Global food security demands a significant increase in crop production by 2050, necessitating a substantial rise in nitrogen fertilizer use. However, the Haber-Bosch process, currently used to produce synthetic nitrogen fertilizers, is energy-intensive and contributes significantly to greenhouse gas emissions. This creates an urgent need for sustainable alternatives. Current organic fertilizers, while improving soil structure, often have low nitrogen content, requiring large quantities and potentially leading to negative effects like increased soil salinity. Microbes, particularly purple non-sulfur bacteria (PNSB), have emerged as potential slow-release fertilizers due to their high nitrogen content and ability to fix nitrogen. This study investigates the potential of *Rhodovulum sulfidophilum*, a marine PNSB, as a source of nitrogen fertilizer, focusing on the utilization of its lysed and dried biomass for plant growth. The researchers aim to assess if the nitrogen within the bacterial biomass can be effectively utilized by plants as a replacement for synthetic fertilizers, thus contributing towards more sustainable agriculture practices and reducing environmental impact.

The literature review highlights the significant environmental impact of current nitrogen fertilizer production, particularly the Haber-Bosch process's contribution to greenhouse gas emissions. It discusses the limitations of existing organic fertilizers like manure and compost, which often possess low nitrogen content and may lead to increased soil salinity or nitrous oxide emissions when applied in large quantities to compensate for the nitrogen deficiency. The review then introduces purple non-sulfur bacteria (PNSB) as promising alternatives, citing their ability to fix nitrogen and produce various plant growth-promoting substances. However, the review points out a lack of definitive evidence on the direct uptake of nitrogen from PNSB by plants and their ability to completely replace mineral fertilizers. Previous work on *R. sulfidophilum* highlights its suitability for heterologous protein expression and polyhydroxyalkanoate production, but its potential as a nitrogen fertilizer source remains unexplored. This gap in knowledge forms the rationale for the current study.

Lysed and dried bacterial biomass (PB) from *R. sulfidophilum* was prepared from 5-day-old cultures. The biomass underwent a lysis process (either high-pressure homogenization or ultrasonication) followed by freeze-drying. Elemental analysis determined the nitrogen, phosphorus, potassium, and carbon content of the PB. Amino acid composition, both free and total, was also quantified. The efficacy of PB as a fertilizer was evaluated using Japanese mustard spinach (*komatsuna*) under various conditions. Different application rates of PB were compared to mineral fertilizer controls (C1 and C2, representing different nitrogen levels) and negative controls (no nitrogen and no fertilizer). Experiments were conducted under three conditions: spring cultivation, summer cultivation (to determine the maximum tolerable PB application rate), and temperature-controlled cultivation (cool and warm conditions). Plant growth parameters, including germination rates, leaf chlorophyll content (SPAD values), maximum leaf length, fresh weight, and dry weight, were measured. The effect of supplementing PB treatments with phosphorus and potassium was also tested. Soil analysis included pH, electrical conductivity (EC), and other parameters before and after cultivation. Total plant nitrogen content was also determined. Statistical analyses (ANOVA, Tukey's post hoc, Pearson correlation) were used to compare the results across different treatments.

The *R. sulfidophilum* biomass (PB) exhibited a high nitrogen content (11%) and a low C:N ratio (4.7), suggesting its suitability as a fertilizer. Application rates of PB up to four times the nitrogen level of the mineral fertilizer control (C1) did not negatively impact plant growth. However, to achieve similar plant growth parameters as C1 and C2, approximately twice the amount of PB was needed, indicating a mineralization rate of about 62%. Under both cool (15-25°C) and warm (22-32°C) temperature-controlled conditions, PB treatments showed comparable or superior growth parameters (SPAD values, leaf length, fresh weight, dry weight) to mineral fertilizer controls, particularly at higher PB application rates (PB4). The positive correlation between plant dry weight and total plant nitrogen with nitrogen input confirmed the effective uptake of nitrogen from PB. The study found no significant improvement in plant growth with added P and K supplementation to PB treatments. The dry weight of plants was higher under warm conditions compared to cool conditions, irrespective of the fertilizer treatment. Soil organic carbon content was comparable between PB and mineral fertilizer treatments.

The study successfully demonstrates the potential of *R. sulfidophilum* biomass as a sustainable nitrogen fertilizer. While requiring a higher application rate compared to mineral fertilizers due to its mineralization rate, the use of PB offers significant environmental advantages by minimizing the harmful effects associated with synthetic fertilizer production and use. The comparable plant growth parameters observed with PB across temperature regimes confirm its viability as a nitrogen source for plants. The high protein content in PB, rich in essential amino acids, might contribute directly to plant growth through uptake of peptides and amino acids by plant roots. The findings support the hypothesis that plants can directly uptake nitrogen from the bacterial biomass, beyond the mineralized nitrogen. The lack of significant benefits from P and K supplementation suggests that this biomass could be a suitable stand-alone nitrogen fertilizer in soils already rich in these nutrients.

This research demonstrates the feasibility of using lysed and dried *R. sulfidophilum* biomass as a sustainable nitrogen fertilizer. While higher application rates are needed compared to conventional fertilizers, the environmental benefits outweigh this aspect. Future research should focus on optimizing PB production, conducting a life cycle assessment to evaluate its overall environmental footprint, and exploring strategies to enhance its nitrogen mineralization rate. Further investigation into the direct uptake of amino acids from PB by plants is also warranted.

The study's scope is limited to one plant species (*komatsuna*) and a specific soil type. The mineralization rate of PB might vary depending on soil conditions and environmental factors. The study didn't fully explore the long-term effects of PB on soil health and nutrient cycling. A comprehensive life cycle assessment is needed to fully evaluate the economic and environmental feasibility of PB as a commercial fertilizer.