In 2009, after accepting a position at the National Science Foundation, I wrangled a postdoctoral fellowship for David Baker. Baker, a graduate student from Cornell University, visited the Geophysical Lab a couple of times asking for a postdoc so he could continue his work on isotopes in corals. Finally, I recognised that he was a determined, smart young man who had a clear vision of how he wanted his career to unfold. He managed to get a matching fellowship at the Smithsonian, which opened up the possibility of going back to Twin and Carrie Bow Cayes.
Baker was interested in studying the effects of nutrient additions to coral symbionts. His field areas included Australia’s Great Barrier Reef, Panama,
from sewage into coral skeletons adjacent to crowded tourist destinations in Mexico and Bermuda. At the Carnegie, he extended his studies to include investigations on coral symbionts called zooxanthellae, photosynthetic dino-
flagellates living inside the coral host. The coral symbiont, Symbiodinium sp.,
are genetically very diverse. The genetic variants, called clades, are associated with particular coral species and with the environmental conditions of the coral reef. For example, clade D is more resistant to heat, whereas clade C, which is much less tolerant to fluctuations in temperature, is more commonly found in a greater variety of corals.
In a set of experiments conducted at Australia’s Great Barrier Reef,
the coral Acropora tenuis was infected with either clade C or clade D symbi-
onts (Baker et al., 2013b). Using enriched 13C and 15N tracers at two different
temperatures (28 °C and 30 °C), we found that clade C outcompeted clade D at 28 °C in terms of carbon and nitrogen uptake. At 30 °C, clade D was able to fix
more CO2 than clade C, but clade C continued to take up more nitrogen, often
a limiting nutrient in oligotrophic environments. We concluded that clade C symbionts will dominate, even though they are less tolerant of thermal stress, owing to their nutrient gathering ability.
Warming of the world’s oceans destabilises the association of corals and their symbionts and reduces the ability of corals to survive climate changes. Dave wanted to test the popular hypothesis that, when coral reefs warm in
polluted waters, Symbiodinium symbionts will sequester more resources for
their own growth and become parasites on their coral hosts. In 2011, Baker, Dartmouth graduate student Derek Smith, Chris Freeman, a graduate student at the University of Alabama, and I travelled to Carrie Bow to carry out our investigations.
Baker and Freeman were certified scientific divers trained to collect corals in a variety of environments. Every few days, we would go off shore or near another coral reef, so Baker and Freeman could collect samples for experiments and isotope measurements. Derek and I were responsible for staying in the boat and figuring out when and where the divers would surface. One particu- larly choppy day just off shore of Carrie Bow, we waited anxiously for the dive “balloons” to surface so that we knew where they were. Divers attach these balloon to their wrists so that people on the surface can see where they are. That day Freeman forgot to do this, and the balloon floated away undetected. The water was rough, we were drifting onto the reef, and we needed to take action fast. I worried that our propellers would hit the divers as they surfaced. Derek was concerned that as we drifted, we’d end up on the reef. After drifting 100 m or so away from the divers, we turned on the engine and circled around. Baker and Freeman were pretty surprised when they surfaced and saw the boat quite a distance from them. Derek and I learned some important boat safety that day and gained some confidence.
After they collected the coral specimens, we set up a series of experiments with small 5 cm pieces of a couple of coral species, placed them in bottles, and incubated them in temperature-controlled plastic pools on Carrie Bow. Each morning and evening dissolved oxygen levels were measured in the exper- imental bottles to determine respiration and photosynthetic rates. At night we extracted chlorophyll from the corals and measured inorganic nitrogen
nutrients. The coral Orbicella faveolata hosted one of two Symbiodinium clades
and was held at ambient (26 °C) or sub-bleaching (31 °C) temperatures. We
added excess nitrate with an enriched 15N content, as well as enriched 13C.
The presence of enriched 13C in the symbiont represents assimilation, whereas
enriched 13C in the coral host tissue represents translocation from the symbiont.
Incorporation of nitrogen for both symbionts and corals allows for growth and cell division (Baker et al., 2018).
Compared to ambient temperature measurements, the symbiotic dinofla- gellate variants (A3 and C7) fixed more carbon and nitrogen than at the higher temperatures. However, warming to 31 °C decreased net primary productivity by dinoflagellate symbionts by 60 % due to increased respiration. Furthermore, the coral host received 15 % less carbon and substantially less nitrogen at the higher temperature of 31 °C. While the symbiont was “happy” with higher temperatures, it cost the coral host substantially. This work has major implica- tions for the resilience of coral reefs under threat from global change.
Baker and Freeman also targeted rouge lionfish (Pterois volitans), an inva-
sive species that escaped from aquaria over the past 25 years. Lionfish are native to the Indo-China region where they are top carnivores. In the Caribbean, they are decimating native fish populations, thereby altering the trophic structure of coral reefs. As Dave and Chris collected their coral samples, they speared lion- fish if they could, collecting over 100 fish. Lionfish have very poisonous spines, which we carefully removed before preparing them to eat. The carbon and nitrogen isotopic compositions of lionfish showed them to be major predators, even potentially cannibalistic, on the reefs.