Improved conidiation from entomopathogenic fungi through 26% oxygen pulses in solid-state culture depends on a balance between headspace volume and substrate amounts.

Abstract

Oxygen-enriched atmospheres applied as periodic pulses increased conidia production from entomopathogenic fungi in agar surface cultures. However, this advantage has not been obtained in solid-state cultures (SSC), probably as a result of different biomass production between both culture systems. In this work, the biomass formation from two Isaria strains was limited in SSC using 5, 2·5 and 1 initial grams of substrate (gds). In the system with 5gds, conidia production decreased in 26% oxygen-enriched pulses compared to the normal atmosphere. Conversely, 26% oxygen pulses increased conidiation up to one order magnitude in systems with 2·5 and 1 gds, respective to the normal atmosphere. These results were explained by oxygen depletion and high CO2 accumulation in the 5 gds system. Whereas in systems with 2·5 or 1 gds, oxygen levels remained high enough to stimulate conidiation. These results were attributed to the headspace volume:gds ratio, which is suggested to be ≥48ml per gds. This ratio is proposed as a scaling-up criterion for bioreactor design when oxygen-enriched pulses are used in SSC for improvement of conidia production. SIGNIFICANCE AND IMPACT OF THE STUDY: Oxygen-enriched atmospheres applied as periodic pulses increase conidiation in entomopathogenic fungi (EF). However, this remained restricted to agar surface cultures, since conidiation decreased when carried out in solid-state culture (SSC) which is used as large-scale production system. We identified that in SSC the ratio between the headspace volume containing 26% oxygen-enriched pulses and the grams of substrate determines the conidiation response to oxygen-enriched pulses. For the first time, oxygen-enriched pulses increased conidiation in SSC respective to the normal atmosphere in four EF. This ratio is proposed as a bioreactor criterion design for large-scale conidia production of EF using oxygen-enriched pulses.