Abstract
Some N2-fixing cyanobacteria form symbiosis with diverse protists. In the plankton two groups of diazotrophic symbioses are described: (1) a collective group of diatoms which associate with heterocystous cyanobacteria (Diatom Diazotroph Associations, DDA), and (2) the microalgal prymnesiophyte Braarudosphaera bigelowii and its relatives which associate with the unicellular cyanobacterium Candidatus Atelocyanobacterium thalassa (hereafter as UCYN-A). Both symbiotic systems co-occur, and in both partnerships the symbionts function as a nitrogen (N) source. In this perspective, we provide a brief comparison between the DDAs and the prymnesiophyte- UCYN-A symbioses highlighting similarities and differences in both systems, and present a bias in the attention and current methodology that has led to an under-detection and under-estimation of the DDAs.
Highlights
Some of the most inconspicuous components of the plankton are partnerships, or symbioses, between diverse unicellular eukaryotic microalgae and prokaryotic cyanobacteria
The more recently discovered is a partnership between a prymnesiophyte, the microalga Braarudosphaera bigelowii and its relatives and several lineages of the unicellular cyanobacterium, Candidatus Atelocyanobacterium thalassa or more commonly called UCYN-A (Zehr et al, 2008; Tripp et al, 2010; Thompson et al, 2012)
There is an additional diatom diazotroph associations (DDAs) described between the pennate diatom, Climacodium frauenfeldianum, and a unicellular cyanobacteria (Carpenter and Janson, 2000)
Summary
Some of the most inconspicuous components of the plankton are partnerships, or symbioses, between diverse unicellular eukaryotic microalgae and prokaryotic cyanobacteria. The DDAs were described over a century ago, yet remain understudied, with only two dedicated reviews in the last two decades (Villareal, 1992; Foster and O’Mullan, 2008), while the prymnesiophyte-UCYN-A symbioses were recently Likewise the sequencing of the draft genomes for the DDA symbionts (RintHH01, RintHM01, CalSC01) revealed a remarkable reduction in genome size and content in the internal symbiont compared to the external symbiont (Hilton et al, 2013).
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