On the red coloration of saltern crystallizer ponds. II. Additional evidence for the contribution of halobacterial pigments

In a previous paper we attempted to assess the contribution of red bacteria of the Halobacterium - Haloferax - Haloarcula group and of the β-carotene-rich green alga Dunaliella salina to the red colour of saltern crystallizer ponds. By means of light absorption measurements, we showed that bacterioruberin contained in the bacteria was mainly responsible for the colour of the brines, in spite of the fact that β-carotene derived from Dunaliella was the pigment present in the greatest amount. This apparent discrepancy was explained by the very small in vivo optical cross-section of β-carotene, which is densely packed in globules inside the D. salina cells. We recently observed that the centrifugation technique used in the previous study to collect biomass from the ponds was unsuitable for this type of measurements, as a substantial part of the Dunaliella cells present did not sediment upon centrifugation due to the low specific gravity caused by the high β-carotene content. Therefore similar measurements were performed with biomass collected by filtration. Again, in vivo absorption spectra were dominated by the absorption peaks of bacterioruberin. The results reported here show that, in spite of the methodological problem associated with the earlier study, all views and conclusions expressed in our earlier paper retain their validity.