Zooxanthellae: not just for corals!

I thought it might be nice to have a change of symbiosis...!

Symbioses are often not well understood due to an incomplete knowledge of the costs and benefits incurred by each of the symbiotic partners. This holds true in the case of Cliona tropical reef sponges and their zooxanthellar symbionts. Like corals, they maintain an intracellular population of Symbiodinium within their pinacoderm (outer layer), which is believed to be a mutualistic relationship similar to that of corals. However, even coral-zooxanthellae relationships are not completely understood and it is hoped that studies such as this may help unravel the complexities generally.

Hill (1996) provided support for a sponge-zooxanthellae mutualism by finding that symbiotic sponges had higher growth rates than non-symbiotic species; and that when shaded, the symbiotic species growth rate was reduced, suggesting that the Symbiodinium provide extra energy for the sponge. However, due to a lack of further research the suggested mutualism may not necessarily be the case.

In this study 3 experiments were carried out to ascertain whether or not the relationship is mutualistic, and also whether depth would play a part due to decreasing light levels. Firstly, carbon and nitrogen (labelled with organismal uptake specific markers) were traced through the symbionts. Secondly, sponges were transplanted to different habitats to examine the effects of light on ¹³C and ¹⁵N isotopes; and thirdly bulk ¹³C and ¹⁵N ratios were taken from sponges at different depths.

The findings provided evidence for a mutualism as the labelled carbon was traced from the zooxanthellae (as only they could uptake it) through the pinacoderm to the choanosome where it could be metabolised by the sponge. However, this mutualism may be questioned as the labelled nitrogen could not be traced. It could be that sponges exploit the zooxanthellae, although perhaps it is more likely that this process is just more complicated than expected as it is one of the main advantages to the zooxanthellae symbiotic lifestyle.

Depth also affected the symbiosis as stable isotope analysis revealed that the carbon contributed by the zooxanthellae decreased with depth, as did the zooxanthellae population. Consequently the sponges at depth had switched to a more heterotrophic feeding mechanism. However, this is an energetically expensive process and not always possible, so to make up for the lack of ‘free’ carbon, some sponges use their stored resources, which explains the loss of mass and health of some sponges in the experiments.

This shows how important the symbiosis is to the sponges’ and also demonstrates how such relationships can determine species ecology; as sponge distribution must be, at least partially, dependent on the conditions needed to maximise their intracellular zooxanthellae populations. This may also be significant evolutionarily as, if this relationship is as important to the hosts as it seems, they may become totally dependent upon it, as in the case of Riftia worms, changing both their physiology and evolution. However, although the authors conclude a mutualism here, they do not show evidence of nitrogen passing to the zooxanthellae, which leads me to wonder what they are benefitting from? More in-depth analysis is clearly needed here, as in the case of corals, to determine whether a nutritional advantage is even the main benefit for zooxanthellae in these relationships.

A review of: Jeremy B. Weisz, Andrew J. Massaro, Blake D. Ramsby and Malcolm S. Hill (2010) Zooxanthellar Symbionts Shape Host Sponge Trophic Status Through Translocation of Carbon. Biol. Bull. 219: 189–197