Introduction
The increasing scarcity and rising cost of fishmeal (FM) challenge the sustainability of aquaculture, particularly for carnivorous species with high dietary protein requirements such as sturgeons. Single���cell proteins (SCPs) produced from microorganisms have gained attention as alternative protein sources due to their high protein content, favorable amino acid profiles, and low environmental footprint. Methanotroph bacterial meal derived from Methylococcus capsulatus (Bath) has demonstrated promising results in several teleost species; however, its suitability for sturgeon nutrition remains poorly understood. This study evaluated the feasibility of partially replacing FM with methanotroph bacterial protein meal in diets for juvenile Russian sturgeon (Acipenser gueldenstaedtii), with emphasis on growth performance, feed utilization, nutrient digestibility, gut microbiota, and intestinal morphology.
Materials and Methods
Juvenile Russian sturgeon (initial body weight 90���±���11���g) were fed four isoproteic, isolipidic, and isoenergetic diets for 10���weeks in a recirculating aquaculture system. A control diet contained 30% FM, while three experimental diets replaced FM at 15%, 30%, or 45% with methanotroph bacterial meal. Growth performance and feed utilization were monitored every two weeks. Apparent digestibility coefficients (ADC) of nutrients and energy were determined using yttrium oxide as an inert marker. Gut microbiota structure was assessed by flow���cytometric fingerprinting, and intestinal morphology was evaluated through histological analysis of mid��� and hindgut tissues.
Results
Growth performance and feed utilization were maintained in fish fed diets containing up to 30% FM replacement, with no differences compared to the control. In contrast, a 45% replacement level resulted in significantly lower final body weight, total length, specific growth rate, and protein efficiency ratio (p<0.05). Survival and organosomatic indices were unaffected by dietary treatment. Protein and lipid digestibility were reduced at the highest inclusion level, while marked reductions were observed in carbohydrate and phosphorus digestibility (p<0.05). Digestible energy was highest in the 15% and 30% replacement diets and lowest at 45% replacement. No significant differences were detected in gut microbiota metrics among treatments, although numerical trends suggested altered microbial structure at the highest inclusion level. Intestinal histology revealed no pathological alterations, with only an increase in enterocyte height at 45% replacement.
Discussion
The present study reveals a clear inclusion���dependent response to methanotroph bacterial meal in juvenile Russian sturgeon diets. Fishmeal replacement up to 30% supported growth performance and feed efficiency comparable to the control diet, whereas a higher replacement level (45%) resulted in reduced growth and protein utilization. This nonlinear response indicates the presence of a nutritional threshold beyond which the digestive and metabolic capacity of Russian sturgeon is exceeded.
The maintenance of growth and feed utilization at moderate inclusion levels is consistent with findings reported for other carnivorous fish species fed methanotroph bacterial protein, where partial fishmeal replacement supported efficient nutrient utilization when inclusion remained below species���specific tolerance limits. In contrast, growth depression at high inclusion levels has frequently been associated with post���ingestive limitations rather than reduced feed intake. In the present study, unchanged feed intake combined with reduced apparent digestibility of protein, lipid, carbohydrate, and phosphorus at 45% replacement supports this interpretation.
The pronounced reduction in carbohydrate and phosphorus digestibility at the highest inclusion level likely contributed to the observed decline in digestible energy, providing a mechanistic explanation for the reduced growth despite similar feeding rates. These effects may be linked to the structural complexity of bacterial cell walls and the increased dietary nucleic acid load associated with high SCP inclusion, which can limit enzymatic access to intracellular nutrients and reduce mineral availability. The stability of digestibility coefficients at 15–30% replacement indicates that moderate levels of bacterial protein remain nutritionally accessible to digestive processes in juvenile Russian sturgeon.
Gut microbiota structure and intestinal morphology were largely unaffected by dietary treatment, indicating that growth impairment at high inclusion levels was not associated with intestinal inflammation or microbial dysbiosis. However, numerical shifts in microbial structure at 45% replacement were consistent with reduced nutrient digestibility, suggesting functional modulation of the gut environment rather than pathological alteration. The absence of histological damage further supports the nutritional safety of methanotroph bacterial meal at moderate inclusion levels.
Overall, these findings demonstrate that methanotroph bacterial protein meal is a viable and sustainable alternative protein source for juvenile Russian sturgeon when applied at moderate fishmeal replacement levels, while excessive inclusion compromises nutrient utilization and growth through digestive and energetic constraints