Metatranscriptomic Analyses of Diel Metabolic Functions During a Microcystis Bloom in Western Lake Erie (United States)

Abstract

This study examined diel shifts in metabolic functions of Microcystis spp. during a 48-h Lagrangian survey of a toxin-producing cyanobacterial bloom in western Lake Erie in the aftermath of the 2014 Toledo Water Crisis. Transcripts mapped to the genomes of recently sequenced lower Great Lakes Microcystis isolates showed distinct patterns of gene expression between samples collected across day (10:00 h, 16:00 h) and night (22:00 h, 04:00 h). Daytime transcripts were enriched in functions related to Photosystem II (e.g., psbA), nitrogen and phosphate acquisition, cell division (ftsHZ), heat shock response (dnaK, groEL), and uptake of inorganic carbon (rbc, bicA). Genes transcribed during nighttime included those involved in phycobilisome protein synthesis and Photosystem I core subunits. Hierarchical clustering and principal component analysis (PCA) showed a tightly clustered group of nighttime expressed genes, whereas daytime transcripts were separated from each other over the 48-h duration. Lack of uniform clustering within the daytime transcripts suggested that the partitioning of gene expression in Microcystis is dependent on both circadian regulation and physicochemical changes within the environment.

Keywords: Lake Erie; Microcystis; cyanobacterial blooms; metatranscriptomics; microcystin.

FIGURE 1

Map of Maumee Bay, western basin of Lake Erie indicating movements of bloom during 48-h survey. The seven sampling events were mapped to the four sites indicated on the map.

FIGURE 2

Physico-chemical water data over the course of the 48-h Lagrangian study. (A) Photopigment and microcystin toxin concentrations; (B) dissolved inorganic nitrogen and phosphorus ratios.

FIGURE 3

Phylogenetic breakdown of transcripts over the course of the 48-h sampling period – community composition by phyla. “Other” refers to bacterial reads that could not be unambiguously assigned to a phylum.

FIGURE 4

Abundances of Proteobacteria (A) and Cyanobacteria (B) reads by order and Bacteroidetes reads (C) by class.

FIGURE 5

Principal Component Analysis (PCA) of transcripts obtained from each time point during the study.

FIGURE 6

Diel transcriptional patterns of Microcystis photosynthesis genes. (A,B) Phycobilisome subunits; (C) Photosystem II; (D) cytochrome b₆/f; (E), Photosystem I; (F), ferredoxin and plastocyanin. Gray shading indicates time periods between sunset and sunrise.

FIGURE 7

Diel transcriptional patterns of Microcystis inorganic carbon acquisition genes. Gray shading indicates time periods between sunset and sunrise.

FIGURE 8

Transcriptional patterns of Microcystis nutrient assimilation genes. (A) GS-GOGAT pathway; (B) nitrite reductase; (C) N regulation genes; (D) phosphorus assimilation; (E) urea assimilation. Gray shading indicates time periods between sunset and sunrise.

FIGURE 9

Transcriptional patterns of Microcystis cell division genes ftsH and ftsZ. Gray shading indicates time periods between sunset and sunrise.

FIGURE 10

Transcriptional patterns of Microcystis microcystin biosynthesis genes. Gray shading indicates time periods between sunset and sunrise.

FIGURE 11

Transcriptional patterns of Microcystis stress response genes (heat shock, high light, phycobilisome stability). Gray shading indicates time periods between sunset and sunrise.

FIGURE 12

Summary of significant Microcystis functions expressed during the 24 h cycle in the Toledo, 2014 bloom event. Lines of the clock indicate sunrise at 0654 h and sunset at 2016 h on August 26, 2014.