The Influence of Genes on the “Killer Plasmid” of Dinoroseobacter shibae on Its Symbiosis With the Dinoflagellate Prorocentrum minimum

Abstract

The marine bacterium Dinoroseobacter shibae shows a Jekyll-and-Hyde behavior in co-culture with the dinoflagellate Prorocentrum minimum: In the initial symbiotic phase it provides the essential vitamins B12 (cobalamin) and B1 (thiamine) to the algae. In the later pathogenic phase it kills the dinoflagellate. The killing phenotype is determined by the 191 kb plasmid and can be conjugated into other Roseobacters. From a transposon-library of D. shibae we retrieved 28 mutants whose insertion sites were located on the 191 kb plasmid. We co-cultivated each of them with P. minimum in L1 medium lacking vitamin B12. With 20 mutant strains no algal growth beyond the axenic control lacking B12 occurred. Several of these genes were predicted to encode proteins from the type IV secretion system (T4SS). They are apparently essential for establishing the symbiosis. With five transposon mutant strains, the initial symbiotic phase was intact but the later pathogenic phase was lost in co-culture. In three of them the insertion sites were located in an operon predicted to encode genes for biotin (B7) uptake. Both P. minimum and D. shibae are auxotrophic for biotin. We hypothesize that the bacterium depletes the medium from biotin resulting in apoptosis of the dinoflagellate.

Fatal affairs – conjugational transfer of a dinoflagellate-killing plasmid between marine Rhodobacterales

“…Therefore, SNPs in the Phaeobacter host and the transconjugants are not associated with the killing effect. Rather, it must be encoded by one or several of the 82 genes that are specific for the 191 kb plasmid and not found on the 126 kb plasmid [ 45 ].…”

“…not present on the 126 kb sister plasmid. A subset of these genes has been investigated for their influence on the growth of the dinoflagellate in co-culture [ 45 ]. The killing phenotype was lost if genes of a putative biotin ABC transporter operon ( bioYMN , Dshi_3685 to Dshi_3687) were inactivated.…”

Identification of Volatiles of the Dinoflagellate Prorocentrum cordatum

“…The large difference in the number of compounds, including several still unidentified, mostly minor components as well as the formation of the dominating compounds 15, 16 and 18, may be a direct consequence of missing bacteria in the strain P. cordatum CCMP 1329. It has been shown that P. cordatum growing symbiotically with the roseobacter-group bacterium Dinoroseobacter shibae obtains vitamins B 1 and B 12 from the bacterium [17,30]. The medium of the axenic strain did contain these vitamins, thus excluding the possibility that the increased VOC production is a direct consequence of a lack of vitamins.…”

“…The axenic P. cordatum strain CCMP 1329 has been studied as a model host for the roseobacter-group bacterium Dinoroseobacter shibae, and it could be shown that the bacteria can provide vitamins to the dinoflagellate but kill the algae during later stages of growth [17]. Killing may be the result of competition between the alga and bacterium for biotin, a vitamin that is essential for both organisms [30]. The microbiome associated to strains of P. cordatum maintained in culture collections, e.g., CCMP 1529, differs according to the biogeographical region and time since isolation (Sanchez-Garcia, in preparation).…”

The Sixth Element: a 102-kb RepABC Plasmid of Xenologous Origin Modulates Chromosomal Gene Expression in Dinoroseobacter shibae

“… Dinoroseobacter shibae DFL 12 is one model organism of the Roseobacteraceae and was originally isolated from the phycosphere of the dinoflagellate Prorocentrum lima ( 28 ). It has been well investigated for its quorum sensing system ( 29 ), gene transfer agent ( 30 ), metabolic adaptations ( 31 , 32 ), and interaction with the algal host ( 28 , 33 – 35 ). The genome of D. shibae comprises five ECRs: two chromids (153 and 72 kb) with chromosome-like GC content and codon usage and three genuine plasmids (191, 126, and 86 kb) showing a largely deviating genomic imprint ( 23 , 27 ).…”