Phaeobacter inhibens DSM 17395 is a heterotrophic member of the ubiquitous, marine Roseobacter group and specialized in the aerobic utilization of carbohydrates and amino acids via pathways widespread among roseobacters. The in vivo responsiveness of P. inhibens DSM 17395 was studied with non-adapted cells (succinate-grown), which were exposed to a single pulse (100-0.01 µM) each of N-acetylglucosamine, mannitol, xylose, leucine, phenylalanine or tryptophan (effectors). Responsiveness was then determined by time-resolved transcript analyses (qRT-PCR) of 'degradation' and 'uptake' genes selected based on previously reported substrate-specific proteome profiles. The transcriptional response thresholds were: 50-100 nM for nagK (N-acetylglucosamine kinase), paaA (ring 1,2-phenylacetyl-CoA epoxidase), and kynA (tryptophan 2,3-dioxygenase), 10-50 nM for xylA (xylose isomerase), and around 10 nM for mtlK (mannitol 2-dehydrogenase). A threshold for leucine could not be determined due to the elevated intrinsic presence of leucine in the exometabolome of succinate-grown cells (no effector addition). Notably, the response thresholds for presumptive carbohydrate-binding proteins of ABC-transporters were in the same range or even lower: 10-100 nM for c27930 (N-acetylglucosamine) and even below 10 nM for c13210 (mannitol) and xylF (xylose). These results shed new light on the sensory/regulatory sensitivity of a well-studied roseobacter for recognizing potential substrates at low ambient concentrations and on the concentration threshold below which these might escape biodegradation ('emergent recalcitrance' concept of DOM persistence).

The Author(s). Published by S. Karger AG, Basel