PHOTOSYNTHETIC PIGMENTS IN FIFTY-ONE SPECIES OF MARINE DIATOMS

The photosynthetic pigments of 51 species (71 isolates) of tropical and sub-tropical diatoms from 13 out of 22 families were examined. These were the Thalassiosiraceae, Melosiraceae, Coscinodiscaceae, Rhizosoleniaceae, Biddulphiaceae, Chaetoceraceae, Lithodesmiaceae, Eupodiscaceae, Cymatosiraceae, Diatomaceae, Naviculaceae, Nitzschiaceae and Phaeodactylinaceae. Pigments were analyzed by cellulose and polyethylene thin-layer chromatography (TLC) and reverse-phase high-performance thin-layer chromatography (HPTLC). All species contained chlorophylls a and c2 and the carotenoids carotene, fucoxanthin, diatoxanthin and diadinoxanthin. In addition, 14 species (20 isolates) contained one or more of four minor carotenoids, which were not identified further. One species, Thalassiothrix heteromorpha, contained small amounts of a 19′-butanoyloxyfucoxanthin-like pigment, in addition to fucoxanthin. Chlorophyll c2 was present in all the diatoms tested and occurred together with chlorophyll c1 in 88% of them. The presence of both chlorophylls c1 and c2 therefore can no longer be considered a universal characteristic of the diatom class. Where chlorophyll c1 was absent or occurred in trace amounts only (8 species, 11 isolates), it was usually replaced by a new chlorophyll c pigment designated chlorophyll c3, recently characterized from several prymnesiophytes and one chrysophyte. Exceptions were Nitzschia closterium (CS-114), which contained only chlorophyll c2, and Nitzschia bilobata (CS-47), which contained all three chlorophylls (c1, c2 and c3) in approximately equal amounts. Five species that contained chlorophylls c1 and c2 also contained chlorophyll c3 in trace quantities Quantitative pigment analyses of the 71 isolates showed that chlorophyll concentrations ranged from 0.02 μg. 106 cells−1 in the smallest diatom, Extubocellulus spinifer, to 174.4 μg. 106 cells−1 in one of the largest diatoms, Coscinodiscus sp. under the standard growth conditions used. The mean molar ratio of chlorophyll a:c in the 72 isolates was 3.33, with a range of 1.65–7.25. The close similarity between diatom and prymnesiophyte pigmentation was confirmed. Each class has three patterns of pigmentation: viz species with chlorophylls c1 and c2 and‘true’fucoxanthin, species with chlorophylls c3 and c2 and‘true’fucoxanthin, and species with chlorophylls c3 and c2 and fucoxanthin derivatives.