Host, substrate, and vertical position variations in microalgal communities on mangrove pneumatophores

Mangrove pneumatophores provide unique habitats for algal assemblages, but previous research has mainly focused on macroalgae rather than microalgae. The variations among plant species and substrates on pneumatophores have also seldom been studied. The present study aimed to compare epiphytic microalgal communities on two substrates (adhered soil and pneumatophore itself) and two vertical segments (upper and lower) of pneumatophores in three mangrove species. Epiphytic microalgae varied between substrates and segments, with higher abundances in adhered soil than on pneumatophore itself and higher species richness on the lower than on the upper segment along a pneumatophore. The distribution of microalgal species differed between segments, with Ankistrodesmus spiralis exclusively observed on the upper segment of pneumatophore itself, although Oscillatoria was generally the most abundant genus in all microhabitats. On both substrates and segments, the diversity of microalgae in Sonneratia caseolaris was higher than that in Avicennia marina and S. apetala, with enriched Nitzschia sigma in S. caseolaris but Hydrosera sp. in A. marina. Water content and salinity were the most critical physicochemical factors affecting the overall microalgal community in adhered soil and on pneumatophore itself, respectively. An increase in salinity in adhered soil enhanced microalgal diversity and favored the growth of Nitzschia in both substrates. These results reveal that pneumatophores provide diverse microhabitats for epiphytic microalgae to colonize, with significant variations in compositions between substrates, vertical positions, and mangrove plant species, whereas the key environmental factors shaping the microalgal community were substrate specific.