look at a good paper showing that sometimes a healthy reef has a healthy, diverse, and dominant algal community. This is important because many coral reef managers equate algae with bad nutrient stress, which isn't always the case.