The Effect of Lignin Content on the Growth and Yield of Two Specialty Mushroom Species: Pleurotus ostreatus and Hericium erinaceus

Abstract

White-rot fungi can convert inedible agricultural waste into highly nutritious, gourmet mushrooms. These fungi have a plethora of enzymes allowing them to degrade some of the most recalcitrant organic molecules: lignin, hemicellulose, and cellulose. However, the mechanisms by which fungi accomplish this under mushroom cultivation conditions are poorly understood. This study examines the effect of various lignin levels on the biological efficiency (BE) and substrate degradation patterns for two specialty mushroom species, P. ostreatus and H. erinaceus in a controlled environment. The goal was to better understand the nutritional requirements of the fungi during different growth stages and the impact of lignin on fungal growth. This work attempts to improve the efficient use of agricultural wastes as growing media for specialty mushrooms and was carried out in a series of three experiments. The first two experiments cultivated P. ostreatus on oak- and wheat-based substrates with increasing lignin contents (5-28%) while attempting to hold the cellulose, hemicellulose, and nitrogen percentages constant. The third experiment grew P. ostreatus and H. erinaceus on oak, wheat, and coco coir each supplemented with 50% soybean hulls. For all experiments, the effect of substrate was quantified by measuring the BE as well as the cell wall components, lignin and cellulose, measured before inoculation, after spawn run, and after harvest. Results from the first two experiments cultivating P. ostreatus revealed that increasing substrate lignin content in oak-based substrates resulted in a significant decrease in BE, but no significant effect of lignin content on BE was observed for wheat-based substrates, although the same trend was evident. Additionally, the initial substrate lignin content had a significant effect on lignin consumption for both oak- and wheat-based substrates but this effect was only apparent for substrates with high initial lignin contents. For substrates with low lignin content, very little lignin was consumed. Interestingly, the acid detergent fiber (ADF) measurement used to quantify cellulose gradually increased over time which was likely due to the ADF detecting both cellulose and chitin, therefore as more plant cellulose was converted to fungal chitin, ADF increased. In experiments comparing the response to substrate lignin by P. ostreatus and H. erinaceus, there was no significant effect of lignin content on BE for both species and similarly, the most lignin was consumed in the substrates with the greatest initial lignin contents for both species. The trend of increasing ADF over time was observed for P. ostreatus, but in contrast, not for H. erinaceus. This observation is likely from the low BEs with H. erinaceus, thus less plant cellulose converted to fungal chitin.