Metal toxicity and recovery response of riverine periphytic algae

Highlights

• Periphyton was grown in situ on metal (Cu and Zn) diffusing substrates.

• Toxic and recovery response were assessed in traditional and newer diatom metrics.

• Removal of metals resulted in periphyton recovery based on all metrics in 3 weeks.

Abstract

In the present study, in situ assessment of metal (Cu and Zn) toxicity followed by their recovery response was examined in periphyton dominated by diatoms. For doing so, metal diffusing substrates (MDS) were constructed and deployed in the river water for 6 weeks (3 weeks stress and 3 weeks recovery after replacing metal solution from the MDS). The use of MDS ensured that colonised periphyton on metal diffusing and control substrates were exposed to similar environmental conditions. The metal toxicity and recovery response of the community was examined in terms of traditional algal community parameters (biovolume, species richness, Shannon index, relative abundance) as well as with the newer non-taxonomical parameters (deformities and lipid bodies in diatoms). Both traditional and non-taxonomical parameters indicated complete recovery (from metal toxicity) of periphytic communities after 3 weeks following the withdrawal of Cu and Zn solution from the diffusing substrates. Newer non-taxonomical parameters, such as, deformities and lipid bodies, provide a new insight to understand metal toxicity and recovery response of diatom assemblages (the dominant autotrophs in the periphyton community) because these features are directly visible in live frustules, need no expertise in identification of diatoms and can be globally assessed with simple protocol. The experimental loss of metal pollutants and the constant immigration of algae (not previously exposed to high levels of metals) in fluvial systems aided periphyton recovery. Lastly, it is found that periphytic biofilms (dominated by diatoms) proved to be good bioindicators of metal toxicity and recovery in fluvial ecosystem.

Introduction

Metal contamination in aquatic ecosystems is a problem of global concern because of metals' toxic, persistent, non-biodegradable and bioaccumulative properties (Pandey et al., 2014, Pandey et al., 2017). Human activities, such as urbanization, industrialization, mining, etc., are the main source of metal pollution in aquatic ecosystems (Pandey et al., 2018b). Waste waters containing heavy metals that are produced by human activities are often released into the waterbodies without any or with only partial treatment. Metal concentrations higher than their permissible limits impact the aquatic flora. For example, Pandey et al. (2015) reported deformities and induction of lipid bodies in the frustules of phytoplanktonic diatoms when exposed to relatively low (Cu ≥1.3 ppm in the contaminated and ≤1.3 ppm uncontaminated water (WHO, 2004); ≥5 ppm Zn in the contaminated and ≤5 ppm Zn uncontaminated water (ATSDR, 2002) concentration of Cu and Zn (100 ppb) (under laboratory conditions for the time period of 14 days). Similarly, Pandey et al. (2018a) reported deformities and lipid body induction in the periphytic diatom frustules collected from chronically contaminated (with Cu: 0.016–0.021 ppm and Zn: 0.30–0.44 ppm) water bodies in South Korea. Although there is an urgent need to investigate the hazards of metal toxicity on living organisms, an understanding of the response of organisms after withdrawal of metal stress in fluvial ecosystems is also needed, both in terms of system recovery after a long-term metal discharge ceases (Moore and Langner, 2012) and recovery after a short-term spill scenario (Proia et al., 2011). Restoration process includes defining the naturally occurring state of an ecosystem, against which human influences can be measured (Smol, 1992), as well as providing remediation possibilities. The use of benchmarks and associated bioindicators and biomarkers to assess recovery during restoration and indicate when target recovery has been reached can help insure positive environmental outcomes (Haase et al., 2013) in a cost-efficient manner.

Periphyton is a solar-powered consortium of microorganisms forming a biofilm on available substrates and is commonly used for early warning systems for environmental degradation (Larned, 2010). Normally, periphyton is dominated by a particular consortium member (e.g., algae, cyanobacteria, bacteria etc.), depending upon the various biotic and abiotic factors (Larned, 2010). For example, diatom dominated periphyton are mainly distributed near the desiccated riverine sides of fluvial ecosystem (Pandey et al., 2014, Pandey et al., 2018a, Pandey et al., 2018b; Yun et al., 2014), mud-flats (Park et al., 2012; Ryu et al., 2014) and anthropogenically stressed localities for example, acid mine drainage areas (Pandey et al., 2016; Bramburger et al., 2017). Cyanobacteria and green algae dominated periphyton were reported from the paddy fields (Yang et al., 2016a, Yang et al., 2016b) while Liu et al. (2017) reported dominance of green algae, diatoms and cyanobacteria in artificially cultured periphyton.

Diatoms are cosmopolitan and are the main primary producers in waterbodies, forming the major energy source fueling aquatic food webs (Pandey et al., 2017). As a taxonomic group with species-specific environmental requirements, high site fidelity and short-life spans, diatom communities integrate habitat conditions and respond more rapidly to environmental and anthropogenic disturbances than do multicellular organisms (Morin et al., 2016). In addition, diatoms are easily sampled and, for all these reasons, are excellent biological indicators for many types of pollution in aquatic systems (Pandey et al., 2017).

Recovery of periphyton following metal exposure may be fast or slow (Steinman and Mcintire, 1990). For example, Rimet et al. (2005) assessed the response of diatoms to water quality changes after the transfer of diatom dominated biofilms from polluted sites to an unpolluted site. They noted that 40 to 60 days were necessary for the diatom indices calculated for the transferred communities to be similar to the indices calculated for the reference communities. Arini et al. (2012a) reported a more delayed return to an improved ecological status of diatom biofilms, even after 56 days of experimental decontamination from Cd and Zn. Analyses of metal bioaccumulation, cell densities, taxonomic composition, and measures of teratological forms showed that Zn and Cd contents were rapidly lost, reaching reference levels 3 and 9 weeks, respectively, after translocation. The in situ situation is different because of persistence of metals in the environment; Moore and Langner (2012) reported that it would take 90 years for average concentrations of As, Cd, Cu, Pb, and Zn to fall below “probable effects concentrations” (PEC), i.e. levels above which we expect to see adverse environmental effects in the Clark Fork River, West Montana, USA.

Although several studies have addressed metal toxicity on periphyton, the recovery of periphyton communities after a reduction in metals has been little examined (Morin et al., 2010; Corcoll et al., 2012; Morin et al., 2012b; Lambert et al., 2012; Arini et al., 2012b). The speed of recovery of periphyton following metal exposure varies (Steinman and Mcintire, 1990). For example, a 40 to 60 day recovery period was needed for diatom-dominated biofilms transferred from polluted sites to an unpolluted site (Rimet et al., 2005) and Arini et al. (2012a) reported incomplete recovery after 56 days of exposure to metals-free water. The in situ situation is different; Moore and Langner (2012) reported that it would take 90 years for average concentrations of metals to fall below “probable effects concentrations” (i.e. levels above which we expect to see adverse environmental effects) in the mining-impacted Clark Fork River, Montana, USA.

In phototrophs (here periphytic algae), Cu and Zn are important part of electron transport proteins and enzymes associated with photosynthesis yet are toxic at higher concentration (Peers and Price, 2006). In the present study, Cu and Zn were selected as the test metals because they are common environmental pollutants in the waterbodies of India (Rai et al., 2010). Furthermore, we also want to explore that how these two metals which have opposite chemical nature (i.e., Cu is redox active while Zn is not) act on periphytic diatoms in the natural fluvial ecosystem. Periphyton was chosen as testing organism because they are the chief primary producers of various waterbodies and have an attached form, as a result they are true representative of waterbodies, and are well suited material for in situ assessment of ecological health of aquatic ecosystems (Pandey et al., 2018a, Pandey et al., 2018b). Periphyton collected from artificial substrates in an uncontaminated site had an intracellular copper content of 0.0–12.8 μg g⁻¹ dw while intracellular zinc content was found to be between 97.5 and 117 μg g⁻¹ dw (Meylan et al., 2003). On the other hand, Pandey et al. (2016) examined periphyton from metalliferous sites of Rajasthan, India and reported intracellular Cu content in the periphyton from uncontaminated sites between 8 and 21 μg g⁻¹ of fw and at severely contaminated sites between 55 and 126 μg g⁻¹ of fw while intracellular Zn content at the uncontaminated sites ranged between 40 and 85 μg g⁻¹ of fw and at severely contaminated sites it ranged between 200 and 295 μg g⁻¹ of fw. Similarly, Pandey et al. (2014) reported intracellular concentration of Cu and Zn lies between 3 and 12 μg g⁻¹ fw in the periphyton under control condition in the river Ganges while under stress the intracellular concentration of Cu and Zn was reported to be between 14.6 and 26.7 and 17.5–55.5 μg g⁻¹ fw, respectively.

The objective of this study was to assess the in-situ recovery response of mature periphytic diatom communities after the withdrawal of metal stress (Cu and Zn). Previous recovery studies have followed recovery after translocation of metal-affected periphyton to either an uncontaminated reference site (Morin et al., 2016) or to laboratory conditions (Arini et al., 2012a). Our in situ approach used metal diffusing substrates (MDS), with the withdrawal of the metal solutions to initiate recovery. These MDS were based on nutrient diffusing substrates (Scott et al., 2009) and have previously been used to study the combined impact of nutrient enrichment and metal stress on periphyton (Pandey et al., 2014). The recovery response of periphytic diatom communities was assessed using a variety of metrics.