1. Introduction
Clay minerals commonly exist in water and sediments [1]. They closely interact with microorganisms, providing them habitats and nutrients [2], adsorbing microbial exudates and promote formation of biofilm [3]. Microalgae form foundations of the aquatic food web and directly influence global carbon and nutrient cycling, as well as the energy flow in ecosystems [4]. Clays may influence a number of microalgal physiological processes, such as flocculation and nutrient consumption [5,6].
Previous studies have shown that clay minerals may protect the microalgal cells from heavy metal stress [7]. Some minerals can also be used as carriers of nutrients to promote the growth and reproduction of algae [8]. Moreover, clay could form aggregates with algae and prevent chlorophyll from decomposition [9]. On the other hand, clay minerals may negatively affect algae. For example, they may inhibit algae growth by reducing their mobility, suppressing nutrient acquisition, or releasing metal cations (e.g., Al3+and Fe3+) [1,10]. Moreover, minerals also absorb or reflect light and may change transparency, color and other optical properties of water. Thus, the growth and reproduction of algae may be inhibited, thereby affecting the stability of aquatic ecosystem [11,12]. As a result, clays have been used as algaecide agents. It was documented that more than 90% of Microcystis aeruginosa in water was removed by clays through netting and bridging with cells, leading to algal death [13].
Clay mineral’ structures may influence their properties and interactions with microalgae [14]. For instance, Montmorillonite (Mt) is a layered phyllosilicate and has a large specific surface area and widely exists in aqueous environments [15]. In comparison, palygorskite (Pal) is a hydrated magnesium aluminum silicate mineral with a unique 3D structure and fibrous morphology [3,16]. However, no comparison was made on the effects of Pal and Mt on microalgal cells.
Chlamydomonas reinhardtii is a model green algae [17] and is involved in multiple element cycles between biosphere and hydrosphere, e.g., carbon (C), nitrogen (N), sulfur (S) and phosphorus (P) [18]. It has been documented that P plays a critical role in algal growth [19]. Previous studies reported that C. reinhardtii could accumulate large quantities of P (up to 2-4% of their cell dry weight) [20]. On the other hand, extracellular polymeric substances (EPS) in algae are enriched in proteins and polysaccharides [21]. However, how minerals affect the uptake of P and secretion of EPS has not been clarified.
The aim of this study was to compare interactions between C. reinhardtii and two types of clay minerals, namely Mt (layered) and Pal (fibrous). Variations of physiological parameters in the algal cells, including growth, photosynthesis, respiration, EPS induction, cell morphology and differentially expressed genes, were systematically investigated upon exposure to Mt and Pal.