Symbiosis Notes

Symbiosis: The Thin Line Between Fighting and Working Together

Marine Invertebrates and Microbial Symbionts

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• Many marine invertebrates derive nutrition from chemosynthetic or photosynthetic microbes.

• These microbes can be:

  • Intracellular endosymbionts: Residing within the tissues of the host.

  • Epibionts: Living on internal or external surfaces of the host.

• The host and its microbial partners form an integrated unit called a holobiont or meta-organism.

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Types of Microbial Symbionts

• Endosymbionts:

  • Intracellular microbes adapted to live inside host cells.

  • The prefix endo- means inside.

• Episymbionts:

  • Extracellular microbes living on the surface of internal ducts or body cavities, like the intestinal tract.

  • The prefix epi- means upon.

• Ectosymbionts:

  • Episymbionts found on the external surface of animals or algae.

  • The prefix ecto- means outside.

Types of Symbiotic Interactions

• Symbiosis: Any close and long-term biological interaction between two different organisms.

• Commensalism: An association where one organism benefits, and the other is neither harmed nor benefits.

• Mutualism: A symbiotic relationship beneficial to both organisms involved.

• Parasitism: A close relationship where one organism (the parasite) lives on or inside another (the host), causing harm to the host. The parasite is structurally adapted to this way of life.

Symbiotic Bacteria in the Marine Environment

• Many bacteria live in symbiosis with marine animals.

• Bacteria can be acquired through:

  • Horizontal transmission: Acquired from the environment.

  • Vertical transmission: Passed down from parent to offspring.

• Chemosynthetic endosymbionts are one example of this.

Tubeworm (Riftia pachyptila) Symbiosis

• Plume: Sucks in oxygen, carbon dioxide, and hydrogen sulfide.

• Trophosome:

  • Organ where symbiotic microbes live.

  • Microbes generate food for the tubeworm via chemosynthesis.

• Tube: A hard cylinder made of chitin proteoglycan/protein complex, protecting the tubeworm.

• Trunk: Tubeworm lacks an anus; waste from microbial reactions is stored within.

Chemosynthesis

• Symbionts fix carbon by making sugar from CO_2 using chemical energy from venting fluid.

• Bacterium converts oxygen, nitrate, CO_2, hydrogen sulfide, and methane into sugar for the animal host.

Tubeworm Infection by Symbiotic Bacteria

• Tubeworms acquire symbiotic bacteria via horizontal transmission during early larval development.

• Hydrothermal vents are ephemeral, so larval dispersal is crucial for tubeworm survival.

• Adult worms rely entirely on symbionts for nutrition.

• Horizontal transmission allows hosts to acquire bacteria best adapted to local conditions, which has played a major role in evolution.

Symbiont Dispersal

• Symbionts reside inside bacteriocytes deep within the tissue with no direct openings to the exterior.

• Symbiont cell density is controlled via coordinated cell cycle, apoptosis, and digestion.

• Symbionts escape when the tubeworm dies, either through active escape via the skin or scavenging by crabs.

Chemosynthetic Symbioses in Deep Sea Hydrothermal Vents

• Many organisms such as crabs, snails, and shrimps participate in chemosynthetic symbioses in deep-sea hydrothermal vents.

Symbioses in Shallow Water

• Seasquirts (ascidians) harbor cyanobacteria.

• Symbionts are transmitted vertically during late embryonic development of the larvae.

Sponges and Their Microbial Communities

• Sponges (Phylum Porifera) are the oldest group of multicellular animals.

• Sponge microbes are diverse, with over 40 different phyla, dominated by Gamma- and Alphaproteobacteria (many heterotrophic).

• Also contain autotrophs:

  • Photosynthetic cyanobacteria, dinoflagellates, or diatoms provide up to half of the sponge’s energy.

  • Some sponges at methane seeps contain methanotrophic symbionts.

• Also contain marine fungi.

• Many mutualistic associations exist.

• Some sponges might cultivate and consume bacteria.

Sponge Microbiome Activities

• Sponges uptake dissolved organic matter (DOM) and particulate organic matter (POM), which leads to metabolic waste.

• Bacteria process the waste and provide food for the sponge.

Stable Core Microbiome in Sponges

• Sponges have a stable core microbiome with particular microbial types that are abundant in all members of a sponge species.

• Specific microbes may have become associated with sponges early in their evolution (over 600 million years ago) and remained associated through evolutionary radiation.

• Vertical transmission (sexual reproduction) passes microbes from one generation to the next (bacteria and yeasts observed in eggs, embryos, or larvae).

• During asexual reproduction, microbes can be passed via tissue buds.

Variability in Sponge Microbiome

• Some parts of a sponge microbiome fluctuate.

• Some microbes may be beneficial at particular times or environments and enter the microbiome through leaky vertical or horizontal gene transfer.

Bioluminescent Symbiotic Bacteria

• Over 460 host species harbor bioluminescent bacterial symbionts.

• Bacterial activities confer behavioral and ecological benefits to the host rather than direct nutritional benefits.

• Bioluminescence is the emission of blue or green light from oxygen-utilizing reactions via luciferase enzymes.

• Bioluminescence requires oxygen and may have evolved originally as an antioxidative mechanism.

Examples of Bioluminescence

• Anglerfish: Use light for prey capture.

• Bobtail squid: Use light for camouflage.

• Newly hatched squid are aposymbiotic and acquire bioluminescent bacteria from seawater, even at very low densities (a few hundred cells per mL, comprising ~0.01% of the total bacterial population).

Eukaryotes as Symbionts: Dinoflagellates and Coral Reefs

• Eukaryotes, such as dinoflagellates, can also act as symbionts.

Zooxanthellae

• Most commonly dinoflagellates of the genus Symbiodinium.

• Reef-building corals are largely dependent on zooxanthellae.

• These golden-brown endosymbionts are found in many marine invertebrates and protists.

The Coral-Dinoflagellate Symbiosis

• Zooxanthellae take up CO_2 and facilitate calcium carbonate deposition.

• They metabolize host waste nitrogen, phosphorus compounds, and CO_2 into forms the coral can use.

• Corals can thus expand into nutrient-lacking habitats, such as tropical surface waters.

Zooxanthellae

• By removing CO_2 for photosynthesis, zooxanthellae facilitate the deposition of calcium carbonate.

Symbiodinium

• Genus containing most dinoflagellate zooxanthellae.

• Morphologically homogeneous (“little-brown-balls”).

• Complex life cycle.

• Molecular data reveals extensive morphologically cryptic genetic diversity.

Other Hosts of Zooxanthellae

• Marginopora vertebralis (foraminiferan)

• Tridacna (giant clam)

• Acanthometra (Radiolarian)

• Pteraeolidia (nudibranch)

• Cassiopeia xamachana (upside-down jellyfish)

• Anthopleura (sea anemone)

Coral Bleaching

• When corals experience high environmental stress, they lose their zooxanthellae via expulsion or digestion. This results in coral bleaching. The corals don’t die immediately, but are debilitated.

• Coral bleaching is occurring more frequently due to warming waters.

The Adaptive Bleaching Hypothesis

• Coral bleaching could be a strategy by the coral to exchange zooxanthellae types to acquire a ‘new’ type that is better adapted to high temperatures.

• * Fautin and Buddemeier 2004, Stat et al 2006

Coral Bleaching Events

• 2016 Coral Bleaching Aerial Surveys: Great Barrier Reef

Temporal Variability in Bleaching

• Cairns, Townsville, and Mackay show varying degrees of bleaching severity between 2016 and 2017.

Why is Coral Bleaching Happening?

• Rising sea surface temperatures are a major factor in coral bleaching.

Global Impact of Coral Bleaching

• Coral bleaching is not just limited to Australia; it's a global issue.

Consequences of Coral Death

• When corals die, macroalgae often take advantage and proliferate.