Competition between phytoplankton and bacteria: exclusion and coexistence

Interactions between Microcystis and its associated bacterial community on electron transfer and transcriptomic processes

Microcystis and bacteria always live together in the mucilage of Microcystis colonies. Extracellular electrons between Microcystis and bacteria can be translated from bioenergy to electric energy. Here, photosynthetic microbial fuel cells (PMFCs) were constructed to make clear the electron transfer mechanism between Microcystis and bacteria. A remarkable enhancement of current density with 2.5-fold change was detected in the coculture of Microcystis and bacteria than pure culture of Microcystis. Transcriptome analyses showed that photosynthesis efficiency of Microcystis was upregulated and may release more electron to improve extracellular electron transfer rate. Significant increase on oxidative phosphorylation of bacterial community was observed according to meta-transcriptome. Bacterial electrons were transferred out of cell membranes by enhancing VgrG and IcmF copies though the type II bacterial secretion system. Not only Microcystis and bacteria attached with each other tightly by filamentous, but also more gene copies relating to pilin and riboflavin production were detected from Microcystis culture. A confirmatory experiment found that riboflavin can upregulate the electron transfer and current density by adding riboflavin into cocultures. Thus, the direct contact and indirect interspecies electron transfer processes between Microcystis and bacteria were observed. Results enlarge knowledge for activities of Microcystis colonies in cyanobacterial blooms, and provide a better understanding for energy transformation.

Optimal Darwinian Selection of Microorganisms with Internal Storage

In this paper, we investigate the problem of species separation in minimal time. Droop model is considered to describe the evolution of two distinct populations of microorganisms that are in competition for the same resource in a photobioreactor. We focus on an optimal control problem (OCP) subject to a five-dimensional controlled system in which the control represents the dilution rate of the chemostat. The objective is to select the desired species in minimal-time and to synthesize an optimal feedback control. This is a very challenging issue, since we are are dealing with a ten-dimensional optimality system. We provide properties of optimal controls allowing the strain of interest to dominate the population. Our analysis is based on the Pontryagin Maximum Principle (PMP), along with a thorough study of singular arcs that is crucial in the synthesis of optimal controls. These theoretical results are also extensively illustrated and validated using a direct method in optimal control (via the Bocop software for numerically solving optimal control problems). The approach is illustrated with numerical examples with microalgae, reflecting the complexity of the optimal control structure and the richness of the dynamical behavior.

Pilot-scale constructed bypass channel for urban river restoration: the remedial efficiency and the variance in biodiversity

Like the blood vessels of the cities, urban rivers play a significant role on maintaining the cities' sustainable development. In addition to the prevention of pollutants discharge and improving the river water quality, the restoration of the urban rivers' ecosystem should be well concerned. To fill this gap, a pilot-scale study with constructed bypass channel (CBC) was conducted. The pollutants reduction by the aquatic plants of the CBC was evaluated, and the similarities/differences of the aquatic biodiversity between the CBC and the natural rivers were analyzed. The results indicated that the mean removal efficiency of TP, NH₃-N, TN, and COD by the CBC was 66%, 60%, 52%, and 36%, respectively. Chlorophyta, Bacillariophyta, and Cyanophyta were the dominant phytoplankton phyla in the CBC which was in accordance with the studies conducted in the Dongjiang River and the Pearl River. During the study period of about 6 months, the population density and the biomass of the phytoplankton and the zooplankton increased over time. The quality of the influent dominated the aquatic organisms' diversity of the CBC. N-element dominated not only the phytoplankton variability, but also the bacterial species of the CBC. The timber pile and the submerged plant root soil were more suitable for the growth of the functional bacteria; thus, the construction of the river restoration infrastructures should avoid using large-scale cement materials. Overall, the study would improve the understanding of urban river restoration practice and provide guidance for future restoration practice especially from the aquatic ecological perspectives.

Global changes may be promoting a rise in select cyanobacteria in nutrient-poor northern lakes

The interacting effects of global changes-including increased temperature, altered precipitation, reduced acidification and increased dissolved organic matter loads to lakes-are anticipated to create favourable environmental conditions for cyanobacteria in northern lakes. However, responses of cyanobacteria to these global changes are complex, if not contradictory. We hypothesized that absolute and relative biovolumes of cyanobacteria (both total and specific genera) are increasing in Swedish nutrient-poor lakes and that these increases are associated with global changes. We tested these hypotheses using data from 28 nutrient-poor Swedish lakes over 16 years (1998-2013). Increases in cyanobacteria relative biovolume were identified in 21% of the study sites, primarily in the southeastern region of Sweden, and were composed mostly of increases from three specific genera: Merismopedia, Chroococcus and Dolichospermum. Taxon-specific changes were related to different environmental stressors; that is, increased surface water temperature favoured higher Merismopedia relative biovolume in low pH lakes with high nitrogen to phosphorus ratios, whereas acidification recovery was statistically related to increased relative biovolumes of Chroococcus and Dolichospermum. In addition, enhanced dissolved organic matter loads were identified as potential determinants of Chroococcus suppression and Dolichospermum promotion. Our findings highlight that specific genera of cyanobacteria benefit from different environmental changes. Our ability to predict the risk of cyanobacteria prevalence requires consideration of the environmental condition of a lake and the sensitivities of the cyanobacteria genera within the lake. Regional patterns may emerge due to spatial autocorrelations within and among lake history, rates and direction of environmental change and the niche space occupied by specific cyanobacteria.

Hydrogenotrophic methanogens of the mammalian gut: Functionally similar, thermodynamically different-A modelling approach

Methanogenic archaea occupy a functionally important niche in the gut microbial ecosystem of mammals. Our purpose was to quantitatively characterize the dynamics of methanogenesis by integrating microbiology, thermodynamics and mathematical modelling. For that, in vitro growth experiments were performed with pure cultures of key methanogens from the human and ruminant gut, namely Methanobrevibacter smithii, Methanobrevibacter ruminantium and Methanobacterium formicium. Microcalorimetric experiments were performed to quantify the methanogenesis heat flux. We constructed an energetic-based mathematical model of methanogenesis. Our model captured efficiently the dynamics of methanogenesis with average concordance correlation coefficients of 0.95 for CO₂, 0.98 for H₂ and 0.97 for CH₄. Together, experimental data and model enabled us to quantify metabolism kinetics and energetic patterns that were specific and distinct for each species despite their use of analogous methane-producing pathways. Then, we tested in silico the interactions between these methanogens under an in vivo simulation scenario using a theoretical modelling exercise. In silico simulations suggest that the classical competitive exclusion principle is inapplicable to gut ecosystems and that kinetic information alone cannot explain gut ecological aspects such as microbial coexistence. We suggest that ecological models of gut ecosystems require the integration of microbial kinetics with nonlinear behaviours related to spatial and temporal variations taking place in mammalian guts. Our work provides novel information on the thermodynamics and dynamics of methanogens. This understanding will be useful to construct new gut models with enhanced prediction capabilities and could have practical applications for promoting gut health in mammals and mitigating ruminant methane emissions.