Abstract
The seasonal and spatial dynamics of diatom-cyanobacteria associations were observed in the coastal waters of the Gulf of Aqaba as well as in the northern and central Red Sea, Saudi Arabia. However, there was an absence of such associated observations in the southern Red Sea. The heterocystous endosymbiotic cyanobacteria
Richelia intracellularis
J. Schmidt, 1901 was found living in association with five centric diatom species, including
Rhizosolenia styliformis
T. Brightwell, 1858,
Rhizosolenia hebetata
Bailey, 1856,
Rhizosolenia imbricata
Brightwell, 1858,
Guinardia cylindrus
(Cleve) Hasle, 1996 and
Proboscia alata
(Brightwell) Sundström, 1986. All five diatom species were observed with
R. intracellularis
in the central Red Sea region, while
R. intracellularis
was not found with
R. imbricata
and
P. alata
in the other Red Sea regions. The density of symbiotic diatom cells showed seasonal variations in the northern and central Red Sea, and the densities were the most conspicuous in these regions compared to the Gulf of Aqaba region. There was seasonal variation in the percent occurrence of the symbiotic diatoms compared to the nonsymbiotic diatoms. Symbiotic associations between the diatoms and
R. intracellularis
were only evident within the central Red Sea region from April to October. Multiple
R. intracellularis
trichomes inside the diatom host cells were common in the central Red Sea, while the other Red Sea regions showed only single trichomes. The size of the individual trichomes of
R. intracellularis
growing inside the host cells was entirely dependent on the size of the respective diatom species (r = 0.781,
P
< 0.01). The complete absence of such specific relations in the southern Red Sea reveals the possibility of different ecological conditions in the region.