Iron-mediated changes in phytoplankton photosynthetic competence during SOIREE

Active fluorescence (fast repetition rate fluorometry, FRRF) was used to fo llow the photosynthetic response of the phytoplankton community during the 13-day Southern Ocean Iron RElease Experiment (SOIREE). This in situ iron e nrichment was conducted in the polar waters of the Australasian-Pacific sec tor of the Southern Ocean in February 1999. Iron fertilisation of these hig h nitrate low chlorophyll (HNLC) waters resulted in an increase in the phot osynthetic competence (F-v/F-m,) of the resident cells from around 0.20 to greater than 0.60 (i.e. close to the theoretical maximum) by 10/11 days aft er the first enrichment. Although a significant iron-mediated response in F -v/F-m was detected as early as 24h after the initial fertilisation, the in crease in F-v/F-m to double ambient levels took 6 days, This response was f ive-fold slower than observed in iron enrichments tin situ and in vitro) in the HNLC waters of the subarctic and equatorial Pacific, Although little i s known about the relationship between water temperature and F-v/F-m, it is likely that low water temperatures - and possibly the deep mixed layer - w ere responsible for this slow response time, During SOIREE, the photosynthe tic competence of the resident phytoplankton in iron-enriched waters increa sed at dissolved iron levels above 0.2 nM, suggesting that iron limitation was alleviated at this concentration, Increases in F-v/F-m of cells within four algal size classes suggested that all taxa displayed a photosynthetic response to iron enrichment. Other physiological proxies of algal iron stre ss (such as flavodoxin levels in diatoms) exhibited different temporal tren ds to iron-enrichment than F-v/F-m during the time-course of SOIREE, The re lationship between F-v/F-m, algal growth rate and such proxies in Southern Ocean waters is discussed. (C) 2001 Elsevier Science Ltd. All rights reserv ed.