Symbiosis refers to any type of close and long-term biological interaction between two different biological organisms. Symbiotic relationships can be mutualistic, commensal, or parasitic in nature depending on how the relationship benefits or harms each organism involved.
What is symbiosis?
Symbiosis comes from the Greek word symbiosis meaning “living together”. It involves a close physical association between two organisms of different species that benefits at least one of the organisms involved. The various types of symbiotic relationships include:
- Mutualism – a mutually beneficial relationship between two organisms
- Commensalism – one organism benefits while the other is unaffected
- Parasitism – one organism benefits at the expense of the other, sometimes leading to harm
Symbiotic relationships are common throughout nature and play an important ecological role. They can be obligate where the organisms entirely depend on each other to survive, or facultative where they can live independently.
Examples of symbiosis
Here are some common examples of symbiosis:
- Mutualism – Bees and flowering plants, cleaner fish and larger fish, bacteria in human gut
- Commensalism – Remoras attached to sharks, barnacles attached to whales, spiders in houses
- Parasitism – Tapeworms in human intestine, fleas on dogs, mistletoe on trees
In mutualism, both organisms benefit from the relationship. Bees gather nectar from flowers while pollinating them in the process. In commensalism, one organism benefits like the remoras getting free transport and food leftovers while sharks are unaffected. In parasitism, the parasite benefits while negatively impacting the host e.g. tapeworms stealing nutrients from humans.
Is symbiosis positive or negative?
Symbiosis cannot be categorized as universally positive or negative. The nature of each symbiotic relationship determines whether it is beneficial, harmful, or neutral for the organisms involved.
Positive effects of symbiosis
Symbiosis is often crucial to the health and survival of many organisms. Positive effects include:
- Provides nutrition – e.g. gut bacteria help digest food in humans
- Enhances defense – e.g. bacteria protect insects against parasites
- Aids reproduction – e.g. fungi help plants absorb water and nutrients
- Allows adaptation – e.g. bioluminescent bacteria help squids camouflage
- Maintains ecosystem balance – e.g. mycorrhizae aid in nutrient cycling
Without symbiotic microbes, larger organisms would often struggle to obtain sufficient nutrition and survive in their environments. Many essential ecological processes like decomposition depend on symbiosis between organisms.
Negative effects of symbiosis
Symbiosis can also be detrimental when one organism exploits the other, such as:
- Parasites harming host health – e.g. parasite-induced diseases in humans
- Imbalance due to invasive species – e.g. invasive symbiotic fungi harming amphibians
- Toxicity due to accumulated metals – e.g. lichens absorbing high metal levels
- Increased vulnerability to stressors – e.g. coral bleaching exacerbated by algal symbionts
- Population destabilization – e.g. overreliance on symbiont leading to ecosystem collapse
In parasitic relationships, the parasite hijacks resources from and can seriously harm the host. Disruption of native symbiosis by invasive species can devastate ecosystems. Even in mutualism, symbionts can become disabled and turn commensal or parasitic.
Factors impacting symbiosis
Several key factors determine whether a symbiotic relationship will be positive, negative, or neutral for the species involved:
| Factor | Impact on symbiosis |
|---|---|
| Type of relationship | Mutualism and commensalism tend to be positive, while parasitism tends to be negative |
| Life strategy | Symbionts with faster life cycles more readily evolve to exploit hosts |
| Genetic adaptation | Co-evolution allows gradual optimization of symbiosis over time |
| Cost-benefit ratio | Symbiosis is more stable if benefits outweigh costs for both partners |
| Environmental changes | Stressors like climate change can destabilize previously positive symbiosis |
No symbiotic relationship remains completely static. Changing environmental conditions and evolutionary shifts continuously alter the cost-benefit ratios between symbiotic organisms.
Examples of shifting symbiotic relationships
Here are some examples of how symbiotic relationships can transform between positive, negative, and neutral states:
Wolbachia bacteria in insects
Wolbachia are reproductive parasites of insects like mosquitoes. However, Wolbachia infection can also limit replication of mosquito-borne viruses like dengue. While originally parasitic, the bacteria have evolved a more mutualistic role in specific contexts.
Hawaiian bobtail squid and bioluminescent bacteria
The squid provides the bacteria nutrients and shelter, while the bacteria camouflage the squid with light. However, over-colonization of tissues by the bacteria can damage the squid’s health, shifting the relationship toward parasitism.
Corals and intracellular algae
The algae supply corals with nutrition via photosynthesis. However, warming oceans can cause the algae to turn deleterious, leading to coral bleaching as the corals eject the symbionts.
Humans and gut microbiome
Gut bacteria are vital for human health through digestion and immunity. But factors like antibiotics and dietary changes can reduce beneficial bacteria, allowing pathogens to multiply.
These examples highlight how environmental fluctuations can push symbionts toward exploiting their hosts for self-preservation. Yet new mutations may also recalibrate the partnerships.
Conclusion
Symbiosis involves intricate cost-benefit tradeoffs between species. While often mutually beneficial, it can also turn commensal or parasitic if the balance shifts toward favoring one symbiont over the other. With evolution favoring self-benefit, symbiosis tends to move toward parasitism unless both partners continuously invest in and gain from the relationship. Active maintenance is thus crucial for ensuring symbiosis remains positive.
