Azolla along a phosphorus gradient: biphasic growth response linked to diazotroph traits and phosphorus-induced iron chlorosis

Abstract

Azolla spp., a water fern often used for phytoremediation, is a strong phosphorus (P) accumulator due to its high growth rate and N2 fixing symbionts (diazotrophs). It is known that plant growth is stimulated by P, but the nature of the interactive response of both symbionts along a P gradient, and related changes in growth-limiting factors, are unclear. We determined growth, and N and P sequestration rates of Azolla filiculoides in N-free water at different P concentrations. The growth response appeared to be biphasic and highest at levels ≥10 P µmol l−1. Diazotrophic N sequestration increased upon P addition, and rates were three times higher at high P than at low P. At 10 µmol P l−1, N sequestration rates reached its maximum and A. filiculoides growth became saturated. Due to luxury consumption, P sequestration rates increased until 50 µmol P l−1. At higher P concentrations (≥50 µmol l−1), however, chlorosis occurred that seems to be caused by iron- (Fe-), and not by N-deficiency. We demonstrate that traits of the complete symbiosis in relation to P and Fe availability determine plant performance, stressing the role of nutrient stoichiometry. The results are discussed regarding Azolla’s potential use in a bio-based economy.

Effects of shading, fertilization and snail grazing on the productivity of the water fern Azolla filiculoides for tropical freshwater aquaculture

“…As suggested by Brouwer et al (2018), regular harvests of Azolla can be used to maintain cultures in a linear growth phase and to obtain predictable and high biomass yields. Our results confirm that the biomass production of Azolla depends on fertilizer concentrations and presumably by P concentrations because this nutrient is the most limiting in Azolla culture (Subudhi and Watanabe, 1981;Kushari and Watanabe, 1991;Temmink et al, 2018). Although production varied with environmental conditions (light and temperature), productivity remained high (median values: 44, 73 and 79 g FW m -2 d -1 for Azolla maintained under a shade net, clear plastic or exposed to direct sunlight, respectively) 15 when plants were cultivated at high fertilizer concentrations (addition of 16, 10 and 7.5 µmol L -1 of N, P and K respectively), regardless of the environmental conditions.…”

“…100 g FW m −2 and surface cover of ~20%). Plants were acclimated for 11 days without harvesting to minimize P history effects (Temmink et al, 2018). During the experiment, water in drums was changed 6 times per week.…”

“…Phosphorus (P) is a major nutrient limiting the growth of Azolla spp., as in many other photoautotrophic aquatic organisms (Sadeghi et al, 2013;Temmink et al, 2018), because the N2-fixation rates are ensured by symbionts (Cary and Weerts, 1992;Kushari and Watanabe, 1991). In experimental conditions, 2 to 10 μmol L -1 P are reported for the sustainable growth of Azolla (Subudhi and Watanabe, 1981;Kitoh and Shiomi, 1991;Temmink et al, 2018), but field observations suggest that higher concentrations are necessary (10 to 33 μmol L -1 , Sadeghi et al, 2013). Based on this information, P may be limiting for Azolla production even in fertilized ponds used for fish production, where P concentrations are usually between 1-17 μmol L -1 (Green and Boyd, 1995;Pengseng and Boyd, 2011;Pouil et al, 2019).…”

Biomass Growth and Composition of Azolla (Azolla Pinnata R. BR.) Supplemented With Inorganic Phosphorus in Outdoor Culture

“…Fresh weight, RGR and dry weight of A. pinnata were enhanced by the supplementation of P and were maximum (5.01 kg/m 2 , 0.82 g/g per day and 0.25 kg/m 2 , respectively) with 10 ppm P. De et al (2015) observed that under P depletion, A. pinnata had recorded a significant restriction of growth by dry matter loss. Temmink et al (2018) in a laboratory experiment observed significant increase in RGR in A. filiculoides, when culture medium was supplied with 0.3 ppm P and further increase in P up to 10 ppm could not improve the RGR. Gerek (2001) reported that 1122 g/m 2 of fresh Azolla mexicana can be harvested after15 days with the initial fresh weight of 300 g/m 2 of Azolla.…”

Physiological Behavior of the Aquatic Plant Azolla sp. in Response to Organic and Inorganic Fertilizers

“…Adding fertilizers to Azolla sp . enhances its nutrition value [ 38 ]. Moreover, it can be used as a biological filter for purifying water as well as providing food and shelter to fish and invertebrates [ 39 ].…”