Exploring the hidden world of zooplankton microbiomes and their response to climate change
Imagine a single water flea, barely visible to the naked eye, darting through the water of a lake. Now envision this tiny creature as a bustling metropolis, home to millions of even smaller bacterial residents, each performing jobs that help regulate the lake's health. This microscopic world holds secrets to understanding how aquatic ecosystems will respond to our rapidly changing climate.
Provide beneficial services to hosts while playing roles in biogeochemical cycling 1 .
Reveal complex relationships with implications for freshwater resources.
Zooplankton, including tiny crustaceans like copepods and cladocerans (such as Daphnia), are fundamental components of aquatic food webs. They consume phytoplankton and serve as crucial food sources for fish, effectively transferring energy through the ecosystem 1 .
Like humans with our gut microbiome, zooplankton host rich bacterial communities that colonize various parts of their bodies—around the oral region, the anus, body appendages, intersegment parts, and the intestine 1 .
These microbial partners provide valuable services including nutrient acquisition, stress protection, detoxification, and habitat provision 1 .
Previous research has shown that zooplankton with intact microbiomes grow, survive, and reproduce significantly better than those without these microbial partners 1 . Specific bacterial taxa, particularly Limnohabitans (β-proteobacteria), consistently associate with certain zooplankton regardless of geographic location, suggesting long-established, beneficial relationships 1 .
To understand how warming affects these microbial communities, scientists needed a research site where they could study temperature effects at the ecosystem scale—something nearly impossible to create in a laboratory. They found such a place in the Konin lakes of Poland 1 .
For over 60 years, these five interconnected lakes have received warm water discharge from two power plants, creating a unique natural laboratory. The heated water is discharged into the lakes through channels, creating two cooling circuits with temperatures approximately 4-5°C higher than normal lakes 1 .
These lakes "mimic the thermal conditions of temperate lakes expected in the next 100 years" according to climate change forecasts 1 .
This long-term warming has allowed the lakes' inhabitants time to acclimate, adapt, or even evolve to the changed conditions, giving scientists a glimpse into the potential future of freshwater ecosystems under climate change 1 .
Higher than normal lakes
The results revealed striking differences between the microbial communities associated with zooplankton, those attached to particles, and free-living bacteria. Each environment represented a distinct microbial neighborhood with its own characteristic residents 1 .
| Habitat | Dominant Phyla | Characteristics |
|---|---|---|
| Zooplankton-associated | Proteobacteria, Bacteroidetes | Lower species richness and diversity; specialized communities |
| Particle-associated (PA) | Mixed composition | Moderate diversity; including methane-oxidizers and nitrifiers |
| Free-living bacterioplankton | Actinobacteria | Highest species richness and diversity |
The zooplankton microbiome showed lower species richness and diversity compared to the other two habitats, suggesting that zooplankton create specific environmental conditions that favor particular types of bacteria 1 .
The free-living bacterioplankton dominated by Actinobacteria represented the most diverse of the three communities.
| Bacterial Group | Function | Significance |
|---|---|---|
| Methane-oxidizing bacteria | Convert methane to other compounds | Potential role in regulating greenhouse gases |
| Methylotrophs | Utilize single-carbon compounds | Contribute to carbon cycling |
| Nitrobacter | Convert nitrite to nitrate (nitrification) | Important in nitrogen cycle |
Perhaps most surprisingly, the researchers found that methane-oxidizing bacteria, methylotrophs, and nitrifiers significantly associated with both zooplankton and particle-associated microbes 1 . This discovery suggests that zooplankton and their microbial partners may play previously unrecognized roles in regulating important biogeochemical cycles in lakes.
From all the abiotic parameters measured, temperature emerged as a significant factor affecting the diversity and composition of all the microbiomes studied 1 . The artificial warming of the Konin lakes had reshaped the microbial worlds in measurable ways.
Complementary research on winter water warming in artificial lakes found that increased temperatures led to complex changes in the zooplankton community 5 . Some crustacean taxa, including Daphnia cucullata and Cyclops vicinus, tended to decrease in biomass under warmer winter conditions, while certain smaller rotifers and protozoans increased their biomass 5 .
These shifts in zooplankton communities inevitably affect their microbial partners, creating a cascade of changes through the ecosystem. The research predicted that winter warming would lead to decreases in chlorophyll a, suspended solids, and total nitrogen in the lakes 5 , indicating broad impacts on lake productivity and nutrient cycling.
| Taxon | Response to Warming | Type of Organism |
|---|---|---|
| Daphnia cucullata | Decreased biomass | Crustacean |
| Cyclops vicinus | Decreased biomass | Crustacean |
| Polyarthra longiremis | Decreased biomass | Rotifer |
| Lecane spp. | Increased biomass | Rotifer |
| Monommata maculata | Increased biomass | Rotifer |
| Centropyxis aculeata | Increased biomass | Protozoan |
The response to warming represents an evolutionary trade-off for microorganisms. Research on marine microbes has shown that in highly variable environments, organisms often favor a strategy of rapid nongenetic adaptations that allow quick responses to change but may delay genetic adaptation. In more stable environments, organisms tend toward faster genetic adaptation but with less flexibility for short-term responses 6 .
The discovery of methane-oxidizing bacteria and nitrifiers associated with zooplankton opens new avenues for understanding how aquatic ecosystems regulate biogeochemical cycles 1 . These microbial partners may be performing essential ecosystem services:
Methane-oxidizing bacteria convert methane—a potent greenhouse gas—into less harmful compounds, potentially reducing methane emissions from lakes to the atmosphere.
Nitrifying bacteria like Nitrobacter play crucial roles in converting nitrogen between different forms, making it available to various organisms in the food web.
The microbial partners likely help their zooplankton hosts break down complex organic compounds, enhancing nutrient acquisition while contributing to nutrient recycling in the water column.
These findings suggest that zooplankton and their microbial companions form integrated units that not only benefit the host but contribute significantly to the ecological functioning of freshwater systems.
The research on zooplankton-associated microbes reveals a fascinating world of interdependence happening right before our eyes—if only we could see it. These complex relationships between animals and their microbial partners represent not just biological curiosities but fundamental components of healthy aquatic ecosystems.
As climate change continues to warm our freshwater resources, understanding how temperature affects these microbial communities becomes increasingly urgent. The findings from the Konin lakes provide a window into the potential futures of lakes worldwide and highlight the intricate connections between climate, organisms, and biogeochemical cycles.
The next time you stand by a lake, remember that beneath the surface lies an invisible world of microbial allies, working in concert with their zooplankton hosts to shape the health and functioning of these precious ecosystems. In the smallest of creatures may lie solutions to some of our biggest environmental challenges.