The plant-microorganism interaction is a well-studied topic in the world of science due to the sustainable management of the ecosystems. The phyllosphere remains the habitat of some microorganisms where several interactions take place. In order to assess whether the mangrove leaves can harbor a bacterial population and analyze the abundance in these leaves microbiotas, leaf samples of mangroves species (Rhizophora mucronata) were collected in the mangroves of Ouroveni in East-Mbandjini, Grande-Comoros. Through the 16S rRNA genes sequencing, the results showed that in the different experimental group, 105303, 110873, 124703, 146954 and 112225 OTUs were identified respectively, where the canopy was open (C1), semi-open (C2), completely closed (C3), and where the plants are submerged (S) and non-submerged (NS) in seawater. The identified OTUs was positively correlated with leaves-wax (p < 0.05, r2 = 0.91), nitrogen (r2 = 0.72), phosphorus content (r2 = 0.62) and the factor “seawater” (r2 = 0.93). It was however highly and negatively correlated with the canopy cover (r2 = 0.93). Considering the factor "seawater", the relative abundance of bacteria in the submerged leaves was significantly higher compared to that from the non-submerged plants. By taking into account the factor “canopy cover”, it was revealed that more the canopy cover was open, the less was the relative abundance of bacteria. Thus, the finding of this present study affirm that the leaves of mangroves can be a major habitat to host a large population of bacteria that can be influenced by local abiotic factor.

Plants and microorganisms can coexist in such a way that each of these two heterospecific organisms benefit from this association. In the environment of plants there are several habitats of bacteria among them the phyllosphere which is the aerial part of the plant. The phyllosphere can be influenced by several factors including hydrocarbons. Thus, polycyclic aromatic hydrocarbons (PAHs) have been used to assess their influence on the phyllosphere microorganisms of the leaves of Gliricidia sepium. The results showed that the atmospheric concentrations of PAHs are rather high in rural areas. The spatial patterns of atmospheric concentrations of PAHs showed higher concentrations of naphthalene in the two experimental group due to the high road traffic. In the different experimental groups, 93626 and 96954 OTUs were identified in the leaves collected on the road (SR) and out of the road (SH), respectively. In this present study, the leaves harvested on the road which are more exposed to PAHs present a strongly elevated relative abundance of Actinobacteria and Bacilli. It can therefore easily deduce that these bacteria could have developed a kind of resistance to these road PAHs. On the other hand, bacteria belonging to the Alphaproteobacteria class are significantly less represented in this rural area.

The interaction of plants and microorganisms in the rhizospheres and non-rhizospheres of plants is well studied and mastered in the terrestrial environment. In general, given the rhizosphere effect exclusively defining the effectiveness of root exudates to promote multiplication, development and microbial growth in the rhizosphere zones, studies unanimously tend to report that the microbial biomass is rather high in the rhizosphere than in the non-rhizosphere. However, the trend may change in the marine environment. This study was conducted in both the rhizosphere and non-rhizosphere of the mangrove species Rhizophora mucronata at different depths ranging from 0-10 cm, to assess the bacterial community in the rhizosphere and non-rhizosphere and to also address the profile of bacterial community changes. The result showed no difference regarding the bacterial abundance in the rhizosphere and in the non-rhizosphere. However, the abundance of bacteria at 0-5 cm depth was significantly higher in rhizosphere and non-rhizosphere. This could be attributed to the large amount of nutrients available in the surface layer. The unequal distribution of nutrients in the rhizosphere and non-rhizosphere of the mangrove species Rhizophora mucronata could be the consequences of mineralization, immobilization of nutrients in the soil and especially root exudation. The general results of this study can be summarized by showing that if the abundance of bacteria in the rhizosphere zones of terrestrial plants is often high, the trend may be different in aquatic plants, more particularly mangroves, which constitute a separate ecosystem.