Small Soldiers, Big Army: The Invasion of Symbiodinium Trenchii

Due to human activities and climate change, the distribution of species is altering at a fast rate. A newly introduced species, often coined as an invasive species, can have dramatic effects on the native ecosystem’s functionality. When one thinks of an invasive species the obvious examples tend to regard macro fauna or flora, as their impacts on the native biodiversity are usually very obvious. However, as this piece of research by Pettay et al.,(2015) shows, invasive microbes have the potential to have just as significant an effect on an ecosystem’s function. Their research documents the spread of an opportunistic coral endosymbiont, Symbiodinium trenchii, from the Indo-Pacific into the Caribbean and begins to investigate the impacts they could have on their ecologically and socio-economically important non-native habitat.

S. trenchii are single celled, photosynthetic dinoflagellates called “zooxanthellae” which usually live inside the body of reef-forming corals. They provide their host with fixed carbon necessary for the coral to survive, grow and produce the calcium carbonate skeletons that form the structure of coral reefs. In exchange the dinoflagellates are protected and receive CO₂ required for photosynthesis. S. trenchii are native to the Indo-Pacific Ocean but are also present in the North Western tropical Atlantic (ie. the Greater Caribbean). Pettay and others suspect that they have been introduced to this region for the following reasons. Firstly, previous phylogenetic analyses are unable to distinguish the Greater Caribbean populations from those in the Indo-Pacific. Secondly, it is the only species present in its clade. In Indo-Pacific regions they co-occur with other host-specified species of the same clade. Finally they behave opportunistically and can invade colonies of coral species that are physiologically stressed.

To test whether the populations of Atlantic S. trenchii represent a recent invasion the team studied interindividual genotypic diversity and divergence and compared it to populations located in the Indo-Pacific using high-resolution microsatellite loci. They only analysed samples which contained a distinct genotype (ie. part of the same clonal lineage). This consisted of 80-90% of the 245 samples collected.

Populations analysed from the Greater Caribbean showed little interindividual genotypic variation across a wide geographic range. The most prevalent of these genotypes was found at 7 of the 8 sample sites, some of which were separated by thousands of kilometers. Samples sourced from the Indo-Pacific region though exhibited relatively high genotypic diversity. Most clonal lineages were found only once and those that were found more than once were living 1-50 m apart. They also possessed a higher diversity of alleles, many of which being unique to the Indo-Pacific populations. No alleles found in the Greater Caribbean were distinct from those present in the Indo-Pacific populations.

The low diversity found in S. trenchii populations in the Greater Caribbean are not characteristic of a long established panmictic population and indicate a reduced effective population size from a founding event. Indeed, not a single clonal lineage found here was not also found in the Indo-Pacific. It was therefore concluded that this species of Symbiodinium was introduced via human-related activities, probably on more than one occasion, and probably via the ballast water of cargo ships.

S. trenchii can tolerate water temperatures 1-2 ^oC higher than the native competitors. As a result of this it is likely to become more prevalent than it already is due to increasing seawater temperatures. It is therefore essential that the ecological implications this could have are clearly understood.

Petay et al. shed some light on this by looking at the dichotomy between photosynthesis rates in different symbionts and the calcification rates in a native coral (Orbicella faveolata). It was found that S. trenchii photosynthesizes at rates indistinguishable from host-typical symbionts but does not contribute as many nutrients. The rate of calcification in corals harboring S. trenchii were reduced to near half of the rates found in corals harboring native colonies. This indicates a maladaption. S. trenchii is inexorably displaced by host-typcial symbionts but this can take months or years. Their persistence means that the short term benefit of stress tolerance for corals may be negated by the long-term effects of hindered growth.

I think it would be interesting to see if this invasive microbe decreases calcification rates in other native coral species. Only one coral species was used in this paper although it is the most abundant in the sampled region. Also a field project monitoring the calcification rates of coral harboring S. trenchii and also the time for it to be displaced would be useful in understanding the impact this introduction will have on the coral reefs in the Greater Caribbean. This paper has however demonstrated the profound effect microbes can have on ecosystems and as a result of S. trenchii’s “stingy” nature, makes you wonder if corals really are ‘the masters of the house’.

Reviewed paper: Pettay, D.T., Wham, D.C., Smith, R.T., Iglesias-Prieto, R. and LaJeunesse, T.C. (2015) ‘Microbial invasion of the Caribbean by an Indo-Pacific coral zooxanthella’, Proceedings of the National Academy of Sciences, 112(24), pp. 7513–7518. doi: 10.1073/pnas.1502283112.