Photoreactivity and bacterioplankton availibility of aliphatic versus aromatic aminoacids and a protein

Abstract

In batch culture experiments with natural bac- erial assemblages collected from a humic-rich lake (Lake Neusiedl, Austria), labile organic nitrogen containing model substrates (alanine (Ala), an aliphatic amino acid, tryptophan (Trp), an aromatic amino acid, and bovine serum albumin (BSA), as protein) were added to 0.2 µm filtered lake water prior to exposure (for 9 h) to surface levels of the full range of solar radiation or being held in the dark. These organic nitro- gen species were chosen to investigate compound-specific differences in the photochemical transformation of labile dissolved organic matter (DOM) and its subsequent availabil- ity to bacteria. 'Photocoloring' or humification in the Trp- amended water exposed to natural solar radiation and signif- icant dissolved organic carbon (DOC) loss (120 µM C over 9 h exposure) resulted, after inoculation of the natural bacterial assemblage, in a lower maximum bacterial abundance in the previously solar radiation-exposed treatments as compared to the dark control. In contrast, the absorbance characteristics of solar-irradiated Ala- and BSA-amended water were almost identical to the unamended control and to the Ala- and BSA- amended treatments kept in the dark. Also, no significant dif- ference in the maximum bacterial abundance of the solar radiation-exposed Ala- and BSA-amended treatments was detectable as compared to the corresponding dark controls. Our data indicate that Trp is a potential source of solar radia- tion-mediated humification in Lake Neusiedl.