Potassium metaborate

Toxicity to aquatic algae and cyanobacteria

Administrative data

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

experimental study

Remarks:

reported in a publication

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH
see target record

Cross-reference

Reason / purpose for cross-reference:

read-across source

Remarks:

target record

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Remarks:

reported in a publication

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

read-across

Justification for type of information:

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
Borates in general dissociate immediately upon contact with water and are converted rapidly into i.a. boric acid. This includes salts of boric acid (borates), metaboric acid (metaborates), hydrated borates (hydroborates) or borax. Boron compounds are highly soluble in water, and upon dissolving form essentially two species, undissociated boric acid (H3BO3) and borate anion (B(OH)4- [Soucek, Environmental Toxicology and Chemistry, Vol. 30, No. 8, pp. 1906–1914, 2011]. Orthoboric acid (Short: Boric acid, H3BO3) is a weak monobasic acid, which does not act as a proton donator but OH- acceptor (Lewis base) according to the following equilibrium:
B(OH)3 + 2 H2O ↔ B(OH)4- + H3O+ pKs = 9.2
Metaboric acid ((HBO2)n is formed during heating >90°C via intermolecular condensation while releasing a water molecule. Upon solubilisation in water, orthoboric acid is formed again. In diluted solutions practically only the monomeric H3BO3 are present [Riedel, Anorganische Chemie, de Gruyter, 1999]. A study by Zhu et al. [Zhu FY, Journal of Molecular Structure, Volume 1070, 24 July 2014, Pages 80-85] shows that the main borate species in aqueous KB(OH)4 solutions is B(OH)4−. This ion is in an equilibrium with H3BO3.
So in aqueous solutions at physiological and acidic pH, low concentrations of simple inorganic borates such as boric acid, disodium tetraborate decahydrate, disodium tetraborate pentahydrate, boric oxide and disodium octaborate tetrahydrate will predominantly exist as undissociated boric acid [WHO, Environmental Health Criteria 204, boron, World Health Organization, Geneva, 1998]. Also borax readily dissolves in water to form undissociated boric acid (H3BO3) and borate anion (B(OH)4-) [Soucek, 2011]. Most of the simple inorganic borates exist predominantly as undissociated boric acid in dilute aqueous solution at physiological pH [Hubbard SA, Biological Trace Element Research Vol. 66, 1998]. In aqueous solution, the metaborate ion is rapidly converted to the borate anion and the weakly dissociated boric acid by the sequential reactions shown by the following equations [Antia NJ, 1975, J. Fish. Res. Board Can. 32: 2487-2494]:
BO2- + 2 H2O → B(OH)4-
B(OH)4- + H3O+ ↔ B(OH)3 + 2 H2O
So if metaboric acid (resp. borates) is dissolved in water, orthoboric acid is formed [Riedel, 1999].
So summarizing, upon contact with water, potassium metaborate dissociates immediately into potassium and metaborate ions, whereas the latter is converted rapidly into boric acid.
As stated above, in diluted solutions and biologically relevant pH values, only undissociated boric acid is present, irrespective of which borate was dissolved in water, which so also applies to potassium metaborate. This is applicable for both ecotoxicity tests (usual limit concentration: 100 mg/l) as well as toxicological studies. Borates are readily absorbed orally in humans and animals [Hubbard, 1998], so the expected plasma levels are maximally as high as the applied dose, which still indicates that the boron species dissolved in plasma is H3BO3.
In consequence, data from boric acid and also all types of borates mentioned above, may be used to cover data gaps for potassium metaborate via read-across.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Target: Potassium metaborate, CAS 13709-94-9, EC 237-262-2, BKO2, MW = 81.9081 g/mol, SMILES [K+].[O-]B=O
Source: Boric acid / Orthoboric acid, CAS 10043-35-3, EC 233-139-2, H3BO3, MW = 61.83 g/mol, SMILES OB(O)O
Source: Borax / di-Sodium tetraborate decahydrate / sodium borate, CAS 1303-96-4, EC 603-411-9, Na2B4O7 *10H2O, MW = 381.365, SMILES (anhydrous) [Na+].[Na+].[O-]B(OB=O)OB([O-])OB=O
Source: Sodium tetraborate pentahydrate / Boron sodium oxide, pentahydrate, CAS 12179-04-3, EC 601-808-1, B4-O7.2Na.5H2-O, MW = 291.291 g/mol, SMILES B(=O)OB([O-])OB([O-])OB=O.O.O.O.O.O.[Na+].[Na+]
Source: Disodium octaborate tetrahydrate / Boron sodium oxide, tetrahydrate, CAS ‎12280-03-4, EC 602-894-3, B8Na2O13
Source: Sodium metaborate tetrahydrate / Boric acid, sodium salt, tetrahydrate, CAS 10555-76-7, EC 600-663-1
Source: Dipotassium tetraborate / boron potassium oxide, CAS 1332-77-0, EC 215-575-5, B4K2O7, MW = 233.4358, SMILES [K+].[K+].[O-]B([O-])[O-].[O-]B([O-])[O-].[O-]B([O-])[O-].[O-]B([O-])[O-]
Source: Diammonium tetraborate tetrahydrate / azane;2-hydroxy-4-[(4-hydroxy-1,3,2,4-dioxadiboretan-2-yl)oxy]-1,3,2,4-dioxadiboretane;tetrahydrate, CAS 10135-84-9; 12228-87-4, B4H16N2O11, MW = 263.371, SMILES B1(OB(O1)OB2OB(O2)O)O.N.N.O.O.O.O
Source: Zinc borate, hydrate / dodecaboron tetrazinc docosaoxide heptahydrate / Boron zinc hydroxide oxide / hexaboron dizinc undecaoxide, CAS 138265-88-0, EC 235-804-2, B12Zn4(OH)14O15, MW = 425.7 g/mol

There are no impurities known in neither target nor source chemical(s) which may affect the feasibility of the read-across approach.

3. ANALOGUE APPROACH JUSTIFICATION
As obvious in detail from the available data matrix, all borates exhibit similar (eco-)toxicological properties.
With regard to ecotoxicity, all available studies on various borates on fish, invertebrates, and algae, both short and long term, consistently indicate that, recalculated from the molecular weight, Potassium metaborate does not need to be classified as aquatic toxic (acute and chronic) according to Regulation 1272/2008 and amendments.
Similarly, with regard to human-relevant endpoints, Potassium metaborate does not need to be classified as acutely toxic, as consistently indicated by various borates in also various species.
Both boric acid and borax do not trigger classification as skin sensitizing, no further study data is available, However, Sodium Borate and Boric Acid are used in cosmetics in various functions, and no sensitizing reactions induced by these cosmetic products have been reported.
Borax and Boric acid were similarly non-mutagenic in the Ames Test, and the non-genotoxic potential is further supported by a negative chromosome aberration test. Both Borax and Boric however gave in different species, although via partially species-specific mode of actions, indication that they interfere via a certain threshold with reproduction. Proof via human data is however not available.
Further, all borates chosen for read-across, incl. the registered substance itself, are highly soluble in water, and upon dissolving form essentially two species, undissociated boric acid (H3BO3) and borate anion (B(OH)4-. Hence, read-across is further based on common breakdown products.
So summarizing, read-across is justified via similar (eco-)toxicological effects and common breakdown products.

4. DATA MATRIX
See attachment

Reason / purpose for cross-reference:

read-across source

Qualifier:

no guideline followed

Principles of method if other than guideline:

- Principle of test: The autotrophic growth of 19 species of marine phytoplankters, from 10 classes of algae, was tested on axenic cultures with boric acid additions of 0-100 mg/liter B to a nutrient-enriched, pH-controlled seawater medium.

GLP compliance:

no

Analytical monitoring:

not specified

Vehicle:

not specified

Details on test solutions:

growth medium, containing 0, 5, 10, 50, 100 mg/liter B,

Test organisms (species):

other: 19 species of marine phytoplankters, from 10 classes of algae

Details on test organisms:

Phytoplankters
Axenic cultures of 19 marine species (listed below) were chosen at random from 10 classes of algae.

Chlorophyceae: DunalieIla tertiolecta
Prasinophyceae: Tetraselmis maculata
Haptophyceae: Emiliania huxleyi; Isochrysis galbana
Chrysophyceae: Monochrysis lutheri
Eustigmatophyceae: Nannochloris oculata; Monallantus salina
Bacillariophyceae (diatoms): Phaeodacytlum tricornutum; Thalassiosira fluviatilis; Thalassiosira pseudonana; Cyclotella cryptica; Skeletonema costatum; BeIlerochea polymorpha
Cryptophyceae: Chroomonas salina; Rhodomonas lens
Dinophyceae: Amphidinium carteri
Rhodophyceae: Porphyridium cruentum
Cyanophyceae: AgmeneIlum quadruplicatum; Anacystis marina

Unless otherwise stated, their strain (or clone) symbols and sources were the same as previously reported (Antia and Cheng 1970,Phycologia 9:179-184). The presently designated Emiliania huxleyi and Thalassiosira pseudonana are nomenclatural revisions, due to Hay et al. (1967, Trans. Gulf Coast Assoc. Geol. Soc. 17:428-480.) and Hasle and Heimdal (1970, Nova Hedwigia Z. Kryptogamenkd. 3l: 559-58 1), of the earlier reported Coccolithus huxleyi and Cyclotella nana, respectively. The species named Bellerochea polymorpha corresponds to Guillard's tropical clone 675 D or 675-d isolated off the coast of Surinam (Carpenter and Guillard 1971, Ecology 52: 183-185; Hargraves and Guillard 1974, Phycologia 13: 163-172), while Monallantus salina corresponds to Maestrini's isolate from the Gulf of Marseille (Berland et al. 1970); both these strains were obtained in axenic culture from their respective isolators. It is pointed out that, whereas Nannochloris oculata was previously placed in the Chlorophyceae (Droop 1955) and Monallantus salina in the Xanthophyceae (Berland et al. 1970,Mar. Biol. 7 : 82-92.), we have transferred them to the Eustigmatophyceae on the basis of more recent observations (Antia et al.1975, J. Phycol. 11: 339-343).

Test type:

static

Water media type:

saltwater

Limit test:

no

Total exposure duration:

36 d

Remarks on exposure duration:

Various, 34-36 days, Growth measurements were made, at intervals of 2-4 days.

Test temperature:

16-20°C

Nominal and measured concentrations:

0, 5, 10, 50, 100 mg/liter B

Details on test conditions:

The tests were made in seawater medium, with analytical reagent grade boric acid purchased commercially and used without further purification. Suitable aliquots (4 ml) of the growth medium, containing 0, 5, 10, 50, 100 mg/liter B, in 8-ml capacity screw-capped culture tubes (chosen to be optically clear) were inoculated with 0.2-ml aliquots of a stock culture (generally 10-15 days old) and incubated stationary in continuous, cool-white fluorescent light at 16-20°C; the tubes were placed inclined at an angle of 22-24° from the horizontal plane and the illumination intensity reaching them, from lights placed horizontally above them, was estimated at ca. 100-150 foot candles. Growth measurements were made, at intervals of 2-4 days, on intact cultures by determining the optical density at 600 nm (Bausch and Lomb Spectronic 20) after vortex-mixing. Exponential growth rates were calculated, as previously described (Cheng and Antia 1970), from the plots of turbidity increase with incubation time and expressed as the percentage of corresponding growth obtained in each control taken without borate. Axenic conditions were ensured by using standard aseptic techniques and by taking periodic sterility checks.
A similar procedure was used for the sequential transfer tests of adaptation to increasing borate concentration, using for inoculum the mature growth obtained on the highest borate concentration initially tolerated. When required as a borate-complexing agent, glycerol was incorporated simultaneously with boric acid into the growth medium before inoculation; the glycerol concentration was calculated to be equivalent to that of the boron used.

Reference substance (positive control):

not required

Duration:

14 d

Dose descriptor:

EC50

Remarks:

DunalieIla tertiolecta

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 757.6 mg/L BHO2.K

Duration:

40 d

Dose descriptor:

EC50

Remarks:

Monallantus salina, Skeletonema costatum, Amphidinium carteri

Effect conc.:

> 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 378.8 mg/L BHO2.K

Dose descriptor:

EC10

Remarks:

all species

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 75.8 mg/L BHO2.K

Remarks:

duration not stated

Dose descriptor:

EC50

Remarks:

D. tertiolecta, N. oculata, M. lutheri, M. salina, T. fluviatilis, T. pseudonana, C. cryptica, S. costatum, B. polymorpha, C. salina, A. carteri, P. cruentum, A. quadruplicatum, A. marina

Effect conc.:

> 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 378.8 mg/L BHO2.K

Remarks:

duration not stated

Dose descriptor:

EC50

Remarks:

Tetraselmis maculata, Emiliania huxleyi, Isochrysis galbana, Phaeodacytlum tricornutum, Rhodomonas lens

Effect conc.:

> 10 - < 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 75.8-378.8 mg/L BHO2.K

Remarks:

duration not stated

Validity criteria fulfilled:

not applicable

Conclusions:

The present study investigated scientifically reasonable the acute boron toxicity in various aquatic marine algae species with sufficient documentation. Hence, the results are considered sufficiently reliable to assess the necessity of classification of boron species. Exposure duration was with 14-40 days way above the stipulated 96 h acc. Regulation 1272/2008, so the obtained effect values may be seen as a worst case scenario which does not underestimate the actual hazard arising from boron, and so the limit values for classification as set out in the regulation may be used. Recalculation from all available LC50 values for boron shows that potassium metaborate does not need to be classified as aquatic toxic acc. Regulation 1272/2008, as all LC50 values were above the limit value for classification of 100 mg/l.

Executive summary:

The autotrophic growth of 19 species of marine phytoplankters, from 10 classes of algae, was tested on axenic cultures with boric acid additions of 0-100 mg/liter B to a nutrient-enriched, pH-controlled seawater medium. All the growth rates were virtually unaffected by the incorporation of 5-10 mg/liter B, while 26% of the species were strongly inhibited by 50 mg/liter B and this proportion was increased to 63% species inhibited at 100mg/liter B. Several species required prolonged periods of adaptation before exponential growth with 50-100 mg/liter B. Both the adaptation period and the degree of inhibition were gradually mitigated on sequential transfer of certain species from lower to higher boron concentration. Such sequential transfer tests showed that the majority of initially inhibited species could recover good growth at 50 mg/liter B but not at 100 mg/liter B, which concentration appeared to be lethal to 37% of the species tested. These results predict that, in the absence of stress from nutrient deficiency and pH adversity, inorganic borate pollution would be facilely tolerated by phytoplankton up to 10 mg/liter B; higher borate concentrations up to 100 mg/liter B are expected to cause species redistribution tending to favour growth of some forms by suppressing that of others. Recalculation from all available LC50 values for boron shows that potassium metaborate does not need to be classified as aquatic toxic acc. Regulation 1272/2008, as all LC50 values were above the limit value for classification of 100 mg/l.

Data source

Reference

Reference Type:

publication

Title:

Unnamed

Year:

1975

Materials and methods

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

- Principle of test: The autotrophic growth of 19 species of marine phytoplankters, from 10 classes of algae, was tested on axenic cultures with boric acid additions of 0-100 mg/liter B to a nutrient-enriched, pH-controlled seawater medium.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

Boric acid

EC Number:

233-139-2

EC Name:

Boric acid

Cas Number:

10043-35-3

Molecular formula:

H3BO3

IUPAC Name:

Boric acid

Test material form:

solid: particulate/powder

Details on test material:

White odourless crystalline powder
Melting point: 168 - 171 °C
Water solubility: 49.5 g/l
pH-value: 3.8 - 4.8
Decomposition temperature: ca. 100 - 130 °C
Formation of metaboric acid by loss of water. At 160 deg C a glassy melt of boron trioxide is formed by further loss of water.
LD50 oral rat: 2660 mg/kg

Specific details on test material used for the study:

analytical reagent grade boric acid purchased commercially and used without further purification

Sampling and analysis

Analytical monitoring:

not specified

Test solutions

Vehicle:

not specified

Details on test solutions:

growth medium, containing 0, 5, 10, 50, 100 mg/liter B,

Test organisms

Test organisms (species):

other: 19 species of marine phytoplankters, from 10 classes of algae

Details on test organisms:

Phytoplankters
Axenic cultures of 19 marine species (listed below) were chosen at random from 10 classes of algae.

Chlorophyceae: DunalieIla tertiolecta
Prasinophyceae: Tetraselmis maculata
Haptophyceae: Emiliania huxleyi; Isochrysis galbana
Chrysophyceae: Monochrysis lutheri
Eustigmatophyceae: Nannochloris oculata; Monallantus salina
Bacillariophyceae (diatoms): Phaeodacytlum tricornutum; Thalassiosira fluviatilis; Thalassiosira pseudonana; Cyclotella cryptica; Skeletonema costatum; BeIlerochea polymorpha
Cryptophyceae: Chroomonas salina; Rhodomonas lens
Dinophyceae: Amphidinium carteri
Rhodophyceae: Porphyridium cruentum
Cyanophyceae: AgmeneIlum quadruplicatum; Anacystis marina

Unless otherwise stated, their strain (or clone) symbols and sources were the same as previously reported (Antia and Cheng 1970,Phycologia 9:179-184). The presently designated Emiliania huxleyi and Thalassiosira pseudonana are nomenclatural revisions, due to Hay et al. (1967, Trans. Gulf Coast Assoc. Geol. Soc. 17:428-480.) and Hasle and Heimdal (1970, Nova Hedwigia Z. Kryptogamenkd. 3l: 559-58 1), of the earlier reported Coccolithus huxleyi and Cyclotella nana, respectively. The species named Bellerochea polymorpha corresponds to Guillard's tropical clone 675 D or 675-d isolated off the coast of Surinam (Carpenter and Guillard 1971, Ecology 52: 183-185; Hargraves and Guillard 1974, Phycologia 13: 163-172), while Monallantus salina corresponds to Maestrini's isolate from the Gulf of Marseille (Berland et al. 1970); both these strains were obtained in axenic culture from their respective isolators. It is pointed out that, whereas Nannochloris oculata was previously placed in the Chlorophyceae (Droop 1955) and Monallantus salina in the Xanthophyceae (Berland et al. 1970,Mar. Biol. 7 : 82-92.), we have transferred them to the Eustigmatophyceae on the basis of more recent observations (Antia et al.1975, J. Phycol. 11: 339-343).

Study design

Test type:

static

Water media type:

saltwater

Limit test:

no

Total exposure duration:

36 d

Remarks on exposure duration:

Various, 34-36 days, Growth measurements were made, at intervals of 2-4 days.

Test conditions

Test temperature:

16-20°C

Nominal and measured concentrations:

0, 5, 10, 50, 100 mg/liter B

Details on test conditions:

The tests were made in seawater medium, with analytical reagent grade boric acid purchased commercially and used without further purification. Suitable aliquots (4 ml) of the growth medium, containing 0, 5, 10, 50, 100 mg/liter B, in 8-ml capacity screw-capped culture tubes (chosen to be optically clear) were inoculated with 0.2-ml aliquots of a stock culture (generally 10-15 days old) and incubated stationary in continuous, cool-white fluorescent light at 16-20°C; the tubes were placed inclined at an angle of 22-24° from the horizontal plane and the illumination intensity reaching them, from lights placed horizontally above them, was estimated at ca. 100-150 foot candles. Growth measurements were made, at intervals of 2-4 days, on intact cultures by determining the optical density at 600 nm (Bausch and Lomb Spectronic 20) after vortex-mixing. Exponential growth rates were calculated, as previously described (Cheng and Antia 1970), from the plots of turbidity increase with incubation time and expressed as the percentage of corresponding growth obtained in each control taken without borate. Axenic conditions were ensured by using standard aseptic techniques and by taking periodic sterility checks.
A similar procedure was used for the sequential transfer tests of adaptation to increasing borate concentration, using for inoculum the mature growth obtained on the highest borate concentration initially tolerated. When required as a borate-complexing agent, glycerol was incorporated simultaneously with boric acid into the growth medium before inoculation; the glycerol concentration was calculated to be equivalent to that of the boron used.

Reference substance (positive control):

not required

Results and discussion

Effect concentrations

Effect concentrations 1

Duration:

14 d

Dose descriptor:

EC50

Remarks:

DunalieIla tertiolecta

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 757.6 mg/L BHO2.K

Effect concentrations 2

Duration:

40 d

Dose descriptor:

EC50

Remarks:

Monallantus salina, Skeletonema costatum, Amphidinium carteri

Effect conc.:

> 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 378.8 mg/L BHO2.K

Effect concentrations 3

Dose descriptor:

EC10

Remarks:

all species

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 75.8 mg/L BHO2.K

Remarks:

duration not stated

Effect concentrations 4

Dose descriptor:

EC50

Remarks:

D. tertiolecta, N. oculata, M. lutheri, M. salina, T. fluviatilis, T. pseudonana, C. cryptica, S. costatum, B. polymorpha, C. salina, A. carteri, P. cruentum, A. quadruplicatum, A. marina

Effect conc.:

> 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 378.8 mg/L BHO2.K

Remarks:

duration not stated

Effect concentrations 5

Dose descriptor:

EC50

Remarks:

Tetraselmis maculata, Emiliania huxleyi, Isochrysis galbana, Phaeodacytlum tricornutum, Rhodomonas lens

Effect conc.:

> 10 - < 50 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Remarks:

boron

Basis for effect:

growth rate

Remarks on result:

other: corresponding to 75.8-378.8 mg/L BHO2.K

Remarks:

duration not stated

Applicant's summary and conclusion

Validity criteria fulfilled:

not applicable

Conclusions:

The present study investigated scientifically reasonable the acute boron toxicity in various aquatic marine algae species with sufficient documentation. Hence, the results are considered sufficiently reliable to assess the necessity of classification of boron species. Exposure duration was with 14-40 days way above the stipulated 96 h acc. Regulation 1272/2008, so the obtained effect values may be seen as a worst case scenario which does not underestimate the actual hazard arising from boron, and so the limit values for classification as set out in the regulation may be used. Recalculation from all available LC50 values for boron shows that potassium metaborate does not need to be classified as aquatic toxic acc. Regulation 1272/2008, as all LC50 values were above the limit value for classification of 100 mg/l.

Executive summary:

The autotrophic growth of 19 species of marine phytoplankters, from 10 classes of algae, was tested on axenic cultures with boric acid additions of 0-100 mg/liter B to a nutrient-enriched, pH-controlled seawater medium. All the growth rates were virtually unaffected by the incorporation of 5-10 mg/liter B, while 26% of the species were strongly inhibited by 50 mg/liter B and this proportion was increased to 63% species inhibited at 100mg/liter B. Several species required prolonged periods of adaptation before exponential growth with 50-100 mg/liter B. Both the adaptation period and the degree of inhibition were gradually mitigated on sequential transfer of certain species from lower to higher boron concentration. Such sequential transfer tests showed that the majority of initially inhibited species could recover good growth at 50 mg/liter B but not at 100 mg/liter B, which concentration appeared to be lethal to 37% of the species tested. These results predict that, in the absence of stress from nutrient deficiency and pH adversity, inorganic borate pollution would be facilely tolerated by phytoplankton up to 10 mg/liter B; higher borate concentrations up to 100 mg/liter B are expected to cause species redistribution tending to favour growth of some forms by suppressing that of others. Recalculation from all available LC50 values for boron shows that potassium metaborate does not need to be classified as aquatic toxic acc. Regulation 1272/2008, as all LC50 values were above the limit value for classification of 100 mg/l.