Resilience to multiple stressors in an aquatic plant and its microbiome

Abstract

1Premise Environments are changing rapidly, and outcomes of species interactions, es-2 pecially mutualisms, are notoriously dependent on the environment. A growing number 3 of studies have investigated responses of mutualisms to anthropogenic changes, yet most 4 studies have focused on nutrient pollution or climate change, and tested single stres-5 sors. Relatively little is known about impacts of simultaneous chemical contaminants, 6 which may di↵er fundamentally from nutrient or climate stressors, and are especially 7 widespread in aquatic habitats. 8 Methods We investigated the impacts of two common contaminants on interactions 9 between the common duckweed Lemna minor and its microbiome. Sodium chloride 10 (salt) and benzotriazole (a corrosion inhibitor) negatively a↵ect aquatic organisms 11 individually, yet commonly co-occur in runo↵ to duckweed-inhabited sites. We tested 12 three L. minor genotypes with and without the culturable portion of their microbiome 13 across field realistic gradients of salt (3 levels) and benzotriazole (4 levels) in a fully 14 factorial experiment (72 treatments), and measured plant and microbial growth. 15Key Results We found that stressors had conditional e↵ects. Salt decreased both 16 plant and microbial growth, but decreased plant survival more as benzotriazole con-17 centrations increased. In contrast, benzotriazole did not a↵ect microbial abundance, 18 and benefited plants when salt and microbes were absent, perhaps due to the biotrans-19 formation we observed without salt. Microbes did not ameliorate duckweed stressors, 20 as microbial inoculation increased plant growth, but not at high salt concentrations. 21Conclusions Our results suggest that multistressor e↵ects matter when predicting re-22 sponses of mutualisms to global change, but that mutualisms may not bu↵er organisms 23 from stressors. 24 dation 26 1 29the fates of interacting species, and most organisms engage in at least one mutualism with 30 another species, be it for nutrition, protection, or dispersal. There is concern that mutual-31 ists may be particularly vulnerable to environmental change because mutualism outcomes 32 are often context-dependent. In other words, the magnitude of benefits exchanged between 33 mutualistic partners typically depends on the external abiotic or biotic environment (Cham-34 berlain et al., 2014), and environments are changing at a historically unprecedented pace. As 35 a result, many mutualisms are now taking place in novel environments where partners face 36 multiple stressors (Kiers et al., 2015). A key question is therefore: how do anthropogenic 37 stressors, singly or in combination, a↵ect mutualism outcomes? And in the same vein, does 38 mutualism mitigate or exacerbate global change (Frederickson, 2017)? 39 There is also currently substantial interest in plant-associated microbiomes and how 40 they may facilitate acclimation or adaptation to plant stressors. The term 'microbiome' 41 refers either to the community of microbes living together in an environment, suc...

Presence of the microbiome decreases fitness and modifies phenotype in the aquatic plant Lemna minor

“…In the wild, the fronds and roots are covered in a species rich assemblage of microbes (Gilbert et al 2018, Acosta et al 2020, which can be removed by sterilisation in the lab when used as an experimental model (Bowker et al 1980). Interest in the L. minor microbiome dates back to the early 20 th C, with the observation of an association with N-fixing bacteria (Bottomley 1920), and has accelerated in recent years (Ishizawa et al 2017a, 2017b, 2019, Gilbert et al 2018, Chen et al 2019, Acosta et al 2020, Iwashita et al 2020, O'Brien et al 2020a, 2020b, Tan et al 2021 with a general consensus that plant-microbe interactions play an important role in mediating plant fitness and function. Although most of this research focuses on identifying specific PGPB strains, recent work has characterised the complete core bacterial assemblage associated with L. minor (Acosta et al 2020), which consists of largely Proteobacteria (Pseudomonas and Actinobacteria) and bears a close resemblance to the leaf microbiome in terrestrial plants like Arabidopsis and rice.…”

The importance of ecotype diversity on duckweed growth with and without salt stress

“…I added NaCl (Sigma-Aldrich, 99.5% purity) to half of the tubs (17.53 +/-0.01 g / 6L) for a final concentration of 50 mM. This salt concentration has been shown to be harmful but not lethal to L. minor (Sree et al 2015;O'Brien et al 2020). The salt was added to pre-labelled replicates 3 and 4 for all experimental cultures.…”

“…In polycultures, ecotypes under attack by such algae could have experienced a reduction in pathogenic load because of a host-dilution effect, similarly to the positive effect of plant species richness on soil pathogens (Schnitzer et al 2011;Maron et al 2011). Interestingly, it has previously been found that L. minor associates with a diverse and mostly beneficial microbial community (Ishizawa et al 2017;O'Brien et al 2020). However, the placing of the L. minor populations in tap water under greenhouse conditions may have selected for a subset of algae, which were on average growth-inhibiting.…”

“…Alternatively, it could be that the associated epimicrobial community that hitchhiked on the surface of the duckweed leaves into the experiment had a very strong effect on duckweed growth and survival. Recent studies have shown that there are strong interactions between the duckweed microbiome and the plants fitness as well as response to stressors (O'Brien et al 2020;Tan et al 2021). Together, these findings suggest that experiments conducted with L. minor in axenic conditions may overestimate growth rates and other fitness components but underestimate the strength of population dynamics over time.…”

Plant–environment interactions from the lens of plant stress, reproduction, and mutualisms

“…O'Brien et al. () investigated the impacts of common city stream contaminants—sodium chloride (salt) and benzotriazole (a common corrosion inhibitor)—on interactions between the duckweed Lemna minor and its microbiome. While they found that salt decreases both plant and microbial growth, benzotriazole provided a slight benefit to plant growth, but only when salt and microbes were absent.…”