Review of Littman R., Bourne D. & Willis B., 2010, Responses of coral-associated bacterial communities to heat stress differ with Symbiodinium type on the same coral host, Molecular ecology, 19 1978-1990.
This paper follows on from Littman et al. (2009) in which the findings can be summarised as follows:
- 9 month old juvenile Acropora tenuis corals that were subject to colonisation by two Symbiodinium clades and had not been through a summer showed no differences in their bacterial community composition.
- 12 month old juvenile Acropora tenuis corals during a summer had significantly different bacterial communities in their tissues between the two Symbiodinium clades. Acropora tenuis with clade D had a high mortality and showed a high prevalence of Vibrio sp. with these two factors possibly being related to each other and temperature.
The background of Littman et al. (2009) bears the same relevance to the most recent paper but I failed to exaggerate the importance of Symbiodinium clade type to the coral holobiont. The benefits of certain clades are highly species specific with some providing increased growth or more resilience to different environmental stressors and this relationship has helped to form the adaptive bleaching hypothesis and has a big impact on the microbial community in the mucus layer. More information including references can be found in both paper’s introductions.
The hypothesis of Littman et al. (2010) was that temperature was related to bacterial community composition and the authors carried out this experiment by exposing juvenile Acropora tenuis harbouring different Symbiodium types to a control of 28 degrees Celsius and an ecologically relevant experimental temperature of 32 degrees Celsius with an aim of consolidating understanding of the ecology of the coral-zooxanthellae-bacteria relationship instead of relying on postulations from Littman et al. (2009). Corals were raised from larvae that were collected then infected with the different Symbiodium clades (C1 & D), returned to the sea on tiles then collected after 12 months (112 C1 juveniles compared to 20 D juveniles survived this period indicating that high mortality found here shows that the authors almost didn’t need to carry out the aquaria experiments!) and placed in experimental aquaria for 14 days (corals harbouring C1 type only showed paling of tissues, corals harbouring D type showed a 44% decline in photochemical efficiency by day 10 and all bleached after 12 days).
There was a high mortality of juveniles harbouring type D Symbiodium but there was enough survivors left to assess bacterial community composition using PCR based DNA extraction, clone library construction based on 16s rRNA gene sequences, DGGE and finally sequence analysis (where a match consisted of over 97% of 16s rRNA cohesion with GenBank. This revealed that coral harbouring C1 type Symbiodium had little difference in community composition after heat stress. Vibrio sp. were the distinguishing factor between corals harbouring clades D and C1 with corals harbouring clade D in the “control” temperature decreasing in Vibrio sp. (the control was slightly cooler than the sea during the 12 month period so the control aquaria allowed corals to recover) and increasing under elevated temperature from 18% in day 1 to 39% in day 12 to the detriment of other bacteria.
To summarise, the paper highlights distinct shifts in bacterial community composition due to heat stress but depending on Symbiodium clade type with clade D being susceptible to Vibrio sp. infection under elevated temperatures. Symbiodium clade C1 allows juvenile corals some resilience to higher temperatures in terms of bleaching and survival compared to clade D. Vibrio sp. found in juvenile corals harbouring D type Symbiodium can be linked to bleaching, mortality and reduced diversity of the bacterial community in the mucus.
Reference:
Littmann R., Willis B., Bourne D., 2009, Bacterial communities of juvenile corals infected with different Symbiodinium (dinoflagellate) clades, Mar. Ecol. Prog. Ser. 389 45-59.
Posted by Tom Greenwood, January 2011
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