BackgroundDuckweed (family Lemnaceae) has recently been recognized as an ideal biomass feedstock for biofuel production due to its rapid growth and high starch content, which inspired interest in improving their productivity. Since microbes that co-exist with plants are known to have significant effects on their growth according to the previous studies for terrestrial plants, this study has attempted to understand the plant–microbial interactions of a duckweed, Lemna minor, focusing on the growth promotion/inhibition effects so as to assess the possibility of accelerated duckweed production by modifying co-existing bacterial community.ResultsCo-cultivation of aseptic L. minor and bacterial communities collected from various aquatic environments resulted in changes in duckweed growth ranging from −24 to +14% compared to aseptic control. A number of bacterial strains were isolated from both growth-promoting and growth-inhibitory communities, and examined for their co-existing effects on duckweed growth. Irrespective of the source, each strain showed promotive, inhibitory, or neutral effects when individually co-cultured with L. minor. To further analyze the interactions among these bacterial strains in a community, binary combinations of promotive and inhibitory strains were co-cultured with aseptic L. minor, resulting in that combinations of promotive–promotive or inhibitory–inhibitory strains generally showed effects similar to those of individual strains. However, combinations of promotive–inhibitory strains tended to show inhibitory effects while only Aquitalea magnusonii H3 exerted its plant growth-promoting effect in all combinations tested.ConclusionSignificant change in biomass production was observed when duckweed was co-cultivated with environmental bacterial communities. Promotive, neutral, and inhibitory bacteria in the community would synergistically determine the effects. The results indicate the possibility of improving duckweed biomass production via regulation of co-existing bacterial communities.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0746-8) contains supplementary material, which is available to authorized users.

Plant growth-promoting bacteria (PGPB) have recently been demonstrated as a promising agent to improve wastewater treatment and biomass production efficiency of duckweed hydrocultures. With a view to their reliable use in aqueous environments, this study analysed the plant colonization dynamics of PGPB and the ecological consequences for the entire duckweed-associated bacterial community. A PGPB strain, Aquitalea magnusonii H3, was inoculated to duckweed at different cell densities or timings in the presence of three environmental bacterial communities. The results showed that strain H3 improved duckweed growth by 11.7–32.1% in five out of nine experiments. Quantitative-PCR and amplicon sequencing analyses showed that strain H3 successfully colonized duckweed after 1 and 3 d of inoculation in all cultivation tests. However, it significantly decreased in number after 7 d, and similar bacterial communities were observed on duckweed regardless of H3 inoculation. Predicted metagenome analysis suggested that genes related to bacterial chemotactic motility and surface attachment systems are consistently enriched through community assembly on duckweed. Taken together, strain H3 dominantly colonized duckweed for a short period and improved duckweed growth. However, the inoculation of the PGPB did not have a lasting impact due to the strong resilience of the natural duckweed microbiome.

Tropical tree species differ widely in their silicon (Si) accumulation patterns, but the implications of such species differences for ecosystem Si flux remain unexplored. We analyzed how biogenic Si flux via leaf litter and soil Si availability in the upper soil vary spatially within a 2 ha area in a lowland Dipterocarp forest in Peninsular Malaysia. Silicon concentration per unit leaf dry mass of 86 dominant tree species in this plot ranged from 0.4 to 126 mg/g. Soil Si availability at 100 sampling points and Si flux via leaf litter at 66 litter traps showed large variations up to 3.3‐ and 17.9‐folds, respectively. However, they correlated neither with each other nor with the expected Si flux from tree species composition in the neighborhood. When we classified tree species to four groups by their leaf Si concentrations (negligible, low, moderate and high), tree species with moderate leaf Si concentrations were significantly more abundant on Si‐rich soils. In contrast, the other three groups did not show any significant spatial relationships with soil Si availability. These results indicate that soil Si availability, Si accumulation by vegetation and Si flux via leaf litter show wide spatial variations in a lowland tropical forest, without a simple relationship with each other.

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