Selected coccal green algae are not affected by the humic substance Huminfeed super( registered ) in term of growth or photosynthetic performance

Humic substances (HSs) have been shown to influence growth, photophysiology, and redox homeostasis in phototrophs. However, many ecological studies deliver controversial results indicating inhibitory or stimulating effects depending on the kind of phototrophic organism or type of HSs applied. Here we analyzed the effect of Huminfeed super( registered ) (HF), a preparation from leonardite associated with lignite deposits, on growth and photosynthetic performance of three different coccal green algae. Concentrations of HF in the range of 2-20 mg l super(-1) dissolved organic carbon (DOC) did neither affect the growth rate nor the light-adapted photosynthetic electron transport. Even photoinhibitory light intensities of 1,600 mu mol photons m super(-2) s super(-1), representing the tenfold of growth light intensity, did not result in a decline of the maximal photosynthetic rate in HF-treated algae. In HF-grown algae, a very subtle decrease by about 10% could be observed in thermoluminescent light emission, a sensitive method to detect changes in photosystem II (PSII) chemistry. However, thermoluminescence (TL) represents only 3% of the light-induced charge separated states in PSII. Hence, rather small changes will not have significant effects on overall photosynthetic performance and growth. Therefore, the physiological effect of HSs on freshwater phototrophs has to be revisited.