Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2018 Jun 5;9:1150.
doi: 10.3389/fmicb.2018.01150. eCollection 2018.

Adaptation of the Freshwater Bloom-Forming Cyanobacterium Microcystis aeruginosa to Brackish Water Is Driven by Recent Horizontal Transfer of Sucrose Genes

Affiliations
Free PMC article

Adaptation of the Freshwater Bloom-Forming Cyanobacterium Microcystis aeruginosa to Brackish Water Is Driven by Recent Horizontal Transfer of Sucrose Genes

Yuuhiko Tanabe et al. Front Microbiol. .
Free PMC article

Abstract

Microcystis aeruginosa is a bloom-forming cyanobacterium found in eutrophic water bodies worldwide. M. aeruginosa blooms usually occur in freshwater; however, they have also been reported to occur in brackish water. Because M. aeruginosa often produces the cyanotoxin microcystin, they are a major concern to public health and environment. Despite this, the ecology, genomic basis, and evolutionary process underlying the M. aeruginosa bloom invasion from fresh to brackish water have been poorly investigated. Hence, in the present study, we have sequenced and characterized genomes of two newly discovered salt-tolerant M. aeruginosa strains obtained from Japanese brackish water lakes (Lakes Shinji and Tofutsu). Both genomes contain a set of genes for the synthesis of osmolyte sucrose (sppA, spsA, and susA), hitherto identified in only one strain (PCC 7806) of M. aeruginosa. Chemical and gene expression analyses confirmed sucrose accumulation induced by salt. A comprehensive genetic survey of >200 strains indicated that sucrose genes are extremely rare in M. aeruginosa. Most surprisingly, comparative genome analyses of the three strains indicated extremely low genetic diversity in the sucrose genes compared with other core genome genes, suggesting very recent acquisitions via horizontal transfer. Invasion of M. aeruginosa blooms into brackish water may be a recent event triggered by anthropogenic eutrophication of brackish water.

Keywords: Microcystis; bloom; brackish water; ecotype; genomics; horizontal gene trasnfer; salt tolerance; sucrose.

Figures

Figure 1
Figure 1
Geographic location and genotypic composition of M. aeruginosa in Japanese brackish waters. Different colors in the pie charts indicate clones belonging to different phylogenetic groups (defined in Figure 2). Sj belongs to none of these groups and is indicated in a specific color (purple). The genotypic composition is based on ftsZ (Tanabe et al., 2007). Note that group A in Lake Abashiri is represented by a single genotype. Detailed geographic location of sampling sites, sampling dates, and salinities are shown in Supplementary Table S1. n indicates number of clones. The data for two locations in Lake Togo are highly similar; therefore, only the data related to Shizen-Koen is shown. Inset: the magnification of Lake Shinji. Sh, Shinji-cho; Oh, Ohashi; Ch, Chidori.
Figure 2
Figure 2
Phylogeny of salt-tolerant M. aeruginosa. A multilocus neighbor-joining (NJ) phylogenetic tree based on seven housekeeping loci (Tanabe et al., 2007). Groups are defined according to a previous report (Tanabe and Watanabe, 2011) with three new designations (groups H, I, and J). Numbers at the selected nodes indicate bootstrap values inferred from RaxML. M. aeruginosa strains for which a whole genome sequence was available are indicated by “°” and the strain name. Strain PCC 9717 is indicated in half-orange color because it has an incomplete microcystin synthetase gene cluster (Humbert et al., 2013). A possible placement of a Lake Abashiri clone (positive for sucrose genes) in group A is indicated with “?.” Scale bar, 0.005 substitutions per sites. Inset: Salt tolerance of three sucrose gene-positive strains and one sucrose gene-negative strain. Note that psu (salt concentration determined from conductivity) and NaCl concentration in MA medium are substantially the same value.
Figure 3
Figure 3
Chemical analysis of intracellular sucrose and sucrose gene expression analyses. (A), Sucrose gene products and their contributions to sucrose metabolism in M. aeruginosa. (B), Sucrose concentrations after 24 h incubation with or without 10 g l−1 NaCl. The dotted line indicates the detection limit of liquid chromatography. The sucrose concentration was >3 times higher in salt-treated cells than controls. (C), Results of RT-qPCR after the same treatment in (B). (D), Results of RT-qPCR after 24 h incubation with or without 10 g l−1 NaCl using salt-acclimated cells as a seed. Bars indicate 95 % confidence intervals calculated from three biological and three technical replicates (by Student's t, df = 16). Statistical values are the results of homoscedastic one-tailed t-tests of ΔCT means: *P < 0.05; **P < 0.01; ***P < 0.001.
Figure 4
Figure 4
Genetic diversity of sucrose genes. (A), DNA polymorphisms within sucrose genes, and the adjacent leuS locus encoding leucyl-tRNA synthetase. A nucleotide site different from that of Sj is indicated by a red bar. PCR targets for sucrose genotyping are indicated below the gene diagram. Genes located downstream of leuS showed a level of genetic diversity similar to that shown by leuS (Supplementary Figure S6). (B), Genetic distances of sucrose genes and selected core genome genes (>650 bps in length) including the seven MLST loci (Tanabe et al., 2007) and leuS. The core genome genes included in this analysis are listed in Supplementary Table S6. X-axis indicates nucleotide length of the genes. Y-axis indicates percent nucleotide difference (p-distance) on a log scale.
Figure 5
Figure 5
Phylogenetic trees. (A), The phylogenetic tree modified from the conserved protein-based tree with the group designations by Shih et al. (2013). M. aeruginosa Sj and NIES-1211 are not included in the analysis. However, whole-genome blast analyses indicated that Sj and NIES-1211 are most closely related to PCC 7806 in the tree (indicated in parentheses). (B), A NJ phylogenetic tree of susA. Bootstrap values (>70) on the basis of 1 000 replicates are indicated at the respective nodes. Numbers in parentheses after the strain name indicate GenBank protein IDs. The color-coding in (B) is according to (A). susB (a homolog of susA; Kolman et al., 2012) is highlighted by the orange box. OTUs in gray in (B) are compressed. Scale bar, substitutions per sites.
Figure 6
Figure 6
A schematic of the hypothesized M. aeruginosa bloom occurrence in Japanese brackish water. In the past, M. aeruginosa blooms were restricted to freshwater systems because M. aeruginosa is salt-sensitive. At some point in time, one M. aeruginosa genotype became salt-tolerant owing to the acquisition of sucrose genes via HGT. Recent eutrophication of brackish water systems caused by human activity has driven the subsequent spread of the sucrose genes in different genotypes of M. aeruginosa. As a result, salt-tolerant M. aeruginosa have become prevalent and can form blooms in brackish water systems. Note that different numbers indicate different genotypes.

Similar articles

See all similar articles

Cited by 7 articles

See all "Cited by" articles

References

    1. Bankevich A., Nurk S., Antipov D., Gurevich A. A., Dvorkin M., Kulikov A. S., et al. . (2012). SPAdes: a new genome assembly algorithm and its applications to single-cell sequencing. J. Comput. Biol. 19, 455–477. 10.1089/cmb.2012.0021 - DOI - PMC - PubMed
    1. Beasley V. R., Cook W. O., Dahlem A. M., Hooser S. B., Lovell R. A., Valentine W. M. (1989). Algae intoxication in livestock and waterfowl. Food Anim. Pract. 5, 345–361. 10.1016/S0749-0720(15)30980-4 - DOI - PubMed
    1. Cohan F. M. (2002). What are bacterial species? Annu. Rev. Microbiol. 56, 457–487. 10.1146/annurev.micro.56.012302.160634 - DOI - PubMed
    1. Cohan F. M. (2011). Are species cohesive? – A view from bacteriology, in Population Genetics of Bacteria: A Tribute to Thomas S Whittam, eds Walk S. T., Feng P. C. H., editors. (Washington, DC: ASM press; ), 43–65.
    1. Curatti L., Flores E., Salerno G. (2002). Sucrose is involved in the diazotrophic metabolism of the heterocyst-forming cyanobacterium Anabaena sp. FEBS Lett. 513, 175–178. 10.1016/S0014-5793(02)02283-4 - DOI - PubMed

LinkOut - more resources

Feedback