Direct and Indirect Effects of Humic Substances of Different Origin on the Green Algae Monoraphidium braunii

摘要

Dissolved natural organic matter (NOM) is widespread and abundant in freshwater ecosystems, exceeding the total living organic carbon by one order of magnitude [1, 2]. A consistent part of NOM (50–80%) is made up by humic substances (HS) [1, 3]. It’s well known that HS, because of their physical and chemical properties, modify the underwater light climate, alter the bioavailability of inorganic micronutrients and xenobiotics and, after their photodegradation, supply carbon and/or energy to heterotrophic organisms. New ecophysiological studies, without disregarding the traditional conception that HS affect indirectly the aquatic biocenosis, consider HS as natural environmental chemicals able to interact also directly with organisms [4]. Due to the low molar mass of their building blocks, HS can be taken up by organisms. Once internalized, they act like xenobiotics inducing several specific or not specific stress responses. Specific reactions include reduction of photosynthetic oxygen production, estrogenicity and chemical attraction. Non specific reactions comprise physical and chemical membrane irritation, induction and modulation of biotransformation enzymes, induction of stress defense proteins and internal oxidative stress [5]. According to recent theories of aging and evolution, a mild stress may be benefit for individuals, training their chemical defense system [5, 6].