MBIO120 Introduction to Marine Biology: Living Light

Microbial Seas: Living Light

Marine microbes are the most abundant life form, accounting for over 90\% of ocean biomass.
Their activities have surprising and far-reaching consequences.

Definition: Light released as energy during a chemical reaction.
Key Components:
- Luciferin: A light-emitting compound.
- Luciferase: An enzyme catalyzing luciferin oxidation.
- Photoprotein: A single unit containing luciferin, catalyst, and oxygen; triggered by ions like calcium (Ca^{2+}).
Diversity: Chemical nature of luciferins and molecular biology of luciferases show great diversity.
Evolution: Thought to have evolved independently at least 40\% times.
Distribution: Found in all major groups of organisms (bacteria to fishes), primarily in marine environments; rare in freshwater and terrestrial systems.
Origin: Can be produced by organisms themselves or by symbiotic bacteria; ingested bacteria/protists can cause bioluminescence in filter feeders.

Stable Conditions: Comparatively stable environmental conditions with a long evolutionary history.
Optical Clarity: Ocean is optically clearer than rivers and lakes.
Dark Habitat: Majority of the habitat is completely dark.
Interspecies Interactions: Critical for interactions between diverse taxa (predators, parasites, prey).

Bioluminescence: Chemical reaction produces light.
Fluorescence: Absorption of electromagnetic radiation at one wavelength and re-emission at a lower energy (longer) wavelength.
Phosphorescence: Energized electron is stable, emitting light long after the stimulating light source is removed.

Photoprotection of zooxanthellae (reducing UV inhibition of photosynthesis).
Photoenhancement (converting short-wavelength light to longer wavelengths for photosynthesis).
Antioxidant effect (removing Reactive Oxygen Species).
Attraction of zooxanthellae, prey.
Camouflage or signalling.

Discovered in the bioluminescent jellyfish Aequoria victoria.
Involves a tight association between light-emitting reaction and GFP, which converts blue light to green light.
Gene for GFP is widely used in developmental biology and transgenics to monitor gene expression.

Mechanism: Movement in water creates an action potential that travels along the vacuole membrane to scintillons (pockets with luciferase). This opens proton channels, allowing protons into scintillons, which interact with luciferase to cause a light flash.
Often seen as 'sea sparkle' during red tides when disturbed.

Anglerfish (Ceratoidei): Possess a bioluminescent lure (esca) containing symbiotic Vibrionaceae bacteria (uncultured).
Flashlight Fish (Anomalopidae): Have external light organs under each eye with symbiotic Candidatus Photodesmus bacteria (uncultured).
Euprymna scolopes – Vibrio fischeri Symbiosis (now Aliivibrio fischeri):
- Squid's adapted ink sac contains $> 10^{9}$ extracellular V. fischeri.
- Highly specific horizontal acquisition of the bacterium.
- Bioluminescence depends on a critical population density of bacteria.
- Nocturnal predator uses 'counter-illumination' to match its environment.

Definition: Controlled production, release, and detection of threshold concentrations of signal molecules.
Mechanism (LuxI/LuxR system):
- At low cell density, signal molecules (acyl-homoserine lactones, AHLs) are produced by LuxI but diffuse away.
- At high cell density, AHLs accumulate and bind to LuxR.
- The AHL-LuxR complex binds to the lux box upstream of the promoter, enhancing transcription of luciferase and LuxI genes.
- This leads to a population-wide switch-on of bioluminescence at high cell density.

Large areas ($> 15,000$ km^{2}) in the ocean exhibit constant light emission, often from high concentrations of Vibrio harveyii.
Bacteria occur in association with Phaeocystis (microalgae) blooms, where algal colonies provide niches for quorum sensing accumulation.
Unlike dinoflagellate luminescence, bacterial light emission is constant day and night.
This phenomenon has been observed and described for centuries (e.g., Jules Verne's 20,000 Leagues Under the Sea).