UCYN-A; More Than Meets the Eye.

UCYN-A are a clade of obligate symbiotic cyanobacteria widely distributed throughout the sub/tropical oligotrophic oceans, they have important roles in fixing N₂ in the ocean. UCYN-A shows genome reduction, having lost genes for photosystem II, carbon fixation pathways and the TCA cycle. It therefore depends on a host to provide it with carbon and in exchange provides its host with N₂. In 2014 a second type of UCYN-A was discovered and a study by a group at the Scripps Institute of Oceanography has investigated the genetic diversity of the new UCYN-A (A2) clade and its host.

The group’s article presents a number of interesting findings; primarily the discovery of new, distinct clades: UCYN-A1, A2 and A3 .This means calculations for nitrogen fixation by UCYN-A have been underestimated until now as we could only detect one type. DNA sequences were collected in the south Pacific gyre that didn’t cluster with A1, A2 or A3, so there may be further clades yet undiscovered. The study shows that these three types live in overlapping environments, but A2 and A3 do occupy some areas exclusively: A2 in the gulf of Santa Catalina and A3 in the south Pacific gyre.

UCYN-A2 expressed the nifH gene 2 orders of magnitude more than UCYN-A1 and seems to associate with larger hosts too. While this may be confounded by A2 and host being taken from perhaps richer coastal waters and A1 from oligotrophic open ocean, I can’t help but wonder if the higher nifH expression and larger hosts are linked and if so, is this a result or even a demonstration of co-evolution between host and symbiont?

The study showed that A1 and A2 have 96% shared genes, but the amino acid sequences have “only” 86% similarity. This is a high similarity though I am curious if these two types are genetically diverging whilst also being obligate symbionts. The strains studied live in different ecosystems and interact with genetically distinct hosts (different genotypes of B. bigelowii) so I don’t think the idea sounds too unrealistic; the divergence of A1 and A2 may also be being influenced by the divergence of the hosts which occupy different habitats and have different genomes yet are of the same species. Interestingly the paper presents a phylogenetic tree which shows that UCYN-A is not one bacterium, as was previously declared in the paper (and earlier studies by the same authors), but instead 3+ groups of unidentified marine bacteria. This is an important discovery and may necessitate revision of previous work by authors who worked with UCYN-A to determine which clade they were actually investigating.

Seawater for this study was collected from the Scripps Institute of Oceanography over a series of months between 2010 and 2013. The water was analysed with flow cytometry to sort through the particles collected. QPCR was used to identify the UCYN-A2 clade and its host then a qPCR assay was used to quantify nifH expression in the samples. 

Reference paper: 

Thompson, A. Carter, B.J. Turk-Kubo, K. Malfatti, F. Azam, F. and Zehr, J. (2014) Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophye host. Environmental Microbiology, 16 (10) 3238-3249.

http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12490/pdf

Whilst not related to the blog, I would like to wish a happy birthday to my father who celebrated his birthday on the 25th of this month.