Return of the Slime-nets

Another milestone has been reached in the Catalogue of Organisms - this will be the first Taxon of the Week post where the taxon in question has already been covered in the series. So what group of organisms is illustrious enough to be worth introducing twice? In this case, that honour goes to the Labyrinthulea, the slime-nets, and I don't think you could find a more deserving target. The biggest challenge, I suspect, will be finding enough good images online to illustrate to post - for some reason, good labyrinthulean images seem to be a little thin on the ground.


Vegetative cells of Labyrinthula terrestris within epidermis of Poa trivialis. Scale bar = 10 µm. Photo from Bigelow et al. (2005).

In my previous post on labyrinthuleans, I introduced you all to the group and gave a brief rundown of their division into three morphological if not necessarily phylogenetic groups, the labyrinthulids, thraustochytrids and the Diplophrys group. I recommend reading that post before this one, but this time round I'd like to focus more on the ecological role of Labyrinthulea. As alluded to in the previous post, the vast majority of known labyrinthuleans are marine, a habitat in which they appear to be pretty much ubiquitous (Raghukumar, 2002). It is thought that within the marine habitat labyrinthuleans mostly act as decomposers, living by breaking down and extracting nutrients from dead plant, algal and animal matter. They are very effective in this role - for instance, thraustochytrids are capable of breaking down sporopollenin, the extraordinarily resistant polymer that coats pollen grains. However, because labyrinthuleans in such a role have little direct effect on humans (though their significance to nutrient cycles vital to other organisms that are significant to humans is probably considerable), this is not how they have been most studied. Instead, much more attention has been given to their interactions with living organisms as commensals or pathogens.

Despite numerous records of labyrinthuleans as pathogens, it seems likely that in the majority of cases they are not primarily so, but merely facultative. Despite their rapid growth on necrotic algal tissue, growth of thraustochytrids on live algae is minimal or non-existent, even when said algae have been deliberately innoculated with thraustochytrid spores. Raghukumar (2002) postulated that antimicrobial substances produced by healthy plants might effectively keep thraustochytrid growth down. Labyrinthulids are somewhat more aggressive in their relationships with marine plants and algae, but are not necessarily harmful - the species Aplanochytrium minutum, for instance, has been recorded living within the tissues of brown algae without any noticeable adverse effects on the host. Thraustochytrids are also known as pathogens of animals - in many cases, the effects of thraustochytrid infection are not severe except in young animals, but the QPX organism that has caused mass mortality in the clam Mercenaria mercenaria has been shown using molecular means to belong to the thraustochytrids.


Lottia alveus, the seagrass-inhabiting limpet driven to extinction by the seagrass wasting disease epidemic of the 1930s. Reconstruction from here.

A dramatic exception to the generally low-key nature of labyrinthulean pathogenicity reared its ectoplasm in the 1930s. In the early part of that decade, a wasting disease of then-unknown cause devastated populations of the seagrass Zostera marina in the North Atlantic. More than 90% of the seagrass beds on both sides of the Atlantic were wiped out. In some places, the effect was so severe that the ecology of the area affected was completely altered and the seagrass never returned (Short et al., 1987). The indirect effects of this devastation on other organisms, needless to say, were also severe. Migratory waterfowl populations declined, while several commercial fisheries were hard hit - in particular, the scallop fishery on the American eastern seaboard collapsed entirely. At least one species dependent on the seagrass, the limpet Lottia alveus, became extinct as a result of the epidemic. By the 1940s, the epidemic had run its course, and seagrass populations began to recover by the 1950s. The reasons for the wasting disease remained unknown, though it was suggested that abnormally high sea temperatures may have been a factor. It wasn't until a smaller scale outbreak of seagrass wasting disease in 1986 that Short et al. (1987) were able to show that it was caused by a Labyrinthula species. While the cataclysmic levels of the 1930s have never, thankfully, returned, wasting disease remains a concern for the maintenance of seagrass populations.


Grass infected with rapid blight. Photo by D. Bigelow.

It is a pathogenic species that also provides the sole exception yet known to the otherwise entirely aquatic lifestyle of Labyrinthulea. Rapid blight of cool season lawn turf was first described in California as recently as 1995, and has since been recorded from multiple locations across the southern United States. Once again, the identity of the causative organism was difficult to resolve, but it was eventually described as a new species of Labyrinthula in 2005. How a representative of an otherwise exclusively marine genus came to be parasitising grass in a terrestrial environment remains a complete unknown, though Bigelow et al. (2005) suggested that Labyrinthula may be more common in soil than previously thought, but overlooked due to the difficulty in culturing it.

REFERENCES

Bigelow, D. M., M. W. Olsen & R. L. Gilbertson. 2005. Labyrinthula terrestris sp. nov., a new pathogen of turf grass. Mycologia 97 (1): 185-190.

Raghukumar, S. 2002. Ecology of the marine protists, the Labyrinthulomycetes (thraustochytrids and labyrinthulids). European Journal of Protistology 38 (2): 127-145.

Short, F. T., L. K. Muehlstein & D. Porter. 1987. Eelgrass wasting disease: cause and recurrence of a marine epidemic. Biological Bulletin 173 (3): 557-562.