Investigating the Nature of a Symbiosis between Astrangia poculata and Symbiodinium B2
Laura Newcomb, Randi Rotjan, Daniel J. Thornhill
Bowdoin College, Department of Biology, Brunswick, ME
Introduction
Symbioses are close and sustained interactions between two different species. These relationships include: (1) mutualisms in which both
species benefit from the interaction, (2) commensalisms where one species benefits and the other is unaffected, and (3) parasitisms wherein one
species benefits to the detriment of the other. Although symbioses are typically viewed as these distinct categories, in reality, mutualisms,
commensalisms, and parasitisms are merely points on a continuum; the nature of a symbiosis may shift between these categories depending on
the relationship’s ecological context (Sachs et al., 2006).
Symbiodinium
coral
Methods
Corals were collected from the southern Gulf of Maine and transported back to the Marine lab. They were sorted by color (indicative of symbiotic or
asymbiotic state) and size, and then randomly assigned to each treatment.
Experiment 2:
Experiment 1:
Experimental Conditions:
Experimental conditions:
• Two warms tank at 23˚C
• Two warms tank at 21.6˚C
• Two cold tanks at 7.2˚C
• Two cold tanks at 7.7˚C
One prominent example of a marine symbiosis occurs on tropical coral reefs. Here, a mutualism between reef-building corals and singlecelled photosynthetic algae, also known as Symbiodinium, enable this ecosystem to thrive. Symbiodinium live with the digestive cells of the coral
where they are actively photosynthetic. In fact, Symbiodinium cells actively pass sugars and other energy rich compounds to their host, thereby
providing up to 90% of the coral’s energetic needs. Thus, the coral host derives energy, while the symbiotic algae receive nutrients and a safe
environment.
Although symbioses between corals and Symbiodinium are most prominent on coral reefs, there are also symbiotic corals that live in nontropical ecosystems. One local example is the coral Astrangia poculata. This coral occurs all along the U.S. east coast, from the Gulf of Mexico to
the Gulf of Maine (Thornhill et al. 2008). Interestingly, A. poculata can be found both with and without symbionts, a phenomenon known as
“facultative” symbiosis (Dimond and Carrington 2007).
• All tanks under light conditions
• One warm and cold tank in light;
one warm and cold tank in the dark
• One warm and cold tank fed twice a week;
Symbiodinium
Measurements:
Measurements:
• PAM flourometry taken weekly
• Polyp counts taken every two weeks
• Photographs to analyze symbiont
• Photographs to analyze symbiont infestation
infestation taken monthly
• Tissue thickness
Experimental set-up at the
marine lab
taken monthly
• Polyp counts taken monthly
• Rate of photosynthesis and respiration
• Cell density
It is highly unusual to find symbiotic corals this far from the tropics. The Gulf of Maine is approximately the northern limit of where
Symbiodinium can occur. Here, only a single species of Symbiodinium, known as “type B2”, can be found (Thornhill et al. 2008). Unlike most
Symbiodinium, “type B2” is able to withstand periods of low temperature and recover when temperatures are raised (Thornhill et al. 2008).
However, when temperatures decrease below about 15ºC, these symbionts nearly stop photosynthesizing (Thornhill et al. 2008). Considering the
nearly year-round cool temperatures of the Gulf of Maine, it is unclear how Symbiodinium “type B2” could be providing energy to A. poculata.
the other warm and cold tank starved
Astrangia poculata
• Cell density
• Tissue thickness
In light of the information presented above, the facultative symbiosis between A. poculata and Symbiodinium “type B2” provides a
fascinating model for studying the costs and benefits of symbiosis. Living inside of the host coral’s tissue, Symbiodinium cells take up space and
demand nutrients. When these algae are not photosynthesizing, they require energy to survive, energy which they may take from the host corals.
Thus, if Symbiodinium B2 is not actively photosynthetic under most conditions, this symbiosis may shift from a beneficial mutualism to a costly
parasitism (e.g., Sachs and Wilcox 2005). We tested this hypothesis at Bowdoin’s Coastal Studies Center (CSC) over the past summer.
Discussion
Experiment 1 Results
Polyp Counts
PAM Measurements
Cell Density
• These preliminary data indicate that under warm light
conditions, the relationship between Astrangia and B2 is
mutualistic.
• In the cold treatments there was no clear benefit to being
symbiotic or asymbiotic. This data suggests a weak
parasitism or commensalism.
• The state of the symbiosis between Astrangia and
Symbiodinium B2 will be further investigated by analyzing
the data from tissue thickness for experiment 1
• For experiment 2 the corals will be processed for tissue
biomass and symbiont density
• Additionally for experiment 2 we will aim to better
understand Symbiodinium’s contribution to growth through
analysis of respiration versus photosynthesis.
References
Dimond J. and Carrington, E. 2007. Temporal variation in the symbiosis and growth of the
temperate scleractinian coral Astrangia poculata. Mar. Ecol. Prog. Ser. 348:161-72
Figure 1: Change in polyp number per unit area from time zero for all experimental
groups. All treatments except warm light white show a non-statistically significant
increase in polyp number. N=16 per treatment
• In warm light treatment, the symbiotic colonies increased in
polyp number while the asymbiotic colonies declined in number
(Figure 1)
• In all other treatments there was no statistically significant
difference between symbiotic and asymbiotic corals (Figure 1)
Figure 2: Maximum efficiency of PSII of Symbiodinium B2 under the
experimental temperature and light conditions for symbiotic corals. The warm
light displayed the highest efficiency and the cold light the lowest. N=16 per
treatment
Figure 3: Symbiont density of the different experimental
treatments. The cold light symbiotic treatment had the highest
density followed by the cold dark symbiotic and warm light
symbiotic. All of the asymbiotic treatments and the warm dark
symbiotic had non-significant low levels of symbionts. N=9 per
treatment
•The cold light symbiotic treatment displays the lowest photochemical efficiency and highest
symbiont density (Figures 2 and 3)
• The warm light symbiotic treatment has the highest photochemical efficiency and the
second highest cell density of symbionts (Figures 2 and 3)
Sachs, Joel L., and Thomas P. Wilcox. 2005. A shift to paracitism in the jellyfish symbiont
Symbiodinium microadriaticum. Proc. Royal Soc B 273:435-439.
Thornhill, Daniel J., Dustin W. Kemp, Brigitte U. Bruns, William K. Fit, Gregory W. Schmidt. 2008.
Correspondence between cold tolderant and temperate biogeography in a western
Atlantic Symbiodinium (Dinophyta) lineage. J. Phycol 44:1126-1135.
Acknowledgements
I would like to thank my lab mates, Will Hatelberg, and Katharine Doubleday and the
New England Aquarium for their help with the experiment and in collecting the corals.
For their help at the marine lab, Amy Johnson, Jon Allen, Marko Melendy, and Mark
Murray. For help with respiration measurements, Barry Logan. For funding I would like
to thank the Rusack Coastal Studies Fellowship.