Planctomycetes & Kelp, possible Symbiosis?

Species from the phylum “Planctomycetes” are not typical of most bacteria; they posses cell compartmentalisations with membrane bound organelles. Additionally their cell wall’s lack peptidoglycan, a trait solely shared with members within Chlamydiae, which are obligate intracellulars. Planctomycetes are also found to be prolific through a plethora of environments across the planet. It has also been found that planctomycetes favour a biofilm lifestyle, where they may adhere to surfaces such as those of seaweeds, sediments or particulates of marine snow.

Genome sequences of a well-studied planctomycete, Rhodopirellula baltica, have shown a considerable number of genes involved in the breakdown of sulfated polysaccharides. These are produced by a number of marine photoautotrophs, such as seaweeds, for instants Laminaria hyperborea. A hypothesis was developed expressing that heterotrophic planctomycetes are specialist degraders of sulphated polymeric carbon because of the overrepresentation of these genes in R. baltica and other species. 

Bacteria associated with the kelp Laminaria hyperborea are believed to be important in the carbon and nitrogen cycling of kelp forest. A recent study investigating biofilms of L. hyperborea found planctomycetes through out the year, however their abundace and phylogenies were not examined. Moreover this paper by Bengtsson and Øvreås, (2010) attempts to:

(1) Enumerate planctomycete abundance and distribution within the biofilms on the Laminaria hyperborea.

(2) Construct clone libraries to make clear phylogenetic relationships, community composition and diversity at different times of year. 

(3) And assess culturability of planctomycetes from kelp surfaces. 

(1) On the kelp surface cells were found in uneven distributions growing in strait lines, rings or clusters. Planctomycetes within these aggregations were cocci of small and medium sizes displaying typical ring shaped cell organization. These were evenly intermingled with cells of different morphologies, from rods to long filaments. 

(2) Samples of planctomycetes from all three months, (July 2007, February 2007 and September 2008) were found to have high abundance within the kelp biofilm community. In July there was high abundance but low diversity. In February there was the greatest diversity but abundance was lowest. September showed high abundance, similar to July, and intermediate diversity.

This may be linked to the age of the kelp tissue, as the kelp lamina is older in February compared to in July and September due to the seasonal growth cycle of the kelp. Aging of the kelp tissue could be associated with lowered antibacterial chemical defense by the kelp, as the old kelp lamina is to be shed soon after February, and does therefore not need to be defended against microbial colonization. Without the presence of chemical defenses, the planctomycetes could loose their competitive advantage over other bacterial groups. Specifically the lack of peptidoglycan in their cell walls may leave them exempt from the affects of antibiotics that inhibit other species. This may explain their lower abundance in February.

(3) One strain of planctomycetes, named “P1” was isoloated from the kelp biofilm. It was found be related to R. baltica and posses similar morphological characteristics, though it was not closely related to any of the clone libraries.

Final notes

This paper shows that this species of bacteria may be more closely associated with eukaryotes than previously thought. The findings discussed above lend support to the hypothesis of an algal holobiont. The genetic evidence for planctomycetes as degraders of sulphated polysaccharides, and their preference to a biofilm life style and the knowledge that kelps produce these polysaccharides can be viewed as fledgling evidence for symbiosis. 

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Bengtsson M & Øvre#as L (2010) Planctomycetes dominate biofilms on surfaces of the kelp Laminaria hyperborea. BMC Microbiol 10: 261–273. 

Available at: http://www.biomedcentral.com/1471-2180/10/261