Producer, Grazer and Predator - an Unlikely Alliance

Tritrophic mutualistic interactions are commonly seen in the plant-insect systems. A plant will secrete volatiles in response to a herbivore, which in turn attracts a predator and relieves the grazing. These interactions have not been widely studied within the marine environment; however the release of dimethylsulfide (DMS) has been shown to be an important infochemical within the marine systems for the grazers and predators, which could potentially mean that there may be a tritrophic link. For this to be mutualistic, the producer must also benefit.

A recent study conducted by Savoca & Nevitt (2014) suggests that within the Southern Ocean marine ecosystem, DMS-producing phytoplankton (eg. Phaeocystis antarctica) will be grazed upon by primary consumers (eg. Antarctic krill, Euphasia superba) and so will produce more DMS. This in turn will attract many carnivorous species, in particular the Procellariiform seabirds, which relieve the grazing.

However, the study also suggests that the marine ecosystem benefits due to iron being made available to the upper ocean surface in the form of the faecal matter from carnivores. In this way, the phytoplankton would benefit from the influx of vital iron, providing further evidence of tritrophic mutualism.

Over 3,000 individuals of 18 procellariiforms (with the phylogeny taken into account to control potential effects) had their stomachs analysed and it was found that DMS-tracking species foraged more on crustacea (as mentioned in the example above) than any other food group, whilst the non DMS-tracking species had an equal proportion of every food group.

The body mass, manoeuvrability and diet were taken into account, and the results suggested that “the DMS behavioural responsiveness is linked to the consumption of primary consumers, which themselves consume DMS-producing phytoplankton”. As the predators consume the grazers, it is implied that the phytoplankton benefit by no longer suffering grazing damage, thus supporting the idea that this interaction is an example of tritrophic mutualism.

However, the study also wanted to consider how iron from the carnivores’ faeces plays a role in supporting the growth of phytoplankton. As iron is vital for primary production in phytoplankton, the carnivores supplying a high proportion of iron (not usually abiotically introduced into the Southern Ocean) is necessary for the well being of the ecosystem.

This study used data from South Georgia to collate previous knowledge as well as their own, concluding that their work has supported the idea that seabird excrement has a beneficial effect on phytoplankton growth. The data can also provide further evidence for the tritrophic mutualistic interactions seen between primary producers and the top predators in the Southern Ocean.

I believe that this paper is introducing a new way of studying the interactions of the micro- and macro-scale interactions present in the oceans, as well as highlighting how important it is to look at the processes occurring at the microbial levels. I think that this study may be a useful starting point in understanding how microbial communities impact on the ecological hierarchy of the oceans, and how this could affect the marine environment in the future.

Savoca, Matthew S., and Gabrielle A. Nevitt. "Evidence that dimethyl sulfide facilitates a tritrophic mutualism between marine primary producers and top predators." Proceedings of the National Academy of Sciences 111.11 (2014): 4157-4161.

http://www.pnas.org/content/111/11/4157.full