# Macrophyte Extracts Promote the Growth of the Microbial Community Associated With Microcystis aeruginosa Alleviating Allelopathic Effects

## TL;DR

Macrophyte extracts inhibit cyanobacteria but also boost their microbial communities, which in turn reduce the harmful effects of the extracts.

## Contribution

This study reveals that microbial communities associated with cyanobacteria can mitigate the allelopathic effects of macrophyte extracts.

## Key findings

- Macrophyte extracts inhibited cyanobacterial growth and photosynthesis but increased bacterial abundance.
- Different macrophyte extracts stimulated distinct bacterial community compositions.
- Reintroducing stimulated microbial communities reduced the allelopathic effects of the extracts.

## Abstract

Macrophytes such as
Pistia stratiotes
and
Pontederia crassipes
can release allelopathic compounds and reduce cyanobacteria biomass. Cyanobacterial cells interact with heterotrophic bacteria, which contribute to nutrient uptake and antioxidative responses, among other functions. However, the role of microbial communities in allelopathic interactions between macrophytes and cyanobacteria remains unexplored. We investigated how the bacterial community associated with
Microcystis aeruginosa
influences the effects of aqueous macrophyte extracts. Both extracts inhibited cyanobacterial growth and photosynthetic activity (99% for
P. stratiotes
and 55% for
P. crassipes
) while increasing bacterial abundance (threefold). The composition of the bacterial communities stimulated by extracts shifted: whereas original cultures were rich in Methyloversatilis and Rhodobacter, the
P. stratiotes
extract promoted the growth of Shinella, Flavobacterium, and Comamonadaceae, and the
P. crassipes
extract favored Enterobacterales. When these stimulated communities were reintroduced into
M. aeruginosa
cultures, allelopathic inhibition was reduced (40% for
P. stratiotes
and 12% for
P. crassipes
). We concluded that the growth of the associated microbiota attenuated the allelopathic effects, partially preserving cyanobacterial cells. Bacterial groups favored by the treatments may participate in allelochemical degradation and antioxidant protection or activate other types of metabolism beneficial to cyanobacteria, mitigating the harmful effects of the extracts. These results highlight the importance of considering the role of microbial communities in cyanobacterial allelopathic interactions.

Macrophyte extracts inhibited
Microcystis aeruginosa
but promoted growth of associated microbial communities.Distinct macrophyte extracts shaped specific microbial community compositions associated with
Microcystis aeruginosa
.Pronounced bacterial functions associated with degradation and antioxidative response.Supplementation of
Microcystis aeruginosa
cultures with microbial communities reduced the deleterious effects of the extracts.

Macrophyte extracts inhibited
Microcystis aeruginosa
but promoted growth of associated microbial communities.

Distinct macrophyte extracts shaped specific microbial community compositions associated with
Microcystis aeruginosa
.

Pronounced bacterial functions associated with degradation and antioxidative response.

Supplementation of
Microcystis aeruginosa
cultures with microbial communities reduced the deleterious effects of the extracts.

Aqueous extracts from
Pistia stratiotes
and
Pontederia crassipes
inhibit
Microcystis aeruginosa
while stimulating its associated microbiota. Reintroduction of microbiota into fresh cyanobacterial cultures reduced allelopathic effects, likely through allelochemical degradation and antioxidative responses. These results highlight the protective role of microbial communities in allelopathic interactions involving cyanobacteria.