The Thermotoga maritima phenotype is impacted by syntrophic interaction with Methanococcus jannaschii in hyperthermophilic coculture

Abstract

Significant growth phase-dependent differences were noted in the transcriptome of the hyperthermophilic bacterium Thermotoga maritima when it was cocultured with the hyperthermophilic archaeon Methanococcus jannaschii. For the mid-log-to-early-stationary-phase transition of a T. maritima monoculture, 24 genes (1.3% of the genome) were differentially expressed twofold or more. In contrast, methanogenic coculture gave rise to 292 genes differentially expressed in T. maritima at this level (15.5% of the genome) for the same growth phase transition. Interspecies H2 transfer resulted in three- to fivefold-higher T. maritima cell densities than in the monoculture, with concomitant formation of exopolysaccharide (EPS)-based cell aggregates. Differential expression of specific sigma factors and genes related to the ppGpp-dependent stringent response suggests involvement in the transition into stationary phase and aggregate formation. Cell aggregation was growth phase dependent, such that it was most prominent during mid-log phase and decayed as cells entered stationary phase. The reduction in cell aggregation was coincidental with down-regulation of genes encoding EPS-forming glycosyltranferases and up-regulation of genes encoding beta-specific glycosyl hydrolases; the latter were presumably involved in hydrolysis of beta-linked EPS to release cells from aggregates. Detachment of aggregates may facilitate colonization of new locations in natural environments where T. maritima coexists with other organisms. Taken together, these results demonstrate that syntrophic interactions can impact the transcriptome of heterotrophs in methanogenic coculture, and this factor should be considered in examining the microbial ecology in anaerobic environments.

FIG. 1.

Growth of T. maritima in pure culture and in coculture with M. jannaschii. (A) Epifluorescence micrographs of pure culture and coculture corresponding to the growth curves are shown from left to right. After inoculation, the pure culture grew without aggregating to a density of above 10⁸ cells/ml before entering a prolonged stationary phase, during which time the T. maritima cell morphology changed from rods to cocci. The coculture began to aggregate once cell densities reached approximately 5 × 10⁸ cells/ml until entering stationary phase (∼10⁹ cells/ml). In stationary phase, cells detached from aggregates, and by 24 h they displayed coccoid morphology similar to what is seen in the pure culture during late stationary phase. (B). Growth curves for T. maritima grown in pure culture and in coculture with M. jannaschii. Error bars indicate standard deviations. (C) Volcano plots comparing expression profiles of pure T. maritima culture (top) and coculture of T. maritima with M. jannaschii (bottom) during the transition of growth phases from mid-log to early stationary phase. The x axis is the log₂ fold change from mid-log to early stationary phase, and the y axis is the −log₁₀ P value for the calculated fold change.

FIG. 2.

Relative expression as shown by the least-squares mean expression of putative sigma factors in T. maritima under conditions of hydrogen removal (C, coculture) compared to that of hydrogen accumulation (P, quiescent pure culture) for mid-log (ML), early-stationary (ES), and late-stationary (LS) phases transitions.