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Microbial Ecology

, Volume 76, Issue 1, pp 49–51 | Cite as

Egalitarianism in Biofilms

  • Dawen Gao
  • Yu Tao
  • Yuan Fu
  • Hong Liang
Notes and Short Communications

Abstract

Microbial biofilms are multicellular communities of sessile microorganisms encased by the hydrated polymeric matrix. They have significant influences on both aquatic/terrestrial ecosystem and anthropogenic activities. Taking advantage of the governing features of selective stress (Tan and Ng in Water Res 42:1122–1132, 2008; Wei in Water Res 45:863–871, 2011; Dereli in Water Res 59C:11–22, 2014), the evenness of microbial communities in a membrane-centered mesocosm was successfully manipulated. By measuring the biofilm growing rates under different evenness levels of communities, an evenly distributed community favors the formation of biofilms was observed. This finding is not only a new evidence linking biofilm diversity to its functionality but also a clear suggestion on controlling a biofilm-based process via a simple and smart way.

Keywords

Biofouling Biofilm Microbial ecology Community evenness 

In 1912, Corrado Gini presented the Gini coefficient (GC) to define the inequality of social income. Since then, the interpretation of this economic concept has been extended to a wider area of science such as medicine [13], chemistry [14], and ecology [15]. Ecologists adopted the concept of GC to describe the equality of species in an ecosystem [10]. It is a single value that describes a specific degree of evenness ranging from 0 to 1 [21]. An absolutely even community (GC = 1) is defined as a one in which each species is equally abundant, while the community is extremely uneven (GC is close to zero) when a single species is the only dominant one in the community [10].

Microorganisms always choose to develop an intricate aggregate (biofilms) onto a surface with supply of nutrients and water [8, 12, 16, 17]. It is ubiquitous in many ecosystems and has been hot topics in microbial ecology for a long time [3, 4]. A lot of factors have been proved to affect the growing features of biofilms [15]. However, less is known on how the community biodiversity, as a response to environmental perturbations, reflects the growing status of biofilms. Recent studies have revealed a close relationship between community evenness and its functional stability [11], indicating that a community evenness could be an important factor governing the growing of biofilms. In this study, the growing rates of biofilms under different evenness levels were measured by manipulating the evenness levels of their communities in a mesocosm based on the selective stresses principle.

The 8-L mesocosm (Fig. 1) consisted of hollow fiber membrane sheets, which not only supplied spaces for biofilms to grow on but also retained nearly all the microbial cells in the mesocosm. Different from natural biofilm harbors where microorganisms were basically fed with randomly transported substrates, the biofilms in our mesocosm received a continuous supply of substrates due to the negative pressure applied on the membranes. The controlled environment of the mesocosm allowed the microorganisms existing at each depth of the microarchitecture to have equal opportunities to receive substrates, so the microbial diversity was less governed by bioavailability [2]. Another advantage of applying the mesocosm is that the growing rate of biofilms can be reflected by the changing pressure created inside the fibers, and such changes in pressure can be continuously measured by a transmembrane pressure gauge (Fig. 1). The microbial assembly was determined by applying terminal restriction fragment length polymorphism (T-RFLP) analysis [6], and the evenness of each community was plotted in the form of the Lorenz evenness distribution curves [9] (Fig. 2a).
Fig. 1

A brief diagram of the membrane-centered mesocosm. A polyethylene hollow fiber membrane module (Mitsubishi Rayon Co., Ltd. Tokyo, Japan) was located at the bottom with a total area of 0.11 m2 and an average pore size of 0.4 μm, which was smaller than the size of most bacterial cells. The mesocosm was inoculated with activated sludge biomass from a bioreactor treating raw domestic wastewater

Fig. 2

The Lorenz distribution curves (a) and microbial abundance under changing selective stresses (b) based on T-RFLP fingerprints. The 45° diagonal is the theoretical perfect evenness line representing an absolutely even microbial community. The normalized area between a curve and 45° diagonal stands for the Gini coefficient of a given community. The details of each selective stressor are the following: O1, the dissolved oxygen concentration of 0.5 mg L−1; O2, the dissolved oxygen concentration of 2.0 mg L−1; P1, the transmembrane flux of 5.88 L m−2 h−1; P2, the transmembrane flux of 13.42 L m−2 h−1; R1, the cell retention time of 10 days; R2, the cell time of 60 days

The application of three selective stressors (i.e., oxygen (O), cell retention (R), and pressure (P)) drove clear variations of species distribution in biofilms (Fig. 2b). Intriguingly, the degree of such distribution was highly relevant to biofilm growth rate (Fig. 3). For example, for the evenly distributed biofilms, i.e., the ones under the selective stressors of O1 and P1 (GC < 0.2), the pressure changing rates were as high as > 3.0 Pf d−1, while for the two most uneven communities (R1 and R2), the slowest pressure change rates (< 0.6 Pf d−1) were observed. It suggests that an evenly distributed community favors the formation of biofilms. In other words, the biofilms with a more evenly distributed species assembly gain a stronger expanding capacity. It has been suggested that unevenness usually leads to lower functionality, and it is more significant when under environmental stresses [1, 21]. Since biofilm growth closely links to microbial functionality, it is understandable that biofilm formation is also governed by community evenness. In fact, the even distribution of species may introduce a guarantee that some will associate with each other for a rapid growth.
Fig. 3

The correlation between the evenness level and biofilm growing rate. The level of evenness is presented by GC, and the biofilm growing rate is shown as the changing rates of transmembrane pressure. Pf indicates the pressure (MPa) under a constant flux (LMH, L m−2 h−1) and its unit is MPa LMH−1

This study hints to not ignore the egalitarianism of microorganisms when studying biofilms, and the conclusion would benefit biofilm-based industrial applications. For example, for some wastewater treatment technologies relying on the formation of biofilms [7], we need to maintain the system with high evenness of microbial community. On the opposite, for a membrane bioreactor, a widely applied technique that is hindered by unwanted biofilms (i.e., membrane fouling) [18], an uneven microbial community might contribute to slow down the biofilm-forming processes. An in-depth understanding of the relationship of biofilm formation and its community features enables us to better design biofilm-based bioprocesses in future. Hence, from an ecology perspective, biofilm formation could be controlled based on the evenness of microbial community.

Notes

Author Contributions

D.G. designed the experiments. Y.F. performed the experiments. D.G., Y.T., Y.F., and H.L. analyzed the data. D.G. and Y.T. wrote the manuscript.

Funding Information

This research was supported by the Natural Science Foundation of Heilongjiang Province (No. ZD201412) and the National Natural Science Foundation of China (No. 21177033).

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.State Key Laboratory of Urban Water Resource and EnvironmentHarbin Institute of TechnologyHarbinChina

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