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Back in the 1930s, a pioneer reef researcher named C.M. Yonge noted that zooxanthellate corals (i.e., those that host algal symbionts) are the most specialized
carnivores in the animal kingdom with respect to their “prey-capture area” (i.e.,
tentacles) compared to their total body mass. He and subsequent researchers studying the physiology of corals were perplexed by the observations that such highly
adapted predators capture only a small percentage of the energy they need to live,
grow and reproduce.
When I was a graduate student, one of the professors, who studied phytoplankton
that are the free-living relatives of the algal cells found within corals, asked me
several times, “What do the algae get out of living within the corals?” He clearly
was not convinced by the standard explanation that “the host provides housing and
protection, and the algae recycle the nutrients (nitrogen and phosphorus) that the
animal excretes”.
As a young postdoc, I decided to explore how this paradox might be resolved. I
designed a simple box model (the computer to which I had access had far less capability than what people today have in their cell phones or wrist watches). My goal
was to model the benefit of the symbiosis to the host and the algae when plenty of
sunlight was available for photosynthesis, but the nutrients needed to grow and
reproduce were scarce. With my simple model, I simulated growth of the host, the
algae, and the composite organism, what now is known as the “holobiont”. I examined different inputs, whether by prey capture by the host, or by uptake of dissolved
nitrogen from the environment by the algal cells. I also examined different rates of
metabolism and loss (excretion) by the holobiont. The results resolved the paradox,
at least for me. As nitrogen becomes increasingly scarce in the environment, most
essential nitrogen is already within either other organisms or organic detritus. So,
the more nutrient-starved the system, the more important it is for the coral host to be
adapted to capture whatever food particles that are available. Because the algae can
photosynthesize nearly unlimited amounts of simple sugars, prey capture is not
needed to provide the host with food for energy, but to provide nutrients required for
both the host and the algae to grow and reproduce.
Sometime in about 1980, I attended a lecture on reefs through geologic time,
presented by a world-renowned scientist. Overall the presentation was magnificent,
except for the statement he made with very first slide. He showed a picture of a
beautiful coral reef in crystal clear water and stated the standard belief at the time:
“Coral reefs occur in warm, clear, nutrient-rich ocean waters”. After the lecture, I
introduced myself, told him how much I learned from the lecture, but said I had a
question about his first slide. I said: “If the seawater was nutrient rich, how could it
be so clear?” He just looked at me in surprise, hesitated a minute, and responded. “I
shouldn’t say that, should I?”
During the 1980s and into the 1990s, I wrote a series of scientific papers with the
goal to communicate the implications of nutrient pollution in the modern world, and
explanations of why reef building in the geologic past could have been halted by a
variety of processes ranging from episodes of massive volcanic activity, to meteor
14 What Is Happening to the World’s Coral Reefs?
Back in the 1930s, a pioneer reef researcher named C.M. Yonge noted that zooxanthellate corals (i.e., those that host algal symbionts) are the most specialized
carnivores in the animal kingdom with respect to their “prey-capture area” (i.e.,
tentacles) compared to their total body mass. He and subsequent researchers studying the physiology of corals were perplexed by the observations that such highly
adapted predators capture only a small percentage of the energy they need to live,
grow and reproduce.
When I was a graduate student, one of the professors, who studied phytoplankton
that are the free-living relatives of the algal cells found within corals, asked me
several times, “What do the algae get out of living within the corals?” He clearly
was not convinced by the standard explanation that “the host provides housing and
protection, and the algae recycle the nutrients (nitrogen and phosphorus) that the
animal excretes”.
As a young postdoc, I decided to explore how this paradox might be resolved. I
designed a simple box model (the computer to which I had access had far less capability than what people today have in their cell phones or wrist watches). My goal
was to model the benefit of the symbiosis to the host and the algae when plenty of
sunlight was available for photosynthesis, but the nutrients needed to grow and
reproduce were scarce. With my simple model, I simulated growth of the host, the
algae, and the composite organism, what now is known as the “holobiont”. I examined different inputs, whether by prey capture by the host, or by uptake of dissolved
nitrogen from the environment by the algal cells. I also examined different rates of
metabolism and loss (excretion) by the holobiont. The results resolved the paradox,
at least for me. As nitrogen becomes increasingly scarce in the environment, most
essential nitrogen is already within either other organisms or organic detritus. So,
the more nutrient-starved the system, the more important it is for the coral host to be
adapted to capture whatever food particles that are available. Because the algae can
photosynthesize nearly unlimited amounts of simple sugars, prey capture is not
needed to provide the host with food for energy, but to provide nutrients required for
both the host and the algae to grow and reproduce.
Sometime in about 1980, I attended a lecture on reefs through geologic time,
presented by a world-renowned scientist. Overall the presentation was magnificent,
except for the statement he made with very first slide. He showed a picture of a
beautiful coral reef in crystal clear water and stated the standard belief at the time:
“Coral reefs occur in warm, clear, nutrient-rich ocean waters”. After the lecture, I
introduced myself, told him how much I learned from the lecture, but said I had a
question about his first slide. I said: “If the seawater was nutrient rich, how could it
be so clear?” He just looked at me in surprise, hesitated a minute, and responded. “I
shouldn’t say that, should I?”
During the 1980s and into the 1990s, I wrote a series of scientific papers with the
goal to communicate the implications of nutrient pollution in the modern world, and
explanations of why reef building in the geologic past could have been halted by a
variety of processes ranging from episodes of massive volcanic activity, to meteor
14 What Is Happening to the World’s Coral Reefs?