Ascidians, sessile filter-feeding chordates, are found globally
and are used in bio-applications such as drug discovery due to their production
of secondary metabolites. Described metabolites range from defensive compounds,
e.g. polyketides, to protective compounds, e.g. mycosprine amino acids which
protect against UV irradiation. Bacteria are the only known sources of ascidian
secondary metabolites; cases have shown bacteria to produce defensive compounds
where host animals do not. Metabolites appear to increase survival and be
species and location specific, suggesting chemical and environmental drivers
are involved in assemblage selection. Both horizontal and vertical transmission
of symbionts takes place. In tropical species, Prochloron producers of cynobactins (toxic cyclic peptides) and
mycosprine (amino acids which act as a UV radiation protectant) seem to be
obligate for host animals despite dominance of horizontal transfer. Contrastingly, the polyketide producing
relationship between ascidians and alpha-proteobacteria Endolissoclinum fulkneri has led to genome reduction, vertical
transmission and co-speciation.
This study aimed to identify factors underlying production
of secondary metabolites in ascidians globally by investigating the microbiomes
and metabolomes of tropical, sub-tropical and temperate ascidians. Specifically,
the study focussed on the link between secondary metabolites and symbiotic
bacteria with the hope of discovering and understanding bio-active natural
products and understanding symbiotic interactions with ascidians to reveal the true
producer of ascidian associated secondary metabolites.
Ascidian samples from Papua New Guina, Fiji, Vanatu,
Catalaina Island, California and Florida Keys were collected during 2006-2011. 18s
and 16s RNA were used for phylogenetic analysis of ascidian and bacterial
species respectively. Compounds were identified using ultra-high pressure
liquid chromatography and mass spectrometry. Ascidians were found to be
predominantly didemnid ascidians from the order Aplousobranchia. Similarly to
sponges, high microbiome diversity was seen across all sample types and
habitats.
Microbiome and chemical diversity were not indicative of
each other; instead, chemical diversity seemed to be down to species specific bacterial
assemblage and ascidian interactions. Ascidian samples were stable over time
and space; most samples had approximately 10 species, described as the ‘Top-10’,
of more abundant bacteria which tended to be more novel with less abundant
microbiome bacteria tending to be more widespread and correlated geographically
to samples. Microbiome and secondary metabolites were strongly species
specific. In addition, tropical ascidians were highly bioactive and toxic,
subtropical less so, and temperate almost not non-toxic due to the production
of patellazoles and pyridoacridines.
This study is the first to compare ascidian metabolmes and
microbiomes across species and location making it valuable to fields involving ascidian
and bacterial interactions and bio-applications. However, it does not detail
any specific relationships. Where compound producers were known, they belonged
to the most abundant species of bacteria in that species specific assemblage.
The authors suggested the most abundant species of bacteria are the priority for
the production of natural products and that there is a strong selection for
defensive microbes. This is a useful starting point however less abundant species
and other microbes must not be overlooked; they may produce or contribute to
production of compounds. Further work must be done to understand the specific interactions
between host and microbes.
Tianero, M. D. B., Kwan, J. C., Wyche, T. P., Presson, A. P., Koch. M., Barrows, L. R., Bugni, T. S., Schmidt, E. W. (2015) Species specificity of symbiosis and secondary metabolism in ascidians. ISME. 9:165-628.
Hi Kat, Interesting study, does it go into any kind of explanation as to why the 'top-10' species in the microbiome tend to be novel?
ReplyDeleteHi Abi, Bacteria associated with geographical location were in low abundance while those specific to an ascidian species were in high abundance suggesting a strongly specific species:species interaction. Where the source of natural secondary metabolites was known these were also from the 'Top-10' most abundant species linking abundant symbionts and natural products.
DeleteUnfortunately, I cant find any data or comment about the abundant bacteria to species level. However, the most abundant phylums appear to be Proteobacteria, Cyanobacteria, Adiciobacteria and Bacterodetes. I think, what they have described as unusual, or novel, is unusual compared to what you would find in the local water- I would be interested the see their 'Top-10'.
Hi Kat, I've got one or two questions.
ReplyDeleteYou mentioned that samples were collected between 2006 and 2011, did they carry out the experiments during this time period too or was it purely sampling? If they were experimenting during the five years, was this large timescale discussed in the paper? Changes in the metabolome, top 10 species etc..
Hi Joss, the samples were collected over that time period, but only once: Papaua New Guinea (2011), Fiji (2006), Vanuatu (2008), Catalina Island, California (2010 and 2011) and Florida Keys (2011). As most were collected once, they didn't mention the time scale. I think it would be a really useful next step to assess the microbial assemblages over time to establish whether the microbiomes are affected seasonally, or if an species are associated with disease.
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