Varied Growth Media Necessitate Different Light Regimes for Indoor Duckweed Cultivation

Controlled indoor cultivation of duckweed plants can support remediation of wastewaters through generation of plant biomass. Despite numerous advantages, indoor cultivation of duckweeds on agri-food wastewaters remains underexplored. Lighting regimes need to be optimised for duckweed growth and affordability of energy consumption, as it has been shown that the composition of wastewater growth medium can alter light utilisation. In the present study, four duckweed (Lemna minor) clones were grown under four different light regimes on either optimised half-strength Hutner's medium or wastewater derived from the liquid fractions of anaerobically digested pig slurry. Cultivation of L. minor was assessed for the four light regimes using a commercial hydroponics plant growth medium in a 3.96 m² multitiered cultivation system. When cultivated on optimised half-strength Hutner's medium or diluted pig slurry under laboratory conditions, it appeared that photoperiod rather than light intensity was more important for duckweed growth. Yet, under moderate flow conditions within a larger scale multitiered cultivation system, greater light intensity appeared to support duckweed cultivation irrespective of photoperiod. These findings emphasise the need to move beyond small-scale and static assessments of duckweed before embarking on larger, industry-relevant scales.