Photosynthetic maximum quantum yield increases are an essential component of the Southern Ocean phytoplankton response to iron

^†Program in Atmospheric and Oceanic Sciences, Princeton University, 300 Forrestal Campus, Princeton, NJ 08544;
^‡Duke University Nicholas School of the Environment and Earth Sciences, 135 Duke University Marine Lab Road, Beaufort, NC 28516;
^¶Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822;
^‖Scripps Institution of Oceanography, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093; and
^††Virginia Institute of Marine Sciences, College of William and Mary, Gloucester Point, VA 23062

Edited by William H. Schlesinger, The Institute of Ecosystem Studies, Millbrook, NY, and approved December 10, 2007 (received for review May 28, 2007)

Abstract

It is well established that an increase in iron supply causes an increase in total oceanic primary production in many regions, but the physiological mechanism driving the observed increases has not been clearly identified. The Southern Ocean iron enrichment experiment, an iron fertilization experiment in the waters closest to Antarctica, resulted in a 9-fold increase in chlorophyll (Chl) concentration and a 5-fold increase in integrated primary production. Upon iron addition, the maximum quantum yield of photosynthesis (φ_m) rapidly doubled, from 0.011 to 0.025 mol C·mol quanta⁻¹. Paradoxically, this increase in light-limited productivity was not accompanied by a significant increase in light-saturated productivity (P_max^b). P_max^b, maximum Chl normalized productivity, was 1.34 mg C·mg Chl⁻¹·h⁻¹ outside and 1.49 mg C·mg Chl⁻¹·h⁻¹ inside the iron-enriched patch. The importance of φ_m as compared with P_max^b in controlling the biological response to iron addition has vast implications for understanding the ecological response to iron. We show that an iron-driven increase in φ_m is the proximate physiological mechanism affected by iron addition and can account for most of the increases in primary production. The relative importance of φ_m over P_max^b in this iron-fertilized bloom highlights the limitations of often-used primary productivity algorithms that are driven by estimates of P_max^b but largely ignore variability in φ_m and light-limited productivity. To use primary productivity models that include variability in iron supply in prediction or forecasting, the variability of light-limited productivity must be resolved.

Footnotes

^§To whom correspondence should be addressed. E-mail: mhiscock{at}princeton.edu
Author contributions: M.R.H., Z.I.J., B.G.M., W.O.S., and R.T.B. designed research; M.R.H., V.P.L., A.M.A., R.R.B., and R.T.B. performed research; M.R.H. analyzed data; and M.R.H., V.P.L., Z.I.J., and R.T.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.