10. Lipids in Water-Surface Micro\ayers and Foams
253
In contrast to foams from lakes and estuaries, foams from coastal marine
environments reveal a very different fatty acid composition (Table 10.4). Foam
formed in the tidal channels and at the edge of the water during the rising tides in
the Bay of Fundy, Canada, showed saturates of mainly 16:0 (17.0%) and 18:0
(7.0%), monounsaturates of especially 18: 1(09 (25.7%) and 16:1(07 (9.3%), and
also PUFA, accounting for 25% of the total fatty acids (Napolitano et aI., 1992).
PUFA consisted of almost equal proportions of fatty acids of the (06 (mainly
18:2(06) and the (03 series (including 20:5(03 and 22:6(03). The 18:2(06 probably
indicates a direct terrestrial plant input, whereas the presence of 20:5(03 obviously
derives directly or indirectly from microalgae, principally diatoms (Napolitano et
aI., 1992).
Several unusual features were reported in the lipids of the surface microlayers
of California rice fields by Gever et ai. (1996). These authors reported relatively
high concentrations of short-chain fatty acids (8:0 and 10:0) and the presence of
very large concentrations of fatty acid methyl esters (FAME; approximately 95%
of the fatty acids 2::20 carbon atoms long were found as methyl esters). FAMEs
are usually a very minor components of the lipids of natural waters, and their
presence, with a few exceptions (Conte et aI., 1994), is normally regarded as an
artifact of the lipid extraction or a result of environmental contamination. Considering that rice lipids (Gunstone et aI., 1994) and monocotlyledonean cuticular
waxes in general (Kolattukudy et aI., 1976) do not contain appreciable quantities
of FAME and owing to the absence of other obvious natural sources of these
compound, the origin of FAME remains unknown. One can speculate that FAMEs
are the product of contamination of the surface microlayer via deposition of
particles from an unidentified distant source or, more likely, that they may be an
ingredient in the formulation of pesticides applied to the rice field.
10.7. Research Needs
Testing and validation of a standard method for sampling of the water micro layer
for lipid analysis is probably the more critical research need. Devices based on the
principle of the rotating drum, equipped with PTFE-coated surfaces and accessories of the same material, are the more appropriate and accepted tools for the
collection of the surface micro layer. Data are still needed, however, to fully
evaluate their performance, especially those aspects concerning the selective absorption of different lipidic and lipophilic substances.
Information presented in the preceding sections clearly showed the marked
qualitative and quantitative differences between the chemical composition of the
surface microlayer and the underlaying water. As inferred from studies performed
in thin water films (Metsik et aI., 1973; McCafferty et aI., 1970), the dielectric
constant of the top few micrometers of the water is much lower than that of the
bulk of the water. Therefore, it is expected from the surface micro layer to present
a much higher affinity for hydrophobic substances. To the best of our knowledge,
the measurement of this unusual property of natural waters and the resulting
geochemical implications have not been evaluated.
253
In contrast to foams from lakes and estuaries, foams from coastal marine
environments reveal a very different fatty acid composition (Table 10.4). Foam
formed in the tidal channels and at the edge of the water during the rising tides in
the Bay of Fundy, Canada, showed saturates of mainly 16:0 (17.0%) and 18:0
(7.0%), monounsaturates of especially 18: 1(09 (25.7%) and 16:1(07 (9.3%), and
also PUFA, accounting for 25% of the total fatty acids (Napolitano et aI., 1992).
PUFA consisted of almost equal proportions of fatty acids of the (06 (mainly
18:2(06) and the (03 series (including 20:5(03 and 22:6(03). The 18:2(06 probably
indicates a direct terrestrial plant input, whereas the presence of 20:5(03 obviously
derives directly or indirectly from microalgae, principally diatoms (Napolitano et
aI., 1992).
Several unusual features were reported in the lipids of the surface microlayers
of California rice fields by Gever et ai. (1996). These authors reported relatively
high concentrations of short-chain fatty acids (8:0 and 10:0) and the presence of
very large concentrations of fatty acid methyl esters (FAME; approximately 95%
of the fatty acids 2::20 carbon atoms long were found as methyl esters). FAMEs
are usually a very minor components of the lipids of natural waters, and their
presence, with a few exceptions (Conte et aI., 1994), is normally regarded as an
artifact of the lipid extraction or a result of environmental contamination. Considering that rice lipids (Gunstone et aI., 1994) and monocotlyledonean cuticular
waxes in general (Kolattukudy et aI., 1976) do not contain appreciable quantities
of FAME and owing to the absence of other obvious natural sources of these
compound, the origin of FAME remains unknown. One can speculate that FAMEs
are the product of contamination of the surface microlayer via deposition of
particles from an unidentified distant source or, more likely, that they may be an
ingredient in the formulation of pesticides applied to the rice field.
10.7. Research Needs
Testing and validation of a standard method for sampling of the water micro layer
for lipid analysis is probably the more critical research need. Devices based on the
principle of the rotating drum, equipped with PTFE-coated surfaces and accessories of the same material, are the more appropriate and accepted tools for the
collection of the surface micro layer. Data are still needed, however, to fully
evaluate their performance, especially those aspects concerning the selective absorption of different lipidic and lipophilic substances.
Information presented in the preceding sections clearly showed the marked
qualitative and quantitative differences between the chemical composition of the
surface microlayer and the underlaying water. As inferred from studies performed
in thin water films (Metsik et aI., 1973; McCafferty et aI., 1970), the dielectric
constant of the top few micrometers of the water is much lower than that of the
bulk of the water. Therefore, it is expected from the surface micro layer to present
a much higher affinity for hydrophobic substances. To the best of our knowledge,
the measurement of this unusual property of natural waters and the resulting
geochemical implications have not been evaluated.