Nanomolar Responsiveness of Marine <b><i>Phaeobacter inhibens</i></b> DSM 17395 toward Carbohydrates and Amino Acids

Abstract

<i>Phaeobacter inhibens</i> DSM 17395 is a heterotrophic member of the ubiquitous, marine <i>Roseobacter</i> group and specializes in the aerobic utilization of carbohydrates and amino acids via pathways widespread among roseobacters. The in vivo responsiveness of <i>P. inhibens</i> DSM 17395 was studied with nonadapted cells (succinate-grown), which were exposed to a single pulse (100–0.01 µM) each of <i>N</i>-acetylglucosamine, mannitol, xylose, leucine, phenylalanine, or tryptophan (effectors). Responsiveness was then determined by time-resolved transcript analyses (quantitative reverse transcription-PCR) of “degradation” and “uptake” genes selected based on previously reported substrate-specific proteome profiles. The transcriptional response thresholds were: 50–100 nM for <i>nagK</i> (<i>N</i>-acetylglucosamine kinase), <i>paaA</i> (ring 1,2-phenylacetyl-CoA epoxidase), and <i>kynA</i> (tryptophan 2,3-dioxygenase), 10–50 nM for <i>xylA</i> (xylose isomerase), and around 10 nM for <i>mtlK</i> (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: 0.1–1 µM for <i>c27930</i> (<i>N</i>-acetylglucosamine) and even below 10 nM for <i>c13210</i> (mannitol) and <i>xylF</i> (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 dissolved organic matter persistence).