Influence of light on endogenous phytohormone concentrations of a nitrogen-fixing Anabaena sp. cyanobacterium culture in open raceways for use as fertilizer for horticultural crops

The influence of photosynthetically active radiation (PAR) on endogenous phytohormone concentrations of a heterocystous N-fixing Anabaena sp. cyanobacterium was studied under 14-day batch cultivation at the CSU Horticultural Research Farm during 2016. The two-factorial experiment compared phytohormone concentrations in 52-L Anabaena sp. cultures grown in CO2-supplemented Allen and Arnon N-free growth medium within and outside polyethylene-covered high tunnels. Inoculation densities were designed to capture 91% and 84% of day 1 maximum outdoor PAR. Across two trials, LC-MS/MS analysis detected mean concentrations of 0.137 +/- 0.062 (standard deviation) ng mg(-1) dry weight (dw) abscisic acid (ABA), 0.189 +/- 0.049 ng mg(-1) dw indole-3-acetic acid (IAA), 0.134 +/- 0.048 ng mg(-1) dw indole acetamide (IAM), 0.116 +/- 0.032 ng mg(-1) dw indole carboxylic acid (ICA), and 0.022 +/- 0.017 ng mg(-1) dw trans-zeatin riboside (tZr). Salicylic acid (SA) was detected at significantly different concentrations of 0.449 +/- 0.229 ng mg(-1) dw in trial 1 and 1.066 +/- 0.262 ng mg(-1) dw in trial 2. ICA and SA have not been reported as cyanobacterial metabolites. Regression equations based on treatment variables and production parameters found that solar irradiance, likely through light stress or energy availability, was a significant variable in predicting ABA, IAA, IAM, and SA concentrations. Anabaena sp. culture inoculation density was determined to be a significant variable in predicting IAA, IAM, and SA concentrations. IAA and SA concentrations were comparable to those reported to have beneficial effects on plant growth when applied as a constituent of cyanobacterial biomass.