Mixotrophic growth of a consortium of microalgae on waste effluents for the production of fatty acid esters

Context

This PhD work is supported by the PEPR B-BEST project WAEster, “Environmental biorefinery (EB) approach for the production of fatty acid esters and co-products from microalgae cultivated on waste fermentation effluents”, that involves several research laboratories (LBE Narbonne, IATE Montpellier, TBI & LGC Toulouse) and transfer technology centres (CRITT-Bio-industries and CRITT GPTE Toulouse).
Dark fermentation (DF) is a process where organic waste is converted by bacteria into hydrogen gas and organic compounds such as volatile fatty acids, and results in a liquid nutrient-rich effluent (DFE). These effluents can be a cheap source for the growth in photobioreactors of mixotrophic microalgae such as Chlorella sorokiniana and Euglena gracilis. The lipids accumulated by the algae can be transesterified into fatty acid esters (FAEs) and used as biofuel or platform molecules. However, during algal cultures on DFEs, the fermentative bacteria compete with the microalgae for substrate which can negatively affect the growth yield and valorization potential of the biomass. An innovative approach to this problem, subject of the WAEster project, is the use of the phagotrophic microalga Ochromonas danica, which can in fact consume these bacteria. Since O. danica does not efficiently fix the CO2 released by the bacteria, consortia with Chlorella type algae can maximize carbon capture from the effluents and optimize growth yields. The conditions strongly influence the growth of the algae, with light intensity being crucial for Chlorella spp. and oxygen (aeration) for O. danica. Stress caused by aeration and/or agitation is a factor to be considered since O. danica is relatively fragile due to the lack of a cell wall. In addition, the lipid accumulation in the algae can be strongly affected by the C:N:P ratio of the DFE, and may be modified to optimize lipid production. Since FAEs are the final product of the WAEster process, the level at which lipids can be accumulated and functionalized will determine the valorization potential of the produced biomass. The development of a digital twin, based on a mechanistic model of the photobioreactor by the IATE partner of the project, will help to better understand and control the factors that determine the mixed algal culture. 

Objective and approach

Cultures of O. danica and Chlorella species on different DFEs were so far done in standard culture conditions. The aim of this study is to optimize growth on DFEs of O. danica and C. vulgaris in consortia, focusing in particular on the main growth parameters such as light, aeration and oxygen levels, pH and temperature. DFEs will be produced and used untreated, including bacteria, for microalgae cultivation, and therefore organic loading rate/hydraulic retention time need to be evaluated. Experimental data will be evaluated for use in the development of the digital twin, and some dedicated experiments may be called for. Cultures will be analyzed for algal growth (dry weight, algae cell counting) and biomass composition (lipids, sugars), as well as metabolites in the culture medium (HPLC, GC). Carbon can be analyzed via TIC/TOC or elemental analyzer. Potentially, omics type analyses may be employed to better understand observed physiology. The essential performance parameters are biomass productivity and carbon-based growth yield, with ultimate finalities being lipid content and transesterification into FAEs, the final product of the WAEster process. 
The PhD student will collaborate with 3 other PhD students working at the project partners IATE, TBI and LGC. He/she will also interact with a postdoc working with Dr. Brais Martínez López on the modelling of the PBR, and with Prof Maeva Subileau on certain aspects of lipid analyses  (IRB-IATE, Institut Agro Montpellier). 

Location

The PhD work will take place at the Laboratory of Environmental Biotechnology (LBE – INRAE), located in Narbonne in the south of France (Occitanie). The research carried out at the LBE aims to develop the concept of environmental biorefinery, which consists of treating by-products of human activities (waste, agricultural residues, effluents) and valorizing them into resources of industrial interest (bioenergies, biomolecules, organic fertilizers) while minimizing their environmental and health impact. The working environment encourages exchanges and collaboration between researchers, doctoral students and interns. In addition, the region offers a particularly pleasant living environment during most of the year, with several accessible activities (sailing, climbing, hiking, etc.).

Candidate profile 

This PhD position is aimed at master level students with good knowledge of general microbiology and bioprocesses, interested in applied research but also in understanding metabolic pathways (omics analyses). Good skills in reading and writing English is required. Practical experience in the culture of microorganisms (particularly algae) would be a plus point as well as an interest (or experience) in mathematical modeling of bioprocesses.

Application

Please send a motivation letter and CV before the end of June 2025 to Robert van Lis and Brais Martinez-Lopez

Duration

3 years starting in October 2025

Domaine

Microbiology, microalgae bioprocesses

Remuneration

€2233 gross salary per month