Sample derivatization
In laboratory, weight samples of sponges were placed into glass tubes with anhydrous
methanol containing 2 M HCl. The tubes were filled
with argon, then securely closed, and
heated for 2 h 90
o
C for complete methanolysis (Meier et al., 2006). After cooling to room
temperature, the tubes were opened and the methanol was evaporated under a stream of
nitrogen, and distilled water was added to reduce the solubility of the FA
methyl esters
formed, which subsequently were extracted with hexane.
Fatty acid analysis
Fatty acid analysis was performed using an Agilent 6890 gas chromatograph equipped with a
mass-selective detector 5973 N, HP-5MS column with a thickness of 0.2 μm and helium as the
mobile phase.The oven was programmed as follows: 90 °C for 4 min, 30 °C min
−1
up to 165 °C,
then 3 °C min
−1
up to 225 °C, and kept isothermal at this final temperature for 10.5 min before
cooling for the next run.
The chromatographic peaks of the methyl ester derivatives were identified by retention time
and mass spectra (NIST 14.0). A standard mixture of 37 FAMEs (Supelco 37 component FAME
Mix, USA) and 26 bacterial acid methyl esters (Bacterial Acid Methyl Esters CP Mix (Supelco,
USA)) was chromatographed for each tenth sample.
Results
B. bacillifera is an endemic large-sized mushroom-shaped sponge
that is widespread in Lake
Baikal. The FA compositions of lipids from studied sponges were compared by using GC with
mass spectrometry. More than 40 FAs from lipid extract of sponges were identified and main
fatty acids are listed in the table 1.
Location
Barguzin bay Southern part of Lake Baikal
Depth, m
745
15
aquarium
Temperature,
о
С
3.7
4.5
10.5
14:0
1.93
2.17
2.40
i15
1.92
0.38
0.33
16:0
4.12
7.05
12.00
i17
1.60
0.11
0.60
a17
1.42
1.14
1.45
17:0
1.39
1.53
1.8
18:0
2.41
1.45
0.51
14:1n11
4.14
0.13
0.49
16:1n7
0.34
0.12
1.13
16:1n9
2.94
1.67
0.92
17:1n8
0.12
2.2
5.35
18:1n9
18.05
16.24
25.5
18:1n7
2.60
1.81
0.14
18:1n11
0.21
0.13
0.52
20:1n9
2.17
0.34
1.23
24:1n9
3.18
3.7
2.94
11
18:3n3
8.22
7.12
10.3
18:2n6
2.72
2.28
6.45
22:6n3
0.15
0.22
0.12
26:3n3
18.99
14.65
10.11
Table 1. Contents of main fatty acids in
Baicalospongia bacilifera, rel. %
Discussion
It was revealed that deep water sponges
B. bacilifera inhabiting at a depth of 750 m contain
higher levels of unsaturated FAs. Structural modifications of membrane lipids are
acknowledged to play a central role in the thermal adaptation of hydrobionts’ tissues, and the
physical properties of the lipids are largely determined by their FA composition (Hazel 1995).
To achieve enhanced membrane fluidity hydrobionts can utilize dietderived unsaturated FAs or
increase the degree of their FA unsaturation by inserting new double
bonds into the existing
acyl chains by using desaturase enzymes (Trueman et al. 2000; Hsieh et al. 2007). As shown
previously,
В. bacillifera contains a sufficient amount of eicosapentaenoic acid (Dembitskii,
1981) and their isomers, whereas the majority of other marine and freshwater sponges
contain only a small amount of these compounds.
In our experiments, this acid was detected
only in trace amounts. The pool of polyunsaturated acids in
B. bacillifera is mainly represented
by demospongic acid 26:3(5, 9, 19), which contains a characteristic 5,9-diene fragment of the
carbon chain. The content of this acid is reduced as the temperature increases which indicates
that the sponge is depressed.
Acids 18:1n9 and 18:2n6 are the most prominent markers sensitively responding
to rise in
ambient temperature. Linoleic acid actively participates in the functioning of chloroplasts, so it
can be assumed that the process of photosynthesis in symbiotic algae is sharply intensified as
the temperature increases. The total content of polyunsaturated fatty acids in the samples
decreased by 10.5 rel.% upon a 6°С temperature rise. The relative content of iso- and antheiso-
branched i15:0, i17:0, ai17:0 acids which are of bacterial origin (Kaneda, 1991) are much higher
in deep water sponges. Probably, the contribution of symbiotic algae in sponges’ diet at great
depths at low temperature, high pressure and absolute darkness is insignificant and the
contribution of symbiotic microorganisms increases.
The results of this study may be used for further research on biochemical adaptations in
hydrobionts, including taxa of lower phylogenetic level. Specific features of biosynthesis and
metabolism under temperature stress in sponges, which often synthesize biologically
active
compounds via nonstandard biosynthetic pathways, may attract the attention of many
ecologists who study the problems of inland waters.
Acknowledgements
The reported work was funded by RFBR according to the research project № 18-35-00439
mol_a.
References
Dembitskii, V.M., 1981. Fatty acid composition of freshwater sponges of
the class Demospongiae from
Lake Baikal: 2. Genera
Swartchewskia and
Baicalospongia // Khimiya Prirodnykh Soedineniy, 4, pp.
12