Introduction

Microalgae are currently cultivated commercially for human nutritional products and biofuels around the world in several dozen small- to medium-scale production systems, producing a few tons to a several hundreds of tons of biomass annually (Benemann 2008). Total world production of dry algal biomass is estimated at about 10,000 t per year, of which about half is produced in mainland China, with most of the rest in Japan, Taiwan, the USA, Australia, India and a few small producers in some other countries (Benemann 2008). Due to high lipid content (20–50% and up to 80% in some cases), microalgae are also considered a promising feedstock for biofuel and next-generation renewable energy source (Chisti 2007; Benvenuti et al. 2016). Algal biofuel production has been extensively researched for over three decades, and a number of challenges have been identified in commercial microalgae production (Lin et al. 2011). These include lipid content, optimum growth condition, and productivity of the microalgae.

Iron (Fe) is an important nutrient for phytoplankton metabolism where it is required for photosynthetic and respiratory electron transport, nitrate reduction, chlorophyll synthesis, and detoxification of reactive oxygen species (Sunda and Huntsman 1995; Marchetti & Maldonado 2016). Iron is the fourth most abundant element in the earth’s crust (Anderson and Morel 1982); however, the oxidized form of Fe, Fe(III), is sparingly soluble in oxygenated waters and, therefore, occurs at extremely low concentration (Wells et al. 1995). This suggests that phytoplankton may be Fe-limited in some aquatic systems that may affect the growth of phytoplankton. Studies have shown that limited Fe availability impairs phytoplankton growth by as much as 40% of the open oceans (Moore et al. 2001), notably in the high-nutrient low-chlorophyll (HNLC) and oligotrophic waters near the Equator and further south (Beardall et al. 2001; Blain et al. 2007).

The economic feasibility of algal culture for nutritional products and/or biofuel production greatly depends on the high biomass productivity and appreciable lipid yields that are significantly affected by Fe nutrition (Liu et al. 2008). Synthetic aminopolycarboxylates (APCs), such as ethylenediaminetetraacetic acid (EDTA), were introduced during the 1950s as a metal ion buffer in seawater media for algal culture (Kean et al. 2015) and since then have been commonly used in order to mimic low Fe concentrations in algal culture (Gerringa et al. 2000). Most of the research has been performed in laboratory cultures using phytoplankton; EDTA had been added in either a synthetic medium or filtered natural seawater in order to buffer the metal chemistry (Anderson and Morel 1982; Sunda et al. 1991; Sunda and Huntsman 1995; Gerringa et al. 2000). In the past decades, there have been many experiments investigated the response of marine phytoplankton productivity to Fe additions in natural sea water without or with low addition of EDTA (Martin et al. 1991; Coale et al. 1996a,b; Boyd et al. 1998) and in laboratory media (Gerringa et al. 2000). Anderson and Morel (1982) investigated Fe uptake in coastal diatom Thalassiosira weissflogii under well-defined chemical conditions imposed by a large excess of the APCs (EDTA, cyclohexanediaminetetraacetic acid (CDTA), diethylenetriaminepentaacetic acid (DTPA), and nitrilotriacetic acid (NTA)). Results showed that the Fe uptake rate was a function of the activity of the Fe3+ ion in the absence of light. However, in the absence of the APCs, Fe uptake rates were dependent on the total Fe concentrations for both Fe(II) and Fe(III) additions to the medium.

EDTA has a fairly high affinity for transition metals and was introduced as a trace metal buffering reagent for algal cultures by Hutner et al. (1950) and Myers et al. (1951). One of the important purposes of the use of EDTA in algal culture medium was to obtain a constant and controlled supply of Fe (Gerringa et al. 2000). However, the fact is that EDTA and other synthetic non-ready biodegradable APCs lead to considerable amounts in the aquatic environment if they are used for algal culture (Pinto et al. 2014). It is true that industrial settings in which environmental release can be controlled or adequately treated may, however, benefit from a chelate that is active for longer. In the past decades, discussion on the environmental consequences of non-ready biodegradable APCs has been raised. The biodegradation of synthetic APCs such as EDTA and DTPA is very limited, and they can only be biodegraded in very particular conditions, with specific isolated bacterial strains (Bucheli-Witschel and Egli 2001; Nörtemann 2005). For example, in a closed bottle test, Bucheli-Witschel and Egli (2001) showed that EDTA can be completely degraded at pH 8. They also showed that the more stable forms of EDTA (such of Fe-EDTA) have to dissociate before degrading and, hence, are recalcitrant to microbial attack under many conditions. With this finding, still these compounds do not follow the Organization for Economic Cooperation and Development (OECD) criteria for biodegradability (Knepper 2003). Therefore, the replacement of these compounds by biodegradable alternatives has been the object of studies in the last three decades.

Recently, we have extensively studied the influence of non-ready biodegradable APCs and biodegradable chelating ligands on Fe mobility and bioavailability in growth medium and its uptake in plants (Rahman et al. 2009, 2011; Hasegawa et al. 2010, 2011, 2012). From these studies, we concluded that the concentration of chelating ligands and the stability constant of Fe-L complex (KFe-L) in the medium are important factors for Fe bioavailability to and uptake in plants. Iron exists predominantly as soluble [Fe(OH)2+, Fe(OH)2+, Fe-L complexes] and apparently soluble (colloidal) forms at moderate concentration of the chelants, which is favorable for Fe bioavailability and uptake (Hasegawa et al. 2012). Furthermore, the biodegradable ligands (e.g., hydroxyiminodisuccinic acid (HIDS)) are more efficient than the non-ready biodegradable APCs (e.g., EDTA) for Fe uptake in plants (Rahman et al. 2009; Hasegawa et al. 2012). However, the effects of biodegradable chelating ligands on Fe uptake in and growth of phytoplankton have not been demonstrated.

In the present study, the effects of synthetic chelating ligands on Fe uptake (using a radioactive tracer 55Fe) and growth (using spectrophotometer) of three coastal phytoplankters (Heterosigma akashiwo, Prymnesium parvum, and Skeletonema marinoi-dohrnii complex) were investigated by growing them in artificial seawater at different concentrations of biodegradable chelating ligands (HIDS, methylglycinediacetic acid (MGDA), and iminodisuccinate (IDS)) and the results were compared with those of non-ready biodegradable APCs (EDTA, ethylenediamine tetra-methylene phosphonic acid (EDTMP), and DTPA). We examined differential effects of chemical properties and ligands’ concentrations in the culture medium on the growth of phytoplankton. The objective of the study was to identify biodegradable chelating ligands that can be used as alternatives to the environmentally persistent APCs for phytoplankton cultivation.

Materials and methods

Reagents

Ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA; > 98.0%, Dojindo, Japan), diethylenetriamine-N,N,N′,N″,N″-tetraacetic acid (DTPA; > 99.8%, Dojindo, Japan), ethylenediamine-N,N,N′,N′-methylene(phosphonic acid) (EDTMP; 95.0%, Dojindo, Japan), hydroxyiminodisuccinic acid (HIDS; purity 90.19%, Nippon Shokubai, Japan), iminodisuccinate (IDS; 34%, Rankusasu, Japan), and methylglycinediacetic acid (MGDA; 82.0–84.0%, BASF, USA) were used as APCs in the present study. Iron chloride hexahydrate (FeCl3·6H2O; special grade, Wako, Japan) was used for the preparation of nutrient solution. The standard solution of Fe was prepared by dissolving FeCl3·6H2O (Wako, Japan), and radioactive isotope 55Fe was prepared by dissolving 55FeCl3 (740 MBq, Daiichi Pure Chemicals, Japan) in 1 M HCl. They were diluted to the desired concentrations with E-pure water (EPW, E-pure system, Barnstead, USA).

Phytoplankton

Three coastal phytoplankton (Heterosigma akashiwo 893, Prymnesium parvum, and Skeletonema marinoi-dohrnii complex NIES-16) were used in the present study. The strains of H. akashiwo and P. parvum were gently provided by Prof. I. Imai, Hokkaido University, Japan.

Heterosigma akashiwo is a microscopic alga of the class Raphidophyceae. Red tides of this species have been reported in coastal waters of Japan (Yamochi 1989), Korea (Park 1989), Singapore and New Zealand (Chang et al. 1990; Chang et al. 1993), and Canada (Taylor and Haigh 1993).

Prymnesium parvum is an ichthyotoxic phytoplankton (Haptophyta) growing in geographically widespread areas of the low-nutrient open oceans including temperate waters of both the northern and southern hemispheres (Fistarol et al. 2003), vast areas of temperate and tropical open seas (Edvardsen and Imai 2006), the Baltic Sea (Edler et al. 1984), and many coastal areas (Edvardsen and Imai 2006; Carvalho and Granéli 2010). Haptophyte algae are important primary producers in aquatic environments, and they are probably the main primary carbon source for petroleum products (Andersen 2004). It is tolerant of large variations in temperature and salinity (Lundholm and Moestrup 2006; Baker et al. 2007, 2009). Because of its widespread distribution in low-nutrient open oceans, P. parvum has been used in many oceanographic studies during the last decade (Fistarol et al. 2003; La Claire 2006; Baker et al. 2007; Maki et al. 2008; Carvalho and Granéli 2010; Roelke et al. 2010; Susan et al. 2010; Rahman et al. 2014). Skeletonema marinoi-dohrnii inhabits bottom sediments in the form that allow survival under cold, dark conditions (Itakura et al. 1992, 1997; Shikata et al. 2008). As a cosmopolitan alga, S. marinoi-dohrnii has been used as a model organism in numerous studies.

Pre-culture and maintenance

Axenic strains of coastal phytoplankton were maintained for 2 weeks in 30-mL polycarbonate bottles (Nalge, USA) with artificial seawater containing modified f/2 nutrient solution (Table 1) in which 2-[4-(2-hydroxyethyl)-1-piperazinyl]-ethane sulfonic acid (HEPES; Nacalai Tesque, Japan) was used as a buffer reagent (Watanabe et al. 2000). Axenicity was assessed and monitored by 4′,6-diamidino-2-phenylindole (DAPI) test. Artificial seawater was prepared according to the method of Lyman and Fleming (1940). Deionized water used for the preparation of modified f/2 nutrient solution was purified using E-pure (Barnstead, Thermo Fisher Scientific, Inc., USA) system.

Table 1 Chemical composition of modified f/2 culture medium in artificial seawater used for culturing costal phytoplankton
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Vessels, bottles, tips, and micropipettes used in the test were sterilized by autoclaving at 121 °C for 30 min. Modified f/2 nutrient solution and FeCl3 (10 μM) solution (in 1 M HCl) were sterilized separately to avoid Fe contamination. The FeCl3 solution was then mixed with the modified f/2 nutrient solution. After sterilization, the materials were placed in a clean bench and kept under UV irradiation for 20 min. Phytoplankton were grown in modified f/2 nutrient solution in artificial seawater in 30-mL polycarbonate bottles (Nalge, USA), and the experiment was conducted in a temperature- and light-controlled incubator (Koitotron 3HN-35MLA, Koito Industries, Ltd. Japan). The culture conditions in the incubator were: 12/12 h light/dark photoperiod at a light intensity of 180 μmol photons m−2 s−1 at 20 °C.

Growth experiment under different chelating ligand concentrations

The axenic algal strains were pre-cultured for 2 weeks in 30-mL polycarbonate bottles with 30 mL of artificial seawater containing modified f/2 nutrient solution containing 0.01 μM Fe. Ten milliliter of pre-culture solution of the phytoplankton at logarithmic growth phase was inoculated in 1.8 L of modified f/2 nutrient solution with 1.5 μM EDTA and 1.5 μM FeCl3 in 2-L polycarbonate bottles under laminar air flow. After 24 h, 30-mL aliquots of the cultures were pipetted into 30-mL acid-washed polycarbonate bottles. The phytoplankton were cultured in the same manner as maintenance cultures, and the additional 1.5, 7.5, and 150 μM of biodegradable (HIDS, MGDA, and IDS, respectively) and non-ready biodegradable (EDTA, DTPA, and EDTPO, respectively) ligands were added by adding appropriate amount of 3 mM solutions of the respective ligands at day 4. The pH of the cultures was adjusted to 8.0 using 0.1 M HCl and NaOH. Then, the phytoplankton were grown for 2 weeks.

Iron uptake experiment with different chelating ligands

The axenic strains of the phytoplankton were pre-cultured in 30 mL of artificial seawater containing f/2 nutrient solution containing 0.01 μM Fe without chelating ligands. From this pre-culture, 2.2 mL solution of algal culture (about 2 × 103 cells mL−1) was inoculated in 400 mL modified f/2 nutrient solution without Fe and chelating ligand until the cells were at a logarithmic phase of growth.

Then, 30-mL aliquots of the algal pre-culture of logarithmic growth phase were pipetted into 30-mL acid-washed polycarbonate bottles and were adjusted to 15 μM chelating ligand and 1.5 μM FeCl3 including 1.25 MBq L−1 of 55Fe. The pH of the culture solution was adjusted to 8.0 using 0.1 M HCl and NaOH. The algal cultures were grown for 48 h in the same manner as maintenance culture.

Extra- and intracellular Fe concentrations were determined according to the method of Hudson and Morel (1989). Briefly, 3 mL of aliquots were collected from each bottle and filtered through 3-μm pore size filters (Nuclepore, Sigma-Aldrich) in a clean booth. For intracellular Fe measurement, the filter was successively rinsed with 5 mL of the artificial seawater, 5 mL of 0.047 M Ti(III)/citrate/EDTA solution, and 5 mL of the artificial seawater on a filter holder. For total Fe measurement (corresponding to intra plus extracellular Fe), the filter was rinsed only with 5 mL of the artificial seawater. Then, the filters, on which 55Fe retained as a tracer, were directly added to 5 mL of the liquid scintillation solutions (2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole 3.03 g per 500 mL toluene) in 20-mL vials. A radiochemical activity of 55Fe was measured using the tritium mode of the liquid scintillation counter. The Fe concentration was calculated by the Fe/55Fe ratio in the samples (Okumura et al. 2013).

Instrumentations

The growth of phytoplankton was measured spectrophotometrically at 540 nm and correlated with an established cell density-to-absorbance ratio to estimate cell number. Radiochemical measurements for 55Fe tracer were carried out on a liquid scintillation counter (LSC-6101, Aloka, Japan). An inductively coupled plasma atomic emission spectrometer (ICP-AES; Optima 3300XL, PerkinElmer, USA) equipped with an ultrasonic nebulizer (U-5000AT, Cetac, USA) was used for the determination of total Fe concentration in standard solution.

Statistics

The growth and intra and extracellular Fe concentration data were subjected to analysis of variance (ANOVA) to determine significant differences using SPSS (v17 for windows). A p value of ≤ 0.05 was considered statistically significant.

Results

Effect of EDTA analogues on Fe uptake

For P. parvum, intracellular Fe concentration in control treatment was 1.74 ± 0.15 nmol cell−1, while its concentration increased slightly in EDTA (2.30 ± 0.72 nmol cell−1) and DTPA (2.10 ± 0.22 nmol cell−1) treatments. A decrease in intracellular Fe uptake was observed for EDTMP treatment. Extracellular Fe concentrations did not differ between the control and the chelating ligand treatments for this microalga (Fig. 1). Extracellular Fe uptake was also not affected by EDTA analogues for S. marinoi-dohrnii. For H. akashiwo, intracellular Fe concentration was found to be significantly higher (p < 0.05) in EDTA and DTPA treatments than in control and EDTMP treatments (Fig. 1). An opposite trend of intracellular Fe uptake was observed for S. marinoi-dohrnii. Although extracellular Fe concentration in EDTA treatment for H. akashiwo was higher than the control and other EDTA analogues, they did not differ significantly (p > 0.05).

Fig. 1
figure 1

Intra- and extracellular Fe concentrations of marine microalgae influenced by non-ready biodegradable chelating ligands (EDTA, DTPA, and EDTMP). Iron and APC concentrations in the growth medium were 1.5 and 15 μM, respectively. Iron concentration in the phytoplankton cells was determined after 48 h of incubation. Error bars are the standard deviations of three replicated samples

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Effect of biodegradable chelating ligands on Fe uptake

Biodegradable chelating ligands have significant influence on intra- and extracellular Fe uptake in marine phytoplankton. For P. parvum, intracellular Fe concentration was highest in MGDA treatment compared to control, HIDS, and IDS treatments. The lowest extracellular Fe concentration was in MGDA and HIDS treatments for both P. parvum and S. marinoi-dohrnii (Fig. 2). For S. marinoi-dohrnii, intracellular Fe concentration was highest in IDS treatment compared to control, HIDS, and MGDA. The biodegradable chelating ligands did not have significant effect on intracellular Fe uptake for H. akashiwo. However, for this microalga, extracellular Fe concentration was highest in IDS treatment followed by MGDA treatment (Fig. 2). The results indicate that the effect of biodegradable chelating ligands on intracellular Fe uptake differ with microalgal strains, and MGDA and IDS increased intracellular Fe uptake significantly compared to HIDS.

Fig. 2
figure 2

Intra- and extracellular Fe concentrations of marine microalgae influenced by biodegradable (HIDS, MGDA, and IDS) and non-ready biodegradable (EDTA) chelating ligands. Iron and APC concentrations in the growth medium were 1.5 and 15 μM, respectively. Iron concentration in the phytoplankton cells was determined after 48 h of incubation. Error bars are the standard deviations of three replicated samples

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Compared to non-ready biodegradable chelating APCs (EDTA and its analogues), the ratio of intra/extracellular Fe concentrations where substantially higher in biodegradable chelating ligands (Fig. 3). Among biodegradable chelating ligands, the ratio of intra/extracellular Fe concentration was highest in MGDA treatment for P. parvum, while the ratio was almost similar in MGDA, IDS, and HIDS treatments for H. akashiwo (Fig. 3).

Fig. 3
figure 3

Influence of biodegradable and non-ready biodegradable chelating ligands on Fe uptake in marine microalgae. The concentrations of Fe and chelating ligands in the growth medium were 1.5 and 15 μM, respectively. Iron concentration was determined after 48 h of incubation

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Effect of EDTA analogues on microalgal growth

Non-ready biodegradable EDTA and its analogues have significant effect on microalgal growth. EDTMP inhibit the growth of phytoplankton substantially, and the inhibitory effect microalgal growth appeared to be stronger at higher concentrations of the APCs (Fig. 4). The growth of P. parvum was not affected by EDTA; however, its growth was reduced substantially by 7.5 μM of EDTMP and 150 μM of DTPA and EDTMP. The growth of H. akashiwo was not affected by EDTA too, but its growth was completely suppressed by 7.5 and 150 μM EDTMP and 150 μM DTPA (Fig. 4). All the three APCs suppressed the growth of S. marinoi-dohrnii at 1.5, 7.5, and 150 μM concentrations, and a complete stop of the growth of this microalga was observed at 7.5 and 150 μM EDTMP (Fig. 4). Irrespective of the microalgal strain, the overall trend of growth inhibition of phytoplankton by the EDTA analogues was EDTMP > DTPA > EDTA.

Fig. 4
figure 4

Influence of non-ready biodegradable chelating ligands (EDTA, DTPA, and EDTMP) on the growth of marine microalgae. Initially, the microalgae were grown with 1.5 μM Fe and 1.5 μM of EDTA in artificial seawater containing f/2 culture solution. After 4 days, additional 1.5, 7.5, and 150 μM of EDTA, DTPA, and EDTPO were added to the growth medium

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Effect of biodegradable chelating ligands on microalgal growth

Irrespective of the ligands’ concentrations in the culture medium, the growth of P. parvum and H. akashiwo increased gradually until day 14 (Fig. 5). Both concentrations and chemical properties of the biodegradable chelating ligands influenced the growth of S. marinoi-dohrnii substantially. For 1.5 and 7.5 μM HIDS and IDS treatments, the growth of S. marinoi-dohrnii increased initially (until day 8), and then their growth stopped. However, 1.5 and 7.5 μM MGDA treatments did not have inhibitory effect on the growth of S. marinoi-dohrnii. Initial increase in S. marinoi-dohrnii growth was observed at 150 μM HIDS, IDS, and MGDA treatments, but growth inhibition occurred by the chelating ligands for culturing them for longer time.

Fig. 5
figure 5

Influence of biodegradable chelating ligands (APCs; HIDS, MGDA, and IDS) on the growth of costal phytoplankton. Initially, the microalgae were grown with 1.5 μM Fe and 1.5 μM of EDTA in artificial seawater containing f/2 culture solution. After 4 days, additional 1.5, 7.5, and 150 μM of HIDS, MGDA, and IDS were added to the growth medium

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Discussion

Effects of chelating ligands on Fe uptake in and growth of phytoplankton

Regardless of the microalgal species, growth inhibition occurred at high concentrations of synthetic non-ready biodegradable APCs. In addition, the growth inhibition effect of the synthetic APCs appeared to vary depending on the microalgal species (Fig. 4). With the addition of non-ready biodegradable APCs to the culture medium, the growth of phytoplankton increased initially for few days, and then their growth stopped (Fig. 4). Except H. akashiwo, this growth inhibitory effect of synthetic APCs on the phytoplankton was associated with their influence on Fe uptake in microalgal cells (Fig. 1). However, the essential micronutrients such as Se, Co, Zn, Mn, Cu, and Mo remained at > 50% of initial concentrations in the test medium (data not shown). Generally, the uptake of colloidal and soluble iron, i.e., dissolved iron, is the norm of microalgae (Naito et al. 2008). However, it is well established that Fe solubility in oxic waters is too low that phytoplankton can be Fe-limited under such aquatic systems (Anderson and Morel 1982). Synthetic APCs are commonly used in microalgal cultures in order to mimic low Fe concentrations (Gerringa et al. 2000). In previous studies, we reported that the concentrations of chelating ligands and stability constant of Fe-L complex (KFe-L) have significant effect on Fe3+ solubility, bioavailability, and uptake by higher plants (Hasegawa et al. 2011, 2012). The present study also showed that chemical properties and concentrations of synthetic APCs have significant influence on Fe uptake in and growth of phytoplankton.

Growth inhibition of the phytoplankton at high APC concentrations (Fig. 4) and inhibition of intracellular Fe uptake by the phytoplankton for the addition of 15 μM APCs to the culture medium (Fig. 1) indicate that high concentrations of APCs may increase the solubility of Fe3+ in the culture medium, but it does not favor Fe uptake in and growth of the phytoplankton. Thus, being a good Fe fertilizer for phytoplankton (Gerringa et al. 2000), the concentration of APCs in the growth medium can be an important controlling factor for Fe bioavailability and uptake in phytoplankton. Ferric ions and their complexes, which have low solubility in water, are extensively buffered by chelation (Morel and Hering 1993) and increase their dissolved concentrations. In this study, the increased growth of phytoplankton at lower APC concentrations (1.5 and 7.5 μM) than of higher concentration (150 μM) is likely due to the fact that the organisms take up Fe as ferric chelants. However, most of the Fe 3+ in the culture medium formed may soluble Fe-L complexes at high APC concentration (Hasegawa et al. 2012), which may not be bioavailable for the phytoplankton. In addition, the bioavailability of Fe3+ at high APC concentrations may be decreased because of the lopsided complex formation equilibrium reaction (CFER) between Fe3+ and APCs (Hasegawa et al. 2012) that results in the decrease of Fe uptake (Fig. 1) and growth of the phytoplankton (Fig. 4). The suppresses of marine phytoplankton growth, > 50%, at higher concentration of EDTA, 100 μM inhibit cell division, reported from the Northeast Subarctic Pacific (Muggli and Harrison 1996). Iron uptake by marine microalgae decreases with the increase of affinity constant of Fe-ligand complex and ligand-Fe ratios (Sutak et al. 2012). Hutchins et al. (1999) reported that the addition of strong chelating ligand (desferrioxamine B) resulted in artificial Fe limitation to the phytoplankton community in high Fe areas of upwelling region, which is in agreement of the results of our present study.

Although Fe uptake in and growth of phytoplankton were found to be inhibited by non-ready biodegradable APCs, the opposite was observed for biodegradable chelating ligands. The results of the present study showed that, except S. marinoi-dohrnii, the growth of the phytoplankton increased at all concentrations of HIDS, IDS, and MGDA (Fig. 5), and intracellular Fe uptake was substantially higher for the biodegradable chelating ligands than for the non-ready biodegradable APCs (Fig. 3). The extended growth of microalgae after addition of biodegradable chelants in the growth medium was also supported by growing them initially for 4 days with strong chelant, e.g., EDTA. These differential effects of non-ready biodegradable APCs and biodegradable chelating ligands on Fe uptake in and growth of phytoplankton can be explained by their chemical properties and nature of complexation with Fe3+ in aqueous solution (e.g., protonation, stability constant). The protonation and stability constant of synthetic APCs with Fe3+ are higher than those of biodegradable chelating ligands (Pinto et al. 2014). For example, at 25 °C, the stability constant of EDTA with Fe3+ (Fe-EDTA) is 25.1 (Martell and Smith 2004), which is substantially higher than that of biodegradable HIDS (15.0) (Begum et al. 2012), IDS (13.9) (Hyvönen et al. 2003), and MGDA (16.5) (Pinto et al. 2014). In the present study, Fe is likely exists in the culture medium predominantly as inorganic [Fe3+, Fe(OH)2+, Fe(OH)2+, etc.] and colloidal (apparently soluble) forms at 1.5, 7.5, and 150 μM of the biodegradable chelating ligands due to their low stability constants. On the other hand, most of the Fe in the culture medium is likely form Fe-L with EDTA and its analogues (DTPA and EDTMP) at higher (150 μM in this instant) concentration due to their high stability contestants (Hasegawa et al. 2012). As inorganic and colloidal forms of Fe is more bioavailable than Fe-L complex, Fe uptake in and growth of the phytoplankton were higher for biodegradable chelating ligands than those of non-ready biodegradable APCs.

Iron uptake model of phytoplankton under conditions induced by chelating ligands

Composition of Fe(III) species in oxygenated seawater is controlled by complexation and precipitation reaction. Before phytoplankton take up Fe into the cells, Fe(III) species in the medium are supplied on the cell surface, transformed into suitable forms, and then transported across the cell membranes (Marchetti & Maldonado 2016). From the view point of Fe transport systems in phytoplankton, many studies have been made on the two common strategies for Fe acquisitions: biologically mediated reduction of Fe(III) compounds by reductases located on the cell surface followed by Fe(II) uptake (Anderson and Morel 1982; Hudson and Morel 1990) and excretion of iron transporters such as siderophores and citrate that form specific Fe compounds and are subsequently taken up (Trick et al. 1983; Sandy and Butler 2009). There is no firm information to date the latter transport system for eukaryotic phytoplankton. The chelated Fe(III) are not taken up directly across the cell membranes. Berne and Levy (1998) reported that synthetic chelating ligands are too large and polar to move through the plasmalemma lipid bilayer.

Effects of the chelating ligands on the Fe speciation in the medium and the Fe uptake system of eukaryotic phytoplankton are presented in Fig. 6. The chelating ligands, L, complex with inorganic Fe(III) though chelation and produce a soluble species, Fe(III)L. The concentrations and stability constants of the chelating ligands decide the composition of Fe species in the culture medium. The solubility of inorganic Fe(III) in oxygenated seawater without chelating ligands is extremely low (Wells et al. 1995), which limits the supplied amount of Fe to phytoplankton. Addition of chelating ligands have been used to enhance the Fe(III) solubility and to increase the Fe supply to phytoplankton (Morel and Hering 1993).

Fig. 6
figure 6

Conceptual model of iron uptake in marine microalgae when grown with chelating ligands

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In this study, the biodegradable chelating ligands, IDS, HIDS, and MGDA, showed some positive effect on the growth of phytoplankton (Fig. 5). Compared to the control and EDTA, the ratio of intra/extracellular Fe concentrations for biodegradable chelating ligands is substantially higher than that for the non-ready biodegradable chelating APCs (Fig. 3). According to the Fe-binding abilities of the biodegradable ligands (log KFeL = 13.9, 15.0, and 16.5 for IDS, HIDS, and MGDA, respectively), the dissolved Fe in the medium is not only Fe(III)L but also unchelated Fe(III), Fe(III)′, at chemical equilibrium. Dissolved Fe(III)′ is partially found in the colloidal size fraction. Dissolved Fe(III) species in the presence of the biodegradable ligands could be precipitated on the algal cells as colloidal form of Fe(III)′. Recent studies have demonstrated that weak ligands, such as saccharides and other exopolymeric substances, could facilitate access of phytoplankton to iron by more readily disassociating and releasing Fe(III)′ to the cell surface (Hassler et al. 2011, 2015). The biodegradable chelating ligands have characteristic properties of moderate complexing ability and microbial degradability, similar to the exopolymeric substances. Another pathway is direct adsorption of dissolved Fe(III)L on the cell surface. Partial framework of amino acid which the biodegradable chelating ligands compose of might facilitate Fe(III)L adsorption on the secretion-rich cell surface. The composition of the extracellular Fe seems to be different by inorganic and organic structures of the cell surface because the contents of the extracellular Fe are influenced by phytoplankton species and chemical forms of chelating ligands (Figs. 1 and 2).

The feature of biodegradable chelating ligands for enhancement of Fe supply is to keep stable growth of phytoplankton (Fig. 5). The Fe pool on the cell surface is potentially important for the uptake of iron, although microalgal growth is supported by the solubility of organically complexed Fe(III) in the media. The uptake pathway (the way of accumulation) on the cells is more entropically favored than the direct uptake of dissolved Fe species into the cells. The microalgae can utilize the Fe(III)L and Fe(III)′ as the Fe source through the reduction to Fe(II) at the cell surface (Shaked et al. 2005). Several mechanisms have been proposed by which phytoplankton could access Fe(III)L and Fe(III)′, including reduction of the Fe(III) by membrane-bound enzymes or by light (Shaked and Lis 2012). Laboratory experiments have shown that the growth rate of phytoplankton is closely correlated with intracellular Fe concentrations (Sunda and Huntsman 1995).

On the other hand, a large excess of the stronger binding ligands, EDTA, EDTMP, and DTPA, inhibited the microalgal growth (Fig. 4), suggesting that complexation of Fe(III)L in the medium and Fe uptake of phytoplankton are competitive. The growth of phytoplankton was inhibited by the deficient Fe(III)′ due to the addition of higher level of chelating ligands (Shaked and Lis 2012). In another way, the adding of strong chelating ligand may increase iron transport to a cell possibly by transporting ligand-bound Fe through cell membrane, exchanging Fe between ligand and a binding group at cell surface, and supplying enhance Fe at cell surface (Jackson and Morgan 1978). However, the composition of truly dissolved and colloidal Fe(III)′ fractions depends on the concentration of the ligands and stability constant of Fe(III)L. Early works demonstrated that Fe uptake of phytoplankton was proportional to the sum of dissolved inorganic Fe(III) species corresponding to the unchelated Fe(III) (Anderson and Morel 1982; Hudson and Morel 1990; Sunda and Huntsman 1995).

Conclusion

Intracellular Fe concentration for biodegradable chelating ligands was substantially higher than that for non-ready biodegradable EDTA analogues. This reflects the growth reduction of phytoplankton by EDTA analogues compared to biodegradable chelating ligands. The impact of chelating ligands on algal growth is likely to be dependent on the uptake pathway, which depended on types and concentrations of chelating ligands and speciation of iron in the medium. A large excess of the stronger binding ligands such as EDTA, EDTMP, and DTPA had a growth inhibition effect on the microalgae, which is likely due to the competitive behavior of complexation of Fe(III)L in the medium and Fe uptake of phytoplankton. The most essential micronutrients such as Se, Co, Zn, Mn, Cu, and Mo were remained > 50% of initial concentrations in the test medium. In the presence of biodegradable ligands, dissolved Fe(III) species in the growth medium is likely to be precipitated on the algal cells as colloidal form of Fe(III)′ that facilitate Fe uptake into the cells.