Functionally uniform monocultures have remained the paradigm in microalgal cultivation despite the apparent challenges to avoid invasions by other microorganisms. A mixed microbial consortium approach has the potential to optimize and maintain biomass production despite of seasonal changes and to be more resilient toward contaminations. Here we present a 3-year outdoor production of mixed consortia of locally adapted microalgae and bacteria in cold temperate latitude. Microalgal consortia were cultivated in flat panel photobioreactors using brackish Baltic Sea water and CO2 from a cement factory (Degerhamn, Cementa AB, Heidelberg Cement Group) as a sustainable CO2 source. To evaluate the ability of the microbial consortia to maintain stable biomass production while exposed to seasonal changes in both light and temperature, we tracked changes in the microbial community using molecular methods (16S and 18S rDNA amplicon sequencing) and monitored the biomass production and quality (lipid, protein, and carbohydrate content) over 3 years. Despite changes in environmental conditions, the mixed consortia maintained stable biomass production by alternating between two different predominant green microalgae (Monoraphidium and Mychonastes) with complementary tolerance to temperature. The bacterial population was few taxa co-occured over time and the composition did not have any connection to the shifts in microalgal taxa. We propose that a locally adapted and mixed microalgal consortia, with complementary traits, can be useful for optimizing yield of commercial scale microalgal cultivation.

Algal lipids have been observed to increase during autumn conditions (low light, low mean temperature, and diurnal shift in temperature), in large‐scale outdoor photobioreactors. In this paper, we tested the effect of diurnal shifts in temperature (DS) and nitrogen (N) limitation on algal BODIPY lipid fluorescence cell−1 (BPF). We show that DS increased BPF in algal biomass up to 28% more compared with N limitation, the standard stressor to boost neutral lipids (NL) in commercial production. Biomass yield was constant, regardless the DS range (6°C‐12°C). A combination of both stressors had an additive effect on algal BPF. A polyculture from an outdoor photobioreactor was cultivated under controlled conditions at different regimes of light, temperature, and N limitation. DSs were mimicking autumn conditions with a difference of 6°C, 10°C, and 12°C between day and night. Biomass and BPF were monitored over one to two weeks, and NLs were stained with a fluorescent marker (BODIPY) and detected with flow cytometry. Results indicate that, during autumn conditions, daily heating and cooling processes in contrast to N limitation do not challenge the trade‐off between biomass production and BPF. During seasons when day temperature is still relatively high, DSs are rapid BPF boosting stressors, while N limitation could be applied to boost BPF further during other seasons.

Microalgal cultivation for biodiesel and feed requires recycled nutrient resources for a sustainable long‐term operation. Whey permeate (WP) from dairy processing contains high organic load (lactose, oils, and proteins) and nitrogen (resources tested for microalgal cultivation) and organic phosphorus (P) that has not yet been tested as a P source for microalgal cultivation. We explored the potential of green algae strains (brackish) and polyculture (freshwater) in exploiting P from WP added to a medium based on either seawater (7 psu) or landfill leachate. Both strains showed a capacity of using organic P in WP with equal growth rates (0.94–1.12 d−1) compared with chemical phosphate treatments (0.88–1.07 d−1). The polyculture had comparable growth rate (0.25–0.57 d−1) and biomass yield (152.1–357.5 mg L−1) and similar or higher nutrient removal rate in the leachate–WP medium (1.3–6.4 mg L−1 day−1 nitrogen, 0.2–1.1 mg L−1 day−1 P) compared with the leachate–chemical phosphate medium (1.2–4.7 mg L−1 day−1 nitrogen, 0.3–1.4 mg L−1 day−1 P). This study showed that WP is a suitable P source for microalgal cultivation over a range of salinities. To date, this is the first study demonstrating that raw WP can replace mineral P fertilizer for algal cultivation. Practitioners Points Whey permeate is a comparable phosphorus source to standard fertilizers used in algal cultivation. Green algae removed phosphorus effectively from whey permeate. Microalgal cultivation is a good approach for treatment of whey permeate in combination with a nitrogen‐rich wastewater.