The research team isolated four bacterial strains from lake sediment and identified them through 16S rRNA gene sequencing, the standard molecular method for pinning down bacterial identity. The four isolates belonged to the genera Serratia, Proteus and Pseudomonas, and were designated Serratia N-1, Proteus N-2, Pseudomonas F5-2 and Pseudomonas F11. Each strain was then tested against two species of green microalgae, Chlorella and Micractinium, under three distinct nutritional regimes: purely autotrophic conditions in which the algae relied entirely on photosynthesis, mixotrophic conditions supplemented with glucose, and systems supplemented with sucrose instead. This experimental design allowed the researchers to isolate the effect of carbon availability and carbon chemistry on the outcome of each bacteria-algae encounter.

Under autotrophic conditions, the picture was one of striking asymmetry. None of the four bacterial strains showed meaningful inhibition of Chlorella, yet all of them suppressed Micractinium by roughly 20 percent. Intriguingly, there was no significant difference between treatments using live bacterial cultures and treatments using only cell-free supernatants, the liquid fraction left after the bacteria were removed. That equivalence carried an important mechanistic clue: whatever was harming Micractinium was not the bacteria themselves through direct contact, but soluble, extracellular metabolites that the bacteria had released into the medium. The algae were being felled by chemistry, not by physical attack.

The addition of glucose transformed the entire dynamic. In mixotrophic cultures containing this simple sugar, algicidal performance against Chlorella, previously negligible, surged. Serratia N-1 became a devastating antagonist, achieving more than 99 percent inhibition of Chlorella growth within just three days. The other strains followed the same direction but at different magnitudes and speeds: Proteus N-2 reached 79 percent inhibition by day seven, Pseudomonas F11 reached 60 percent, and Pseudomonas F5-2 reached 47 percent. Once again, supernatants performed comparably to whole cultures, reinforcing the conclusion that extracellular metabolites, whose production or potency is boosted when the bacteria can feed on glucose, are the dominant weapons in this interaction.

Micractinium, however, told a different story under the same glucose conditions. Only the live culture of Serratia N-1 exhibited strong inhibition against this alga. Its cell-free supernatant had no effect, and neither did any of the other three strains, whether as cultures or supernatants. This pattern suggests that Serratia N-1 attacks Micractinium through a mechanism that requires the living bacterium, possibly contact-dependent activity or a metabolite that is unstable, rapidly degraded, or produced only in the immediate vicinity of intact cells. The contrast with the supernatant-driven attack on the same alga under autotrophic conditions implies that the bacterium can deploy fundamentally different modes of action depending on its nutritional environment and its target.

The most dramatic reversal came when glucose was swapped for sucrose. In the disaccharide systems, the algicidal capacity of all four strains was largely weakened, and in one case the relationship inverted entirely. Serratia N-1, which had annihilated Chlorella cultures in the presence of glucose, switched roles and became a growth promoter, increasing Chlorella biomass fivefold. A microorganism that functioned as a near-total algicide under one carbon regime became, under another, an effective biofertilizer for the very same algal species. The authors attribute these shifts to the presence and type of organic carbon sources in the co-culture systems, which significantly modulate bacterial algicidal phenotypes.

The technical implications of this carbon-dependent switching are considerable. Algicidal bacteria are widely explored as biological control agents against harmful algal blooms, which are increasing in frequency and intensity worldwide as eutrophication and warming waters create ideal conditions for explosive algal growth. Cyanobacterial blooms in particular produce toxins such as microcystins that threaten drinking water supplies, fisheries, recreation and public health. Conventional chemical algicides can leave toxic residues and damage non-target organisms, which is why microbial control, with its promise of environmental sustainability and high efficiency, has attracted growing interest. But the new findings show that a candidate biocontrol strain cannot be evaluated in isolation from the chemistry of the water body it is meant to treat.

Organic carbon is not a uniform background variable in natural waters. Dissolved organic carbon arrives from decaying vegetation, agricultural runoff, wastewater discharge and the algae themselves, and its composition varies enormously across systems and seasons. A lake receiving glucose-rich runoff may behave very differently from one dominated by more complex sugars or by carbon-poor conditions. The study suggests that in carbon-rich environments containing readily metabolizable sugars, bacterial antagonists of algae may become dramatically more lethal, while in systems where the dominant carbon sources resemble sucrose, the same bacteria might lose their killing power or even stimulate the algae they were intended to suppress. Any field deployment of algicidal bacteria would therefore need to account for the carbon fingerprint of the target water body.

The results also illuminate the broader ecology of algae-bacteria interactions, a relationship that ranges from mutualism to parasitism and is increasingly recognized as a central regulator of aquatic ecosystems. Previous research has shown that bacteria can promote algal growth by supplying vitamins, hormones such as indole-3-acetic acid, or quorum-sensing molecules, and the same research group has previously exploited growth-promoting bacteria to boost biomass accumulation of Chlorella for bioenergy applications. The new work demonstrates that a single bacterial strain can occupy multiple points on this mutualism-antagonism spectrum simultaneously, with nutrition acting as the switch. Understanding the metabolic logic behind these switches, which sugars trigger toxin production, which suppress it, and why the two algal species respond so differently, could allow researchers to design interactions rather than merely observe them.

For now, the study delivers a clear and sobering message wrapped in an exciting discovery: the battle lines between bacteria and algae are drawn in sugar. Serratia N-1’s ability to wipe out more than 99 percent of a Chlorella culture in three days marks it as one of the most potent algicidal agents reported against this genus, while its fivefold growth-promoting effect under sucrose shows the same strain could serve biofuel and biomass production instead. The authors present the work as a scientific basis for understanding algae-bacteria interactions and the nutritional regulation of algicidal activity under varying carbon environments. As harmful blooms intensify across the globe, the ability to predict, and perhaps one day deliberately tune, these microbial relationships by managing organic carbon could become a decisive tool in the effort to keep the world’s waters clear.

Subject of Research: Carbon-source-dependent algicidal activity of lake-sediment bacteria against Chlorella and Micractinium

Article Title: Differential responses of Chlorella and Micractinium to algicidal bacteria in varying organic carbon environments

Article References: Zhang, B., Zhang, F., Wang, X., Ji, Z., Li, H., Zhang, Z., & Su, Y. (2026). Differential responses of Chlorella and Micractinium to algicidal bacteria in varying organic carbon environments. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38264-z

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38264-z

Keywords: algicidal bacteria, Chlorella, Micractinium, Serratia, Pseudomonas, harmful algal blooms, organic carbon, glucose, sucrose, mixotrophic culture, microbial control, algae-bacteria interactions