COMPOSITION OF BLUE-GREEN-ALGAL MATS AND WATER CHEMISTRY OF THE BANI-MALIK HOT-SPRING (GIZAN PROVINCE), SAUDI-ARABIA

The ecology of microbial mats, water chemistry and invertebrates in th e Bani Malik hot spring (Gizan Province) Saudi Arabia are presented. V arious forms of thick algal mars were identified. They exhibited signi ficant growths and occurred in a temperature gradient (45-50 degrees C ) in a natural stream. An abundance of Phormidium/Oscillatoria, Synech ococcus lividus and Calothrix/Pleurocapsa occurred as visually-obvious mats, In the mats were discrete groupings which were recognizable by their morphologies and color discriminations. These outgrowths were re garded as a nuisance by the area development authority. A community ap parently not previously recorded in the thermal springs of Saudi Arabi a was abundant in a deep thermal pool. This consisted of clusters of p edicellate nodular mats predominated by Calothrix thermalis (90%) with the remainder contributed by Pleurocapsa minor (10%). A cross-section of the mat revealed striking laminations composed of an outer thin co ver (Synechococcus spp.), followed by a middle thick zone (Calothrix t hermalis/Pleurocapsa minor) and finally a central zone (calcite crysta ls) of a rather firm consistency. The other biological structure consi sted of intermittent old and dead filaments of C. thermalis, with a fe w insect larvae associated within the mat. The overwhelmingly lothic a lgal community in a hydrogen sulfide-rich thermal pool appeared to pla y an active role in the photosynthetic activity, resulting in aeration (O-2 = 3.5 mg/l) of the waterbody. The water may be characterized as slightly to-moderately alkaline with high concentrations of reactive-S i (44 mg/l), Cl- (217 mg/l) and SO2-4 (613 mg/l). The water of the spr ing orifice contained hydrogen sulfide (> 5 mg/l).