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Prochlorococcus predation

Posted by vaulot on 30 Jan 2018 at 00:25 GMT

This paper provides beautiful SEM and EM pictures of photosynthetic picoeukaryotes associated to bacteria but a I believe that the interpretation presented in the paper is wrong and that a more parsimonious explanation is provided by the comment of Jon Zehr. In brief what the authors believe to be Prochlorococcus is in fact UCYN-A because, as pointed out in Zehr's comment, the position of the UCYN-A cell with respect to the Braarudosphaeraceae cell perfectly matches all previous images done using FISH probes. I believe that the Prochlorococcus sequences originate from cells predated by the rest of the picoeukaryotes which sequences appear in Fig. 1.
I would like to add a few points the previous comments by Jon Zehr and Ann Thompson.
1. Only half of the plastid 16S sequences from the sorted sample correspond to a Braarudosphaeraceae, the rest corresponds to other picoeukaryotes including (Ochromonas, 9%; Pelagomonas, 2%; Triparma, 2%; Imantonia, 1%; Chrysochromulina, 1%; Rhizochromulina, 1%). Most of these organisms are very well known mixotrophs (for example see the following papers: Wilken, S., Schuurmans, J.M., and Matthijs, H.C.P. (2014) Do mixotrophs grow as photoheterotrophs? Photophysiological acclimation of the chrysophyte Ochromonas danica after feeding. New Phytol. 204: 882–889; Hansen, P.J. and Hjorth, M. (2002) Growth and grazing responses of Chrysochromulina ericina (Prymnesiophyceae): the role of irradiance, prey concentration and pH. Mar. Biol. 141: 975–983.). So they are more likely predators of Prochlorococcus than the Braarudosphaeraceae. Indeed Frias-Lopez et al. in 2008 (Frias-Lopez, J., Thompson, A., Waldbauer, J., and Chisholm, S.W. (2009) Use of stable isotope-labelled cells to identify active grazers of picocyanobacteria in ocean surface waters. Environ. Microbiol. 11: 512–525) showed that almost of the predators of Prochlorococcus were mixotrophs and not heterotrophs. Among these potential predators they did not recover any sequence of Braarudosphaeraceae (see their Fig. 2).
2. The assignation of the 18S sequence to Braarudosphaera bigelowii seems wrong. As clearly demonstrated in Fig. 5 of Thompson et al. (2014 - Thompson, A., Carter, B.J., Turk-Kubo, K., Malfatti, F., Azam, F., and Zehr, J.P. (2014) Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymnesiophyte host. Environ. Microbiol. doi:10.1111/1462-2920.12490), Braarudosphaeraceae forms 2 clades, one with sequences of Braarudosphaera bigelowii (and known to host UCYNA-A2) and the other by sequence by an uncultured haptophyte represented by FJ537341 (clone BIOSOPE T60_034) known to host UCYN-A1. In their tree (Supp Fig. S4), the sequence recovered by Kamennaya et al. seems to branch with both FJ537341 (corresponding to the uncultured Braarudosphaeraceae clade) and AB847980 which originated from an isolate of Braarudosphaera bigelowii. This seems to contradict all previous trees done for Braarudosphaeraceae that clearly show these two distinct clades. Indeed a tree redone with sequence JC142 (MF MF185178) and an alignment of all existing Braarudosphaeraceae (obtained from Edvardsen, B., Egge, E.S., and Vaulot, D. (2016) Diversity and distribution of haptophytes revealed by environmental sequencing and metabarcoding – a review. Perspect. Phycol. 3: 77–91) clearly demonstrates that JC142 belongs to the uncultured Braarudosphaeraceae and not to Braarudosphaera bigelowii.
3. Finally This work relies on a single sorted sample, which is my opinion raises some problem because Prochlorococcus , UCYN-A and Braarudospharaceae are very ubiquitous in the ocean, so this "pomacytosis" should be wide spread and found in other samples. Why has it not be seen before ?

No competing interests declared.