Adaptive strategies by Southern Ocean phytoplankton to lessen iron limitation: Uptake of organically complexed iron and reduced cellular iron requirements

Abstract

We report results of laboratory studies examining the effect of low levels of iron (Fe) availability on the intracellular Fe concentrations and specific growth rates in Southern Ocean diatoms (Fragilariopsis kerguelensis, Eucampia antarctica, Proboscia inermis, and Thalassiosira antarctica) and Phaeocystis antarctica.

Diagnosing oceanic nutrient deficiency

“…Thus, it might be hypothesized that the majority of new biomass would actually be produced by cells which are not severely nutrient limited and which hence have nutrient quotas towards the higher end of observed ranges (figure 1). However, markedly low Fe quotas within certain SO taxa [31] suggest that considerable taxonomic adaptation to reduced Fe occurs.…”

“…Plot of the stoichiometry of different nutrient elements in phytoplankton cellular material against the dissolved concentration stoichiometry within Antarctic Circumpolar Deep Water (CDW). Open symbols are plotted at a point on the abscissa corresponding to the mean values across a range of different phytoplankton taxa as previously adopted [11,30], with the horizontal bars representing the maximum range of stoichiometric variability which has currently been observed in culture for these elements (see [11] for further discussion), updated where applicable [31]. Open symbols are plotted at a point on the ordinate corresponding to the mean concentration of that element observed within CDW (potential temperature from 1 to 2°C, salinity from 34.62 to 34.73), from the GEOTRACES IDP [20], with vertical bars (where visible) representing 1 s.d.…”

Temporal progression of photosynthetic-strategy in phytoplankton in the Ross Sea, Antarctica

“…Molecular and 34 physiological data further indicate that as nitrate is removed from the surface ocean the 35 phytoplankton community transitions to one displaying an iron-efficient photosynthetic 36 strategy characterised by an increase in the size of photosystem II (PSII) photochemical 37 cross section (σPSII) and a decrease in the chlorophyll-normalised PSII abundance. Fe requirements and light harvesting capacity, studies on Southern Ocean diatoms and 117 P. antarctica in culture suggest the Fe burden of photosynthesis may be significantly 118 reduced for these species through increases in the size rather than the number of 119 photosynthetic units (termed sigma-type acclimation) in response to iron/ and light 120 limitation (Strzepek et al, 2012;Strzepek et al, 2011). Effectively, these Southern 121…”

Diatom traits regulate Southern Ocean silica leakage

“…1 links the wider concepts of physiological traits and tradeoffs with observed temporal trends of diatom species succession in subarctic (13) and Southern Ocean ironstimulated blooms (8). Important adaptive strategies, in addition to predator avoidance, include growth rate and iron requirements (14), capacity for intracellular iron storage (pennates vs. centric diatoms) (15), cell sinking rate (13,16), and how prone cells are to species-specific aggregation (13). As observed for the ratio of silicic acid:nitrate uptake, iron supply influences most of these characteristics (13,14).…”

“…Important adaptive strategies, in addition to predator avoidance, include growth rate and iron requirements (14), capacity for intracellular iron storage (pennates vs. centric diatoms) (15), cell sinking rate (13,16), and how prone cells are to species-specific aggregation (13). As observed for the ratio of silicic acid:nitrate uptake, iron supply influences most of these characteristics (13,14). Hence, the trends from EIFEX (8) also reflect the interplay of additional strategies in terms of diatom physiology, which set the timing of bloom dominance by different diatom species (growth rate), their fate (rapid growth leads to aggregation then export) (13), and the interplay of factors controlling diatom succession (slow growth rate, ability to store iron, body armor) evident in both subarctic and polar waters.…”