title: Surplus photosynthetic antennae complexes underlie diagnostics of iron limitation in a cyanobacterium.
abstract: Chlorophyll fluorescence from phytoplankton provides a tool to assess iron limitation in the oceans, but the physiological mechanism underlying the fluorescence response is not understood. We examined fluorescence properties of the model cyanobacterium Synechocystis PCC6803 and a ΔisiA knock-out mutant of the same species grown under three culture conditions which simulate nutrient conditions found in the open ocean: (1) nitrate and iron replete, (2) limiting-iron and high-nitrate, representative of natural high-nitrate, low-chlorophyll regions, and (3) iron and nitrogen co-limiting. We show that low variable fluorescence, a key diagnostic of iron limitation, results from synthesis of antennae complexes far in excess of what can be accommodated by the iron-restricted pool of photosynthetic reaction centers. Under iron and nitrogen co-limiting conditions, there are no excess antennae complexes and variable fluorescence is high. These results help to explain the well-established fluorescence characteristics of phytoplankton in high-nutrient, low-chlorophyll ocean regions, while also accounting for the lack of these properties in low-iron, low-nitrogen regions. Importantly, our results complete the link between unique molecular consequences of iron stress in phytoplankton and global detection of iron stress in natural populations from space.
authors.affiliation: Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America.
authors.forename: Paul S
authors.initials: PS
authors.lastname: Schrader
authors.forename: Allen J
authors.initials: AJ
authors.lastname: Milligan
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Behrenfeld
mesh_terms: D015153:Blotting, Western; D004591:Electrophoresis, Polyacrylamide Gel; D005453:Fluorescence; D055786:Gene Knockout Techniques; D007501:Iron / Q000378:metabolism*; D010788:Photosynthesis; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism*
chemical_list: D007501:Iron