Overview - ISO 21716-4:2025 (Ships and marine technology - Bioassay methods for screening anti-fouling paints - Part 4: Algae)
ISO 21716-4:2025 specifies a laboratory flow-through bioassay method for screening anti-fouling paints using algae as the test organism. The method is designed to be used alongside ISO 21716-1 (general requirements) and focuses on measuring the colour change of algae on painted test panels versus inert, non-toxic control panels. The test uses a brown macroalga (filamentous Ectocarpus sp.) affixed to nitrocellulose substrates and quantifies results in a colour space (L*, a*, b*) to calculate total colour difference.
Key technical topics and requirements
- Test system: Flow‑through laboratory setup to maintain controlled seawater conditions and continuous exposure.
- Test organism: Filamentous brown macroalga (Ectocarpus sp.); cultured stock and preparation procedures are given (see Annex B and C).
- Substrates and affixation: Algae are affixed by vacuum filtration onto nitrocellulose membranes (recommended: ≤8 μm pore size, ~47 mm diameter).
- Test/control panels: Panels (recommended 50 × 50 mm) - painted test panels vs inert PVC control panels.
- Test seawater: Natural seawater filtered (GF/C 1.2 μm), boiled, salinity adjusted to 35.0 ± 1.0 PSU, pH 7.8–8.2.
- Colour measurement: Colour space reported as L*, a*, b* measured by a spectrocolorimeter (per ISO/CIE 11664-4); calculation of total colour difference between test and control.
- Experimental design & validation: Minimum of three runs; each run includes ≥3 test panels and ≥3 control panels. The procedure includes an algae viability test, bioassay operation, validation criteria, and statistical analysis (Annex A).
- Data treatment: Procedures for averaging colour parameters, calculating differences, and interpreting screening results are specified.
Practical applications and who uses this standard
- R&D and product development: Paint manufacturers and formulators use ISO 21716-4:2025 to screen experimental anti-fouling coatings early in development.
- Testing laboratories: Independent marine test labs implement standardized bioassays for reproducible, comparable screening data.
- Regulatory and risk assessment support: Provides objective, repeatable laboratory data to inform decisions in a tiered testing strategy (screening prior to raft, patch, or full-vessel trials).
- Marine technologists and researchers: Use the method to compare coating effects on algal colonization and to identify potential toxicological concerns.
Note: ISO 21716-4:2025 is a screening tool and is not intended to predict in-service performance; field and raft tests remain necessary for operational efficacy assessment.
Related standards
- ISO 21716-1:2020 - Ships and marine technology - Bioassay methods for screening anti-fouling paints - Part 1: General requirements
- ISO/CIE 11664-4 - Colour measurement and colour space (referenced for L*, a*, b* definitions)
- Other parts of the ISO 21716 series cover screening methods for common fouling organisms (e.g., barnacles and mussels)
Keywords: ISO 21716-4:2025, anti-fouling paints, algae bioassay, flow-through system, Ectocarpus, colour space, screening test, test panels, nitrocellulose substrate.
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