Effects on organisms in the environment
Bacteria are relatively tolerant towards boron. Acute and chronic effect concentrations range between 8 and 340 mg boron/litre, with most values greater than 18 mg boron/litre. More sensitive are protozoa. Tests with Entosiphon and Paramecium yielded 72-h no-observed-effect concentrations (NOECs) and EC3 values between 0.3 and 18 mg boron/litre.
Boron is an essential micronutrient for cyanobacteria and diatoms. Standard chronic tests with freshwater green algae resulted in no-effect concentrations between 10 and 24 mg boron/litre. Blue-green algae appear to be similar in sensitivity, with an 8-day EC3 of 20 mg boron/litre.
Based on acute toxicity values, invertebrates are less sensitive to boron than microorganisms. For several species, 24- to 48-h EC50 values ranged from 95 to 1376 mg boron/litre, with most values in the 100-200 mg boron/litre range. Chronic toxicity studies with Daphnia magna gave NOECs ranging between 6 and 10 mg boron/litre. Slightly lower NOEC values were obtained from laboratory and field biocenosis studies. The 28-day laboratory study consisting of six trophic stages yielded a NOEC of 2.5 mg boron/litre. Long-term outdoor pond and field studies (not including fish) yielded NOECs up to 1.52 mg boron/litre.
Acute tests with several fish species yielded toxicity values ranging from about 10 to nearly 300 mg boron/litre. Rainbow trout (Oncorhynchus mykiss) and zebra fish (Brachydanio rerio) were the most sensitive, providing values around 10 mg boron/litre.
The toxicity of boron to early life stages of fish has been documented for several species in reconstituted water. Embryonic and early larval stages of rainbow trout, largemouth bass (Micropterus salmoides), channel catfish (Ictalurus punctatus), and goldfish (Carassius auratus) were exposed to boron, as boric acid or borax, from fertilization up to 8 days post-hatch in soft or hard water. Neither water hardness nor the form of boron consistently affected embryo-larval survival of fish. Rainbow trout was the most sensitive species. The NOECs for rainbow trout ranged from 0.009 to 0.103 mg boron/litre.
The effect of natural dilution water on boron toxicity was determined by using surface waters collected from three locations, with boron concentrations of 0.023, 0.091, and 0.75 mg/litre. No adverse effects were determined up to 0.75 mg boron/litre. Lowest-observed-effect concentrations (LOECs) ranged from 1.1 to 1.73 mg boron/litre. One test using deep (600 m) well-water, typically used for aquatic toxicity tests, from a contract laboratory located in Wareham, Massachusetts, USA, yielded a NOEC of >18.0 mg boron/litre. Hence, reconstituted water exposures appeared to overestimate the toxicity determined in natural waters, possibly as a result of nutrient deficiency in the former.
Boron has been known since the 1920s to be an essential micronutrient for higher plants, with interspecies differences in the levels required for optimum growth. Boron plays a role in cell division, metabolism, and membrane structure and function. Boron in the form of borates occurs naturally in fruits, nuts, and vegetables. There is a small range between deficiency and excess uptake (toxicity) in plants. Boron deficiencies in terrestrial plants have been reported in many countries. Boron deficiency is more likely to occur in light-textured, acid soils in humid regions because of boron's susceptibility to leaching. Boron excesses usually occur in soil solutions from geologically young deposits, arid soils, soils derived from marine sediments, and soils contaminated by pollutant sources, such as releases from coal-fired power plants and mining operations. Irrigation water is one of the main sources of high boron levels resulting in toxicity in the field.
Mallard (Anas platyrhynchos) duckling growth was adversely affected at dietary levels of 30 and 300 mg boron/kg, and survival was reduced at 1000 mg/kg.