Dynamic changes in community composition of Scenedesmus-seeded artificial, engineered microalgal biofilms

Non-destructive monitoring of microalgae biofilms

Biofilm-based cultivation systems are emerging as a promising technology for microalgae production. However, efficient and non-invasive monitoring routines are still lacking. Here, a protocol to monitor microalgae biofilms based on reflectance indices (RIs) is proposed. This framework was developed using a rotating biofilm system for astaxanthin production by cultivating Haematococcus pluvialis on cotton carriers. Biofilm traits such as biomass, astaxanthin, and chlorophyll were characterized under different light and nutrient regimes. Reflectance spectra were collected to identify the spectral bands and the RIs that correlated the most with those biofilm traits. Robust linear models built on more than 170 spectra were selected and validated on an independent dataset. Astaxanthin content could be precisely predicted over a dynamic range from 0 to 4% of dry weight, regardless of the cultivation conditions. This study demonstrates the strength of reflectance spectroscopy as a non-invasive tool to improve the operational efficiency of microalgae biofilm-based technology.

Microalgal-bacterial biofilms for wastewater treatment: Operations, performances, mechanisms, and uncertainties

Microalgal-bacterial biofilms have been increasingly considered of great potential in wastewater treatment due to the advantages of microalgal-bacterial synergistic pollutants removal/recovery, CO2 sequestration, and cost-effective biomass-water separation. However, such advantages may vary widely among different types of microalgal-bacterial biofilms, as the biofilms could be formed on different shapes and structures of attachment substratum, generating "false hope" for certain systems in large-scale wastewater treatment if the operating conditions and pollutants removal properties are evaluated based on the general term "microalgal-bacterial biofilm". This study, therefore, classified microalgal-bacterial biofilms into biofilms formed on 2D substratum, biofilms formed on 3D substratum, and biofilms formed without substratum (i.e. microalgal-bacterial granular sludge, MBGS). Biofilms formed on 2D substratum display higher microalgae fractions and nutrients removal efficiencies, while the adopted long hydraulic retention times were unacceptable for large-scale wastewater treatment. MBGS are featured with much lower microalgae fractions, most efficient pollutants removal, and acceptable retention times for realistic application, yet the feasibility of using natural sunlight should be further explored. 3D substratum systems display wide variations in operating conditions and pollutants removal properties because of diversified substratum shapes and structures. 2D and 3D substratum biofilms share more common in eukaryotic and prokaryotic microbial community structures, while MGBS biofilms are more enriched with microorganisms favoring EPS production, biofilm formation, and denitrification. The specific roles of stratified extracellular polymeric substances (EPS) in nutrients adsorption and condensation still require in-depth exploration. Nutrients removal uncertainties caused by microalgal-bacterial synergy decoupling under insufficient illumination, limited microbial community control, and possible greenhouse gas emission exacerbation arising from microalgal N2O generation were also indicated. This review is helpful for revealing the true potential of applying various microalgal-bacterial biofilms in large-scale wastewater treatment, and will provoke some insights on the challenges to the ideal state of synergistic pollutants reclamation and carbon neutrality via microalgal-bacterial interactions.

Coordinated proteome change precedes cell lysis and death in a mat-forming cyanobacterium

Cyanobacteria form dense multicellular communities that experience transient conditions in terms of access to light and oxygen. These systems are productive but also undergo substantial biomass turnover through cell death, supplementing heightened heterotrophic respiration. Here we use metagenomics and metaproteomics to survey the molecular response of a mat-forming cyanobacterium undergoing mass cell lysis after exposure to dark and anoxic conditions. A lack of evidence for viral, bacterial, or eukaryotic antagonism contradicts commonly held beliefs on the causative agent for cyanobacterial death during dense growth. Instead, proteogenomics data indicated that lysis likely resulted from a genetically programmed response triggered by a failure to maintain osmotic pressure in the wake of severe energy limitation. Cyanobacterial DNA was rapidly degraded, yet cyanobacterial proteins remained abundant. A subset of proteins, including enzymes involved in amino acid metabolism, peptidases, toxin-antitoxin systems, and a potentially self-targeting CRISPR-Cas system, were upregulated upon lysis, indicating possible involvement in the programmed cell death response. We propose this natural form of cell death could provide new pathways for controlling harmful algal blooms and for sustainable bioproduct production.

Environmental Factors Associated with the Eukaryotic Microbial Community and Microalgal Groups in the Mountain Marshes of South Korea

The diversity indices of eukaryotic microalgal groups in the Jeonglyeongchi, Waegok, and Wangdeungjae marshes of Mount Jiri, Korea, were measured using Illumina MiSeq and culture-based analyses. Waegok marsh had the highest species richness, with a Chao1 value of 828.00, and the highest levels of species diversity, with Shannon and Simpson index values of 6.36 and 0.94, respectively, while Wangdeungjae marsh had the lowest values at 2.97 and 0.75, respectively. The predominant species in all communities were Phagocata sibirica (Jeonglyeongchi, 68.64%), Aedes albopictus (Waegok, 34.77%), Chaetonotus cf. (Waegok, 24.43%), Eimeria sp. (Wangdeungjae, 26.17%), and Eumonhystera cf. (Wangdeungjae, 22.27%). Relative abundances of the microalgal groups Bacillariophyta (diatoms) and Chlorophyta (green algae) in each marsh were respectively: Jeonglyeongchi 1.38% and 0.49%, Waegok 7.0% and 0.3%, and Wangdeungjae 10.41% and 4.72%. Illumina MiSeq analyses revealed 34 types of diatoms and 13 types of green algae. Only one diatom (Nitzschia dissipata) and five green algae (Neochloris sp., Chlamydomonas sp., Chlorococcum sp., Chlorella vulgaris, Scenedesmus sp.) were identified by a culture-based analysis. Thus, Illumina MiSeq analysis can be considered an efficient tool for analyzing microbial communities. Overall, our results described the environmental factors associated with geographically isolated mountain marshes and their respective microbial and microalgal communities.

Interaction between CO2-consuming autotrophy and CO2-producing heterotrophy in non-axenic phototrophic biofilms

As the effects of climate change become increasingly evident, the need for effective CO2 management is clear. Microalgae are well-suited for CO2 sequestration, given their ability to rapidly uptake and fix CO2. They also readily assimilate inorganic nutrients and produce a biomass with inherent commercial value, leading to a paradigm in which CO2-sequestration, enhanced wastewater treatment, and biomass generation could be effectively combined. Natural non-axenic phototrophic cultures comprising both autotrophic and heterotrophic fractions are particularly attractive in this endeavour, given their increased robustness and innate O2-CO2 exchange. In this study, the interplay between CO2-consuming autotrophy and CO2-producing heterotrophy in a non-axenic phototrophic biofilm was examined. When the biofilm was cultivated under autotrophic conditions (i.e. no organic carbon), it grew autotrophically and exhibited CO2 uptake. After amending its growth medium with organic carbon (0.25 g/L glucose and 0.28 g/L sodium acetate), the biofilm rapidly toggled from net-autotrophic to net-heterotrophic growth, reaching a CO2 production rate of 60 μmol/h after 31 hours. When the organic carbon sources were provided at a lower concentration (0.125 g/L glucose and 0.14 g/L sodium acetate), the biofilm exhibited distinct, longitudinally discrete regions of heterotrophic and autotrophic metabolism in the proximal and distal halves of the biofilm respectively, within 4 hours of carbon amendment. Interestingly, this upstream and downstream partitioning of heterotrophic and autotrophic metabolism appeared to be reversible, as the position of these regions began to flip once the direction of medium flow (and hence nutrient availability) was reversed. The insight generated here can inform new and important research questions and contribute to efforts aimed at scaling and industrializing algal growth systems, where the ability to understand, predict, and optimize biofilm growth and activity is critical.

Chlorella vulgaris and Its Phycosphere in Wastewater: Microalgae-Bacteria Interactions During Nutrient Removal

Microalgae-based bioenergy production is a promising field with regard to the wide variety of algal species and metabolic potential. The use of liquid wastes as nutrient clearly improves the sustainability of microalgal biofuel production. Microalgae and bacteria have an ecological inter-kingdom relationship. This microenvironment called phycosphere has a major role in the ecosystem productivity and can be utilized both in bioremediation and biomass production. However, knowledge on the effects of indigenous bacteria on microalgal growth and the characteristics of bacterial communities associated with microalgae are limited. In this study municipal, industrial and agricultural liquid waste derivatives were used as cultivation media. Chlorella vulgaris green microalgae and its bacterial partners efficiently metabolized the carbon, nitrogen and phosphorous content available in these wastes. The read-based metagenomics approach revealed a diverse microbial composition at the start point of cultivations in the different types of liquid wastes. The relative abundance of the observed taxa significantly changed over the cultivation period. The genome-centric reconstruction of phycospheric bacteria further explained the observed correlations between the taxonomic composition and biomass yield of the various waste-based biodegradation systems. Functional profile investigation of the reconstructed microbes revealed a variety of relevant biological processes like organic acid oxidation and vitamin B synthesis. Thus, liquid wastes were shown to serve as valuable resources of nutrients as well as of growth promoting bacteria enabling increased microalgal biomass production.