Microalgae are ubiquitous photosynthetic microorganisms found in nature. They have been reported to synthesize many bioactive chemicals that stimulate plant development. Our experiments aimed to examine the effects of three selected strains of eukaryotic green microalgae on plant growth. Two Chlorella species (MosonmagyarĂłvár Algal Culture Collection (MACC)-360 and MACC-38) and a Chlamydomonas reinhardtii strain (cc124) were examined in Medicago truncatula, A17 ecotype, in the first portion of our investigation. First, using growth curves and microscopy, the growth patterns, cell size, and morphology of the microalgal strains were determined. In addition, their ability to synthesize auxins was evaluated. In the greenhouse, M. truncatula was grown in pots containing a mixture of vermiculite and soil (1:3) with a clay layer at the bottom. Living algae cells were applied to the plants using the soil drench method. The plants’ physiological reactions to adding algal biomass were comprehensively studied. Microalgae substantially boosted the plant’s stem length, leaf size, fresh weight, number of flowers, and pigment content. For most of the investigated factors, there was a strain-specific effect. Overall, the application of Chlorella sp. MACC-360 resulted in more robust plants with greater fresh biomass, larger leaves, and more flowers/pods than the control, which received the same total nutrients.
In the second phase of the investigation, the biostimulant effects of Chlorella sp. MACC-360 and C. reinhardtii cc124 on Solanum lycopersicum (tomato) were studied. This study’s first purpose was to determine whether the two strains had biostimulant effects on tomato plants. The significance of application mode and timing (plant age) was also studied. Thirdly, the strain-specific effects of the two algal strains were evaluated. Finally, transcriptome and metagenomic analyses were conducted to identify the molecular effect of algae and the microbial community of the rhizosphere. Tomatoes were grown in pots with a clay layer at the bottom and a mixture of soil and vermiculate (2:1). In two sets of trials, living algae and algal extract and living algae and spent media plus extracts were applied to the soil and plant leaves, respectively. In the first group, the culture suspension (treatment A) was centrifuged, the algae pellet was re-suspended in water to make (Treatment B), and this was applied weekly to the soil, while the algae extract (cell disrupted algae suspension – Treatment C) was sprayed on the leaves bi-weekly. Analyses were conducted on the blooming process, plant morphology, fruit characteristics, and pigment content. In the second set of tests, culture suspension (A) was administered weekly to the soil, and C was sprayed bi-weekly on the leaves. The kinetics of flowering, reproductive capability, and photosynthetic characteristics were investigated. Both algal strains enhanced the leaf pigments, fruit weight, and fruit diameter. The age of the plant at the onset of treatment was a significant determinant of the outcome; treatments initiated later (week 5) produced superior results than those initiated at a juvenile level (week 1). Chlorella sp. MACC-360 stimulated early blooming and fruit development, whereas C. reinhardtii cc124 greatly slowed these processes. Chlorella facilitated the transformation of light energy into chemical energy, whereas Chlamydomonas boosted the protection of photosynthetic parameters. Both strains increased leaf thickness and leaf temperature differential. Both algal strains enhanced crucial agronomically useful tomato processes.
The upregulation of genes involved in systemic resistance demonstrated that microalgae readied plants to respond to abiotic stress and pathogen attacks, as evidenced by the transcription data. According to soil metagenomics research results, algae influenced the construction of the tomato rhizosphere microbiome. In soils saturated with microalgae, the number of Ascomycota fungus, Streptomyces, Brevundimonas, and other helpful bacteria that provided plant nutrition and defense against dangerous microbes increased.