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What intrinsic and extrinsic factors explain the stoichiometric diversity of aquatic heterotrophic bacteria?

ABSTRACT: The elemental content of microbial communities is dependent upon the physiology of constituent populations, yet ecological stoichiometry has made slow progress toward identifying predictors of how species and strains change the elemental content of their biomass in response to the stoichiometry of elements in resources. We asked whether the elemental content of aquatic bacteria, especially flexibility in elemental content, could be predicted by their phylogeny, maximum growth rate or lake productivity. We examined 137 isolates using chemostats and found that strains differed substantially in how the carbon:nitrogen:phosphorus ratios (C:N:P) in their biomass responded to P-sufficient and P-limiting conditions. The median strain increased its biomass C:N:P from 68:14:1 to 164:25:1 under P limitation. Patterns in elemental content and ratios were partly explained by phylogeny, yet flexibility in elemental content showed no phylogenetic signal. The growth rate hypothesis predicts that P content is positively related to growth rate, but we found weak correlation between maximum growth rate and P content among the strains. Overall, isolates from highly productive lakes had higher maximum growth rates and less flexible biomass N:P than isolates from unproductive lakes. These results show that bacteria present within lake communities exhibit diverse strategies for responding to elemental imbalance.

SUBMITTER: Godwin CM

PROVIDER: S-EPMC5776474 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:It is frequently presumed that heterotrophic bacteria from aquatic environments have low carbon (C) content, high phosphorus (P) content, and maintain homeostasis at low C:P in their biomass. Dissolved and particulate organic matter from primary producers in terrestrial and aquatic environments typically has high C:P ratios, suggesting that heterotrophic bacteria consuming this resource experience stoichiometric imbalance in C and P. The strength of elemental homeostasis is important for understanding how heterotrophic bacteria couple C and P cycles in response to environmental change, yet these generalizations are based upon data from only a few species that might not represent the physiology of bacteria in freshwaters. However, recent research has indicated that some strains of bacteria isolated from freshwaters have flexible C:P stoichiometry and can acclimate to changes in resource C:P. Although it is apparent that strains differ in their biomass C:P and flexibility, the basis for these characteristics has not been explained. We evaluated biomass C:P homeostasis in 24 strains of bacteria isolated from temperate lakes using a uniform relative growth rate in chemostats. Overall, the strains exhibited a range of homeostatic regulation from strong homeostasis to highly flexible biomass stoichiometry, but strains that were isolated using P-rich media formulations were more homeostatic than strains isolated using P-poor media. Strains exhibiting homeostatic biomass C:P had high cellular C and P content and showed little morphological change between C and P limitation. In contrast, stoichiometrically flexible strains had low P quotas and increased their C quotas and cell size under P limitation. Because stoichiometric flexibility is closely coupled to absolute P content in bacteria, anthropogenic inputs of P could lead to prevalence of more homeostatic bacteria, reducing the ability of natural assemblages to buffer changes in the availability of P and organic C.

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What intrinsic and extrinsic factors explain the stoichiometric diversity of aquatic heterotrophic bacteria?

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