Microalgae are microscopic photosynthetic organisms that inhabit marine and freshwater environments including seas, rivers, and lakes. Interest in their commercial and nutritional applications has substantially expanded in recent years. Spirulina, which is widely marketed as superfood, is derived from blue-green microalgae. Microalgae are also widely used in biomass production, prompting the development of cultivation systems ranging from small-scale facilities to industrial operations.

However, dense microalgal colonies may create conditions that favor bacterial growth and facilitate the spread of antibiotic resistance (AR).

A recent review published in Biocontaminants highlights the role of the microalgal microenvironment in promoting bacterial persistence and the propagation of AR genes (ARGs), raising concerns about its implications for environmental and public health.

Phycosphere Reservoir

“Microalgae and bacteria are two dominant microbial groups in the photic zone of natural water bodies, having coexisted in aquatic ecosystems for billions of years. Microalgae can act as carriers for bacteria and interact with them over certain distances, forming a unique microecological environment known as a phycosphere. The phycosphere creates favorable conditions for the propagation of AR genes, making them a potential environmental hotspot,” the authors wrote.

The review examined the phycosphere within watershed basins, focusing primarily on genera such as Microcystis, a freshwater cyanobacterium often referred to as blue-green algae that forms dense blooms commonly described as green tides, and Chlorella, a freshwater green microalga frequently studied in laboratory settings.

Microalgae and bacteria secrete polymeric substances that assemble into a three-dimensional network, promoting cell-to-cell interactions and mediating nutrient enrichment and adhesion. The biofilm-like structure of the phycosphere, analogous to other microbial aggregates, confers greater robustness to microbiota. Attached microorganisms benefit from a relatively stable environment provided by their biofilm structures and attachment surfaces. Some studies have shown that bacteria can horizontally transfer specific genes to microalgae. In different freshwater ecosystems, such as urban rivers, reservoirs, and lakes, the total absolute abundance of ARGs enriched in microalgal niches (phycosphere and attached biofilms) was one to two orders of magnitude higher than that in the surrounding water.

Resistance Drivers

“The development of antimicrobial resistance in the phycosphere is a complex process that involves the interplay of multiple factors. In addition to the misuse of antibiotics, environmental factors, and human activities can all contribute to the development and spread of ARGs,” the authors noted.

For example, higher temperatures appear to enhance the bacterial assimilation of ARGs. Elevated nitrogen and phosphorus concentrations promote microalgal proliferation, thereby increasing the number of ecological niches available for colonization by resistant bacteria. Heavy metal contamination may further select for AR bacteria, which tend to form biofilms more readily than antibiotic-sensitive strains.

Recently, attention has been focused on microplastics. Biofilms formed on plastic particles, referred to as plastispheres, have been identified as potential hotspots for ARG transmission.

The authors emphasized that research on microalgae-mediated AR is still in its early stages. Key questions regarding the movement of resistance genes in aquatic ecosystems persist. Understanding this mechanism is crucial for developing strategies to reduce environmental and public health risks.

This story was translated from Univadis Italy, part of the Medscape Professional Network.