Microbial Community from Mangrove Rhizosphere Mechanistic Study and Plant Probiotic Potential in Duckweed

Chemical fertilizers have been utilized to provide nutrients to plants and resulted in rapid
increase of agricultural productivity, which is important to achieve the growing world
population’s demand for food. However, chemical fertilizers can be harmful, with
negative effects on the environmental ecology and human health. Therefore, biofertilizers
are considered as the best substitute for chemical fertilizers for enhancing the growth of
host plants. In this study, mangrove-associated microbes collected from soils were
analyzed and used to develop biofertilizer. The first objective of this study was to identify
a wide range of microbial species and function of genes from three soil samples collected
at the freshwater riverine mangrove at Sungai Lukut, Negeri Sembilan, Malaysia by using
metagenomic Whole Genome Shotgun Sequencing (mWGS). The second objective was
to isolate and identify bacterial species grown on selective media which are nitrogen
fixing bacteria, potassium solubilizing bacteria and phosphorus solubilizing bacteria by
using PCR and sequencing of the 16s rRNA gene. The last objective was to determine the
selected bacterial species that can affect duckweed plants in terms of growth rate and
protein contents. Results from mWGS analysis show different distribution of bacteria of
the genera Bradyrhizobium, Methyloceanibacter and Desulfobacteaceae were detected in
soil samples collected from the three locations. For culture-depending technique, bacterial
species that grew on selective media were identified as Acinetobacter radioresistens,
Brachybacterium paraconglomeratum and Enterobacter cloacae which are screened as
nitrogen-fixing bacteria, Klebsiella quasipneumoniae, Bacillus tropicus, and
Paenibacillus pasadenensis screened as potassium solubilizing bacteria, and Bacillus
cereus and Bacillus thuringiensis screened as phosphate solubilizing bacteria. Next, three
sets of biofertilizers that include Set A containing A. radioresistens, K. quasipneumonia
and B. cereus, Set B that contains B. paraconglomeratum, B. cereus and B. tropicus, and
Set C containing E. cloacae, P. pasadenensis and B. thuringiensis with 106 cfu/g were
formulated by mixing each set of bacterial species with commercial compost. The results
showed that Set C is the best biofertilizer set compared to Set A and Set B as it can
increase the content of nitrogen, phosphorus and potassium in the soil and increase the
growth and protein of the duckweed plants. This indicates that biofertilizers formulated
with mangrove-associated bacteria could enhance the growth of duckweed as well as its
protein content.