How NMs exert effects on cell growth is an important issue. They may be distributed around cells or penetrate the cell wall to function. Several reviews have summarized the interactions of NMs and microalgal cells, including adsorption, distribution, and ecotoxicity [19,20,21]. Several examples show their contact relationships. NMs can be distributed around cell walls when applied in microalgae cultivation, which limits cell exposure to nutrients and light [22]. Those NMs with a size smaller than that of the cell wall pore were supposed to enter the cells directly [23]. Larger NMs could penetrate the cell wall through embedding [24], membrane perforation, and endocytosis [21]. After entering the cell, NMs could physically contact various organelles and damage or alter their structures and functions [25,26]. Thus, it is critical to examine the mode of interaction (extracellular or intracellular) when discussing the effects of functional mechanisms of NMs applied in microalgal cultivation.

As the main form of energy used by algae, light is one of the most important factors affecting microalgal growth. Light energy is absorbed and transmitted through photosynthetic pigments, including chlorophyll, carotenoids, and phycobiliprotein, in photosynthetic systems. However, these pigments can only cover no more than 10% white light [27]. As the main photosynthetic pigments, chlorophyll a and b have only a dual-absorbance range of blue (450–480 nm) and red (605–700 nm) lights [27]. To maximize the use of solar energy, developing high-performance light conversion materials to improve the absorption efficiency of red and blue lights or utilizing the lights of other wavelengths for growth may be a feasible approach [28,29,30]. A mechanistic illustration of the application of NMs in improving light utilization efficiency and growth of microalgae is shown in Figure 1.

In liquid suspension cultivation, cell shading can cause insufficient absorption of blue and red lights by chlorophyll molecules [31]. Light utilization efficiency can be improved by light-harvesting NMs to maintain normal photosynthetic systems in microalgae [32]. One of the mechanisms is to enhance red or blue light absorption of microalgal cells. It was reported that some NPs selectively enhanced microalgal blue light absorption through plasma light scattering, increasing the growth of Chlamydomonas reinhardtii (Chlorophyta, Chlorophyceae) and Cyanothece (Cyanophyceae) 51142 by more than 30% when exposed to the full spectrum [33]. Additionally, the localized surface plasmon resonance (LSPR) of metal NPs was used to filter light at specific wavelengths within a bioreactor, and the LSPR wavelength could be tuned to the violet-blue or red regions [34]. The photosynthesis of Mung Bean was increased by more than two-fold, and the dry weight was increased by 15.39% [34].

Near-infrared (NIR) light, accounting for approximately 52% of the solar spectrum, is not effectively utilized by photosynthesis [35,36]. The photosynthetic efficiency will be improved if NIR light can be utilized by microalgae. A feasible route is to convert NIR light into visible light with photon-up conversion (UC) materials [37]. It was reported that NaYF₄:Yb,Er, as a UC material, could efficiently transform NIR light to visible light (mainly green and red lights) via multiple-photon absorption [38,39]. Due to the potential of carbon dots (CDs) in light conversion, some studies explored the joint effect of NaYF₄:Yb,Er, and CDs. For example, the construction of NaYF₄:Yb,Er + CD nanocomposites further improved the conversion of NIR light to red light [40]. CDs modulated the UC emission of NaYF₄:Yb,Er by efficient energy transfer [40].

In the photosynthetic system, the wavelengths at 500–600 nm (yellow and green lights) are not well absorbed by microalgae [27]. To improve the absorption efficiency of these lights, light-trapping NMs can be developed to convert poorly absorbed yellow and green lights into highly absorbed visible lights. Among light-trapping NMs, CDs were applied as good electron donors or acceptors in light energy conversion [41,42,43,44] due to their high quantum yield, chemical stability, and superior biocompatibility [45]. At present, these CDs can emit red light under yellow-green or green light. For example, the CDs with tunable emission could directionally shift unutilized yellow-green light (500–600 nm) to red light (580–700) to promote growth by 15% in Chlorella sp. [13]. They could also enhance microalgal photosynthetic activity by redshifting the incident light [46]. However, the synthesized CDs did not emit blue fluorescence under yellow or green light [46]. Under 1 mg/L CDs treatment, the photosystem II activity of Chlorella was significantly enhanced, and the growth rate was increased by 52.7% [46]. In addition, artificially synthesized polyolefin-based fluorescent dyes could convert green light (500–570 nm) to red light (580–650 nm) and contribute to microalgal biomass increase [14].

Ultraviolet (UV) light (200–400 nm) usually has negative effects on microalgal growth and is also poorly adsorbed by microalgal photosynthetic systems [47]. In microalgae, it has rarely been reported that light-trapping NMs can convert UV light into highly absorbed visible light [48]. However, two aggregation-induced emission luminogens (AIEgens) were recently reported to absorb UV/blue light to emit green and yellow lights, which were efficiently used by Cyanobium bacillare (Cyanophyceae) [49]. Under AIEgens treatment, several photosynthetic parameters were significantly improved, and the growth (in terms of cell density) of C. bacillare was boosted five-fold [49]. More reports about UV light conversion are provided in the plant research area. For example, the synthesized CDs were reported to emit blue fluorescence under UV excitation [43,50,51]. Functionally modified CDs could enhance the spectral transformation of UV light, such as increasing graphitic-N and hydroxyl group contents [42], vinyl alcohol encapsulation [52], and amine functionalization [52]. A multifunctional CD was reported to match the chloroplast absorption spectrum (blue and red lights) by strong absorption of UV light [53]. In addition, aggregation-induced emission carbon dots (CD-AIEgens) had strong UV absorption in natural light [54], exhibiting application potential in mass cultivation.

The aggregate state of NMs may cause the quenching effect of fluorophores in water, which could decrease the efficiency of spectral conversion. To solve this problem, sustainable aggregation-induced emission (AIE) materials were prepared from natural resources [55,56,57]. These resources include quercetin, lignin, and rosin, which can eliminate the quenching effect of the existing materials developed to enhance photosynthesis [56,57,58]. In the state of aggregation, AIE materials produced stronger excitation than traditional aggregation-caused quenching luminescent NMs [59,60]. In addition, the coupled application of AIEgens and CDs could obtain unique optical properties, such as high AIE-active fluorescence, efficient harvesting of UV light, and good photostability [55].

To avoid the aggregation-caused quenching in light-harvesting NMs, special scaffolds have also been developed, including macrocycles [61,62,63,64], DNA [65,66], and cyclic peptides [67,68]. The design and synthesis of ideal scaffolds are complex. To greatly simplify the fabrication steps of special scaffolds, the employment of AIE materials was presented [69,70]. In addition, supramolecular polymerization is an excellent strategy for constructing light-harvesting NMs, which can assemble the chromophores together to pack tightly and enhance supramolecular assembly-induced emission of the chromophores [70]. New polymerizations have been reported based on ureidopyrimidinone quadruple hydrogen bonding units [71] and tetraphenyl-ethylene [72]. Organic dyes are classical optical materials that can be used in AIE NPs. A hybrid dye system based on the tetraphenylene-encapsulated organic dye (Nile red) was synthesized, which had a considerable redshift distance (~126 nm), with a high energy-transfer efficiency of 99.37% and an antenna effect of 26.23% [73].

The NM solutions are easily dried or washed away when applied to plants and algae [74]. To gain a prolonged enhancement of photosynthesis by NMs, continuous leaf spraying or hydroponic conditions throughout the plant growth process are needed, but this results in high labor costs or large-scale facility construction [41,75,76]. In vitro spraying of adhesive fluorescent coatings on leaf surfaces with continuous fluorescence emission and good rain–erosion resistance provides a new tool for efficient photosynthesis enhancement [74]. CDs and some monomers could be used to synthesize covalently cross-linked polymers with prolonged fluorescent capacity, rain–erosion resistance, and stability [77]. In addition, a fluorescent polymer coating was developed, which consisted of UV-excited, blue light-emitting nitrogen-doped CDs as the fluorescent body to catalyze the covalent copolymerization of CDs and tannic acid [77]. This fluorescent polymer coating exhibited excellent fluorescence properties, stability, nontoxicity, and rain–erosion resistance [77]. Such light harvesting NMs used in plants should also have good potential to be applied in microalgae cultivation for enhancing photosynthetic efficiency.

Various environmental stressors can induce reactive oxygen species (ROS) in microalgal cells, generating oxidative stress [47]. Intracellular antioxidants from algal cells can mitigate cellular damage. However, the effectiveness of these antioxidants is sometimes limited. Recently, antioxidant NPs were synthesized by amalgamation of material sciences with nanotechnology, including carbon nanotubes (CNTs), metal NPs, and metal oxide NPs [78,79]. In addition, the transport function of NMs was developed to transport antioxidants into cells to oxidative stress damage [80].

Some NMs can scavenge ROS and mimic antioxidant molecules. Pristine CDs reduced the UV light damage by scavenging 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical in Chlorella vulgaris (Chlorophyta, Trebouxiophyceae) [81]. The antioxidant activity is related to the long-conjugated C=C chains of the CDs [82]. Similarly, cerium oxide NPs (CON) exhibited superoxide dismutase (SOD) activity, which reduces the levels of superoxide anions [83,84]. The antioxidant level of CON is related to nanocrystal diameter since smaller diameter nanocrystals are found to be more reactive towards H₂O₂ [85]. CON could be applied multiple times and used for several weeks since the particles remained colloidally stable [85]. To further improve the ROS scavenging ability under broad-scale environmental conditions (e.g., application in biological tissues and cells), CON could be wrapped with biocompatible polymers, such as PEGylated and dextran [86,87]. The dextran-coated CON was applied in the chloroplasts of photosynthetic organisms to improve the antioxidant capacity of their photosynthetic system [35,88].

Functionalized NMs derived from various biological extracts of living organisms, such as proteins, EPSs, and terpenes, show potential antioxidant activity. For example, extracellular protein (from Escherichia coli)-capped gold NPs showed excellent antioxidant ability [89]. The DPPH radical scavenging activity of gold NPs was found to be dose-dependent, with the maximum inhibition being greater than that of the extract alone [89]. The EPS-mediated silver (Ag) NPs also showed excellent antioxidant activities at a suitable concentration [90]. Terpene-rich extracts were used to synthesize the antioxidants of Ag NPs [77]. The obtained antioxidant ability was comparable to that of ascorbic acid [91]. In addition, the crude extracts of living organisms were also used to synthesize NM antioxidants, including leaf extracts [92,93,94], cell-free supernatants [94,95], and other extracts [96,97,98].

Many chemical compounds, either endogenous or exogenous, have been evaluated for their antioxidant properties, which have the potential to modulate oxidative stress [99]. Recently, NPs were found to efficiently enhance antioxidant activity and provide targeted delivery of certain antioxidants that show poor cell membrane permeation and cell internalization [80]. Encapsulation of vitamin E and catechol in Ag NPs facilitated the scavenging of DPPH radical, hydrogen peroxide, and nitric oxide [100]. The Ag NPs were also therapeutically applied for targeted delivery in breast cancer treatment [100]. Biodegradable polyanhydride NPs containing the mitochondrion-targeted antioxidant apocynin were explored for treating neuronal cells, and their pretreatment significantly protected cells against H₂O₂-induced toxicity [101]. The curcumin-encapsulated NPs with dual responses to oxidative stress and reduced pH could efficiently reduce the excess oxidants produced by lipopolysaccharide-stimulated macrophages [102]. These studies provide useful references for decreasing the damage of oxidative stress in microalgal cells generated from adverse environmental conditions. Moreover, precise delivery by NMs to specific locations, such as cell membrane, chloroplast, and nucleus in microalgae, needs to be explored with the aim of advancing the development of microalgal biotechnology.