Organic Iron-binding ligands in the Arctic, Antarctic and subtropical Regions

Abstract

Phytoplankton are the most abundant primary producers in the oceans, and diatoms often dominate the marine phytoplankton assemblage. Phytoplankton and heterotopic bacteria coexist in the marine environment and actively engage in a specific way that influences the nutrient and carbon cycles in the marine environment. Iron (Fe) is an essential micronutrient for primary producers. Despite its importance, it is present at trace level in the ocean as it is rather insoluble at the pH of seawater. However, organic iron-binding is acknowledged to enhance Fe solubility in seawater. Diatom and bacteria have developed mutualistic interactions for Fe acquisition. However, very limited knowledge is available about how phytoplankton and bacteria interact to achieve the most efficient Fe acquisition when there is a lack of Fe resources. We developed an approach to reveal the influence of marine bacteria in diatom cultures of Pseudonitzschia sp. We used a dialysis membrane to physically separate the diatoms from marine bacteria to test whether direct cell contact between bacteria and diatoms is necessary for chemical signalling. Our results showed that bacteria and diatoms might interact chemically, in which the presence of bacteria is able to enhance phytoplankton growth even without direct physical contact. The different Fe treatments used did not have a measurable effect on the growth of phytoplankton, yet affected phytoplankton physiological performance (Fv/Fm) and yielded biomass (Chl-a/cell). The low-Fe treatments resulted in Fv/Fm < 0.25, indicating that phytoplankton was experiencing physiological stress. Consequentially, chl-a/cell ratios in low-Fe treatments were often lower compared to those in higher-iron treatments. Dissolved iron (DFe) drawn down was observed as the incubation progressed along with macronutrients (PO 4 and Si) utilization. Since Fe was sufficient for phytoplankton growth, the production of strong Fe-binding ligands was not likely stimulated as the incubation progressed. Otherwise, ligands may have been degraded as the incubation progressed. The initial [L t ] varied between 2.26 ± 0.3 and 8.69 ± 0.2 nmol/L eq. Fe, and at the end of incubation ranged between 2.62 ± 0.1 and 6.07 ± 0.1 nmol/L eq. Fe. The values of conditional binding constant (log K Fe'L cond ) ranged from 10.8±0.2 to 12.3±0.2. Based on the value of log K Fe'L cond , the detected ligands in our experiment were potentially a degradation product of stronger ligands, or microbial exudates.