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.