Impact of phytoplankton on the biogeochemical cycling of iron in subantarctic waters southeast of New Zealand during FeCycle

Abstract

During austral summer 2003, we tracked a patch of surface water infused with the tracer sulfur hexafluoride, but without addition of Fe, through subantarctic waters over 10 days in order to characterize and quantify algal Fe pools and fluxes to construct a detailed biogeochemical budget. Nutrient profiles characterized this patch as a high‐nitrate, low‐silicic acid, low‐chlorophyll (HNLSiLC) water mass deficient in dissolved Fe. The low Fe condition was confirmed by several approaches: shipboard iron enrichment experiments and physiological indices of Fe deficiency (Fv/Fm < 0.25, Ferredoxin Index < 0.2). During FeCycle, picophytoplankton (0.2–2 μm) and nanophytoplankton (2–20 μm) each contributed >40% of total chlorophyll. Whereas the picophytoplankton accounted for ∼50% of total primary production, they were responsible for the majority of community iron uptake in the mixed layer. Thus ratios of 55Fe:14C uptake were highest for picophytoplankton (median: 17 μmol:mol) and declined to ∼5 μmol:mol for the larger algal size fractions. A pelagic Fe budget revealed that picophytoplankton were the largest pool of algal Fe (>90%), which was consistent with the high (∼80%) phytoplankton Fe demand attributed to them. However, Fe regenerated by herbivory satisfied only ∼20% of total algal Fe demand. This iron regeneration term increased to 40% of algal Fe demand when we include Fe recycled by bacterivory. As recycled, rather than new, iron dominated the pelagic iron budget (Boyd et al., 2005), it is highly unlikely that the supply of new Fe would redress the imbalance between algal Fe demand and supply. Reasons for this imbalance may include the overestimation of algal iron uptake from radiotracer techniques, or a lack of consideration of other iron regeneration processes. In conclusion, it seems that algal Fe uptake cannot be supported solely by the recycling of algal iron, and may require an Fe “subsidy” from that regenerated by heterotrophic pathways.

Physical mixing effects on iron biogeochemical cycling: FeCycle experiment

“…Chlorophyll levels also increased in both the FeCycle patch [ Boyd et al , 2005] and throughout the survey region as seen by MODIS‐AQUA (Figure 3). In patch discrete sampling showed a doubling in the chlorophyll content from 0.4 to 0.8 μ g L −1 of the surface waters during FeCycle [ McKay et al , 2005]. This increase continued after FeCycle as Satellite chlorophyll estimates for 17 February show chlorophyll concentrations in the study region rising to over 1 μ g L −1 overall and included some areas near 2 μ g L −1 .…”

“…The beam attenuation coefficient ( c p ) data were used as a proxy for Particulate Organic Carbon (POC) [ Gardner et al , 2003] to examine whether the increase in chlorophyll observed during FeCycle [ McKay et al , 2005] was related to admixing with higher chlorophyll containing waters from outside the initial SF 6 patch. During the first 2 phases of patch evolution a mixing gradient for POC between the initial SF 6 patch ( c p ∼ 0.08) and out patch waters ( c p > 0.08) was apparent.…”

Elemental quotas and physiology of a southwestern Pacific Ocean plankton community as a function of iron availability

“…Size-fractionated chlo rophyll a (chl a) concentrations (chl a retained on 0.2, 2.0, and 20.0 µm pore size polycarbonate filters) were calculated as the arithmetic mean of measurements from duplicate samples from individual bottles using the non-acidification approach (Welschmeyer 1994). Dissolved nutrients (NO 3 , PO 4 , SiO 3 ) were determined by shipboard automated analysis (after Frew et al 2001), and photosynthetic efficiency (F v /F m ) values were determined as the mean of 5 repeated measures using fast repetition rate fluorometry as described by McKay et al (2005).…”

Cyanobacterial Bioreporters as Sensors of Nutrient Availability

“…Enrichment bioassays, though offering direct experimental evidence for growth limitation, do not completely mimic the undisturbed natural environment: grazing is disrupted, physical mixing is decreased, and the phytoplankton are isolated at a fixed optical depth [59]. To overcome these concerns, several biochemical and molecular approaches have been suggested such as monitoring the expression of Feresponsive genes in environmental samples [60,61], measuring the ratios of the redox catalysts ferredoxin:flavodoxin [62,63] or other suitable Fe-responsive proteins such as IdiA [64], and assaying variable chlorophyll fluorescence [65]. Implementing a living system such as a bioreporter organism can help us gain a better understanding of the availability of Fe from the perspective of a living cell.…”

“…While the use of Fe-responsive bioreporters has offered some evidence of transient Fe deficiency in the Great Lakes (e.g., [9,62]), it is not widespread [35,46]. A new approach whereby the bioreporter is contained in a porous underwater chamber (PUC) during incubation with unfiltered water [44] promises to offer insight into the biological cycling of Fe by regenerative processes such as grazing and viral lysis which may provide 30-80% of the algal Fe demand in surface seawater [62].…”