Breaking the Water Mould: An oomycete pathogen of an Atlantic seagrass

Oomycetes have well-described ecological roles as the saprophytes and pathogens of terrestrial plants. For a long time, the oomycetes were recognised as a fungal taxon but subsequent genetic analyses have demonstrated them to be heterotrophic stramenopiles – the same group as diatoms and kelp. However, much like their fungal look-a-likes, marine oomycetes are very poorly understood relative to their terrestrial cousins. Despite the devastating effects oomycetes can have on terrestrial plants, virtually no work has been carried out to understand their role in the pathogenesis of marine angiosperms. Here, Govers and colleagues (2016) present the first report of a widespread oomycete infection in the seagrass Zostera marina.

Seeds of this species were collected from countries on both Atlantic coasts and visually inspected for oomycete sporangia. Infected seeds were then cultured on an oomycete-selective agar known as ‘ParpH’ and colonies were sequenced for ITS rRNA gene signatures. Strikingly >99% of all the collected seeds were infected by the oomycetes Halophythophthora sp. Zostera and Phytophthora gemini. To understand this phenomenon more comprehensively, Z. marina seeds were subjected to a germination experiment under laboratory conditions in the presence (or absence) of oomycete pathogens. Perhaps most surprisingly, following 110 days of incubation in sediment, two thirds of the infected seeds were no longer infected; they had shrugged off the oomycete pathogen. Those that remained infected, however, exhibited a sixfold lower germination rate than the non-infected control seeds, indicating a deleterious effect of oomycetes on seagrass reproduction.

The authors also introduced physiochemical variation into their experimental design. This was achieved by growing infected seeds in differential sediment treatments (mud or sand) and at low or high temperatures (5 vs 12^oC). These conditions were selected to be ecologically realistic of cool or warm winters in the Dutch Wadden Sea. The authors found that infection rates were higher in sandy sediment and at cooler temperatures. The authors postulate that both organic-rich, muddy sediments and warmer temperatures favour bacterial sulphide production and anoxia (corroborated by physiochemical profiling of sediments from field study sites), which could hinder oomycete growth and reproduction.

These findings centre around a robust hybrid methodology, combining in situ surveys with controlled laboratory experimentation. The molecular work, however leaves much to be desired as the samples sequenced were culture-dependant, having been isolated from colonies grown on agar. As with a multitude of other marine microbial taxa, the oomycetes may suffer from poor culturability and these samples may not be representative of the potentially pathogenic oomycete community. I would have been keen to see metabarcoding conducted on environmental samples from the sample field sites, to gain a holistic oomycete community profile. It is too important to note that Koch’s postulates have not yet been fulfilled, as with many cases of marine disease, and therefore further work is needed.

The identification of marine oomycete pathogens, particularly in a species of conservation interest, is an eye-opening discovery. Previously, the study of seagrass disease was focussed on the Labyrinthulomycetes (also stramenopiles), which cause a devastating wasting disease. This study provides evidence that other taxa may be key players, and a proper understanding of their biology could reap serious conservation benefits. The physiochemical results already suggest that sediment and climate optimisation could be important in mitigating disease in seagrass conservation efforts.

Reviewed Paper: Govers, L. L., Man, W. A., Meffert, J. P., Bouma, T. J., van Rijswick, P. C., Heusinkveld, J. H., ... & van der Heide, T. (2016, August). Marine Phytophthora species can hamper conservation and restoration of vegetated coastal ecosystems. In Proc. R. Soc. B (Vol. 283, No. 1837, p. 20160812). The Royal Society. http://rspb.royalsocietypublishing.org/content/283/1837/20160812