A polyyne toxin produced by an antagonistic bacterium blinds and lyses a Chlamydomonad alga

Abstract

Algae are key contributors to global carbon fixation and form the basis of many food webs. In nature, their growth is often supported or suppressed by microorganisms. The bacterium Pseudomonas protegens Pf-5 arrests the growth of the green unicellular alga Chlamydomonas reinhardtii, deflagellates the alga by the cyclic lipopeptide orfamide A, and alters its morphology [P. Aiyar et al., Nat. Commun. 8, 1756 (2017)]. Using a combination of Raman microspectroscopy, genome mining, and mutational analysis, we discovered a polyyne toxin, protegencin, which is secreted by P. protegens, penetrates the algal cells, and causes destruction of the carotenoids of their primitive visual system, the eyespot. Together with secreted orfamide A, protegencin thus prevents the phototactic behavior of C. reinhardtii. A mutant of P. protegens deficient in protegencin production does not affect growth or eyespot carotenoids of C. reinhardtii. Protegencin acts in a direct and destructive way by lysing and killing the algal cells. The toxic effect of protegencin is also observed in an eyeless mutant and with the colony-forming Chlorophyte alga Gonium pectorale. These data reveal a two-pronged molecular strategy involving a cyclic lipopeptide and a conjugated tetrayne used by bacteria to attack select Chlamydomonad algae. In conjunction with the bloom-forming activity of several chlorophytes and the presence of the protegencin gene cluster in over 50 different Pseudomonas genomes [A. J. Mullins et al., bioRxiv [Preprint] (2021). https://www.biorxiv.org/content/10.1101/2021.03.05.433886v1 (Accessed 17 April 2021)], these data are highly relevant to ecological interactions between Chlorophyte algae and Pseudomonadales bacteria.

Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms

“…Algicidal compounds that have been fully characterized reveal a high diversity in both their structural characteristics and mechanisms of action (see review by Meyer et al, 2017 ). Identifying the genetic components responsible for algicidal activity among bacteria provides clues to the identity of potential algicidal compounds (e.g., Hotter et al, 2021 ; Hu et al, 2021 ; Li et al, 2021d ; Pal et al, 2021 ). This is exemplified by the genomic analysis of algicidal bacterium Brevibacillus laterosporus Bl-zj, which included a number of genes putatively involved in algicidal activity, including virulence factors, proteases and antibiotic resistance genes ( Zhang et al, 2021a ).…”

“…These compounds, Rhizoxin S2, pyrrolnitrin, pyoluteorin, 2,4-diacetylphloroglucinol (DAPG) along with known algae-immobilizing agent orfamide A, acted differentially, with overlapping activities that produced a powerful net effect of non-motility and inhibited growth. Hotter et al (2021) uncovered yet another toxin, the polyyne protegensin, from the antagonism between these same species that degrades carotenoid pigments, damaging C. reinhardtii ’s eyespots and eventually causing lysis. Complementary and synergistic interactions between algicidal chemicals such as these may be optimized for the development of targeted algicidal technologies.…”

Beyond Soil-Dwelling Actinobacteria: Fantastic Antibiotics and Where to Find Them

“…Although less frequent, Pseudomonas also have PKS-derived secondary metabolites, as is the case of the antibiotic mupirocin ( Figure 15 ) [ 364 , 365 , 366 ] or 2,4-Diacetylphloroglucinol (DAPG), a small molecule which can frequently be found in plant-associated Pseudomonas [ 333 , 367 , 368 , 369 ], as well as hybrid PKS-NRPS molecules such as the toxins pyoluteorin, syringolin or coronatine ( Figure 14 ) [ 326 , 370 , 371 , 372 ]. Interestingly, Pseudomonas is also the source of many secondary metabolites with non PKS or NRPS biosynthetic origins, such as the highly diverse and versatile phenazines, which can act as antimicrobials, siderophores and redox acceptors among other functions [ 373 , 374 , 375 ], the phytohormone Indole-3-acetic acid, hydrogen cyanide [ 333 , 376 ], the recently discovered anti-algal polyyne toxin protegencin ( Figure 15 ) [ 377 , 378 ] or the novel pyonitrins ( Figure 15 ), which are the product of spontaneous condensation of two previously known natural products, pyochelin and pyrrolnitrin, just to name a few [ 379 ]. It is in this area, the less easily identified BGCs, where Pseudomonas exhibit big potential for novel chemistry discovery.…”

Ubiquitous bacterial polyketides induce cross-kingdom microbial interactions

“…However, the ecological role for most of these compounds remains obscure. What has been shown is that the ecological context, including the presence of other microorganisms, is important for production of these compounds ( 13–15 ). Consequently, many of the gene clusters in microorganisms encoding the biosynthesis of such NPs are silent under conventional laboratory conditions ( 12 ) and biosynthesis can only be activated when the correct microbial partner is provided.…”

“…However, the ecological role for most of these compounds remains obscure. What has been shown is that the ecological context, including the presence of other microorganisms, is important for production of these compounds (13)(14)(15).…”