Assessment of heat treatment for nutrient preservation in seawater samples

Heat treatments, pasteurization and tyndallization, were tested as potential techniques for preservation of seawater samples for subsequent determination of nutrients (nitrate, nitrite, ammonia and phosphate), either for aquatic studies or for production of reference materials. The advantage over other methods is two-fold: there is no addition of preservative and no special equipment is required for sample storage. The effect of heating on nutrient concentrations was first examined, then long-term homogeneity and stability was assessed. Various types of sample bottles were used. Good physical characteristics of the bottles are essential, in particular air tight closure. Leaks introduce volatile compounds (ammonia, etc.) and bacteria. Nitrate and nitrite show satisfactory behaviour, with insignificant changes under heating and preservation over at least 1 year after pasteurization at a temperature of 65 °C, either in plastic or glass vials. Ammonia shows small concentration changes during the treatment. As ammonia is very sensitive to microbial activity, better stabilization is obtained if pasteurization is undertaken at a higher temperature (80–85 °C). Plastic bottles should be avoided since they are likely to be permeable to ammonia vapors and they favour its microbial consumption. Heat treatment for ammonia should be limited to reference materials, for which homogeneity and stability are the main requirements, with storage of up to several months and adequate checking. Phosphate also shows small changes under heating, but these can be greatly reduced by lowering the pH of the sample to about 7 before the treatment (preferably 80–85 °C). As glass leaches phosphate, plastic bottles are recommended for long-term storage (at least 6 months), but glass can be used for short-term storage or for well controlled reference material.