Marine Microbiology: Extremophiles, Chemolithotrophs and Symbiotic Bacteria

Marine Microorganisms and Their Ecological Roles

Marine environments harbor a wide diversity of microorganisms, including Enterobacter, Vibrio, and the marine bacterium Photobacterium. Many of these organisms can thrive without oxygen, relying on alternative electron acceptors such as sulfate, nitrate, or inorganic compounds.

Energy Metabolism and Iron Oxidation

Some bacteria oxidize Fe²⁺ to Fe³⁺, obtaining energy from this reaction and fixing CO₂ as their carbon source. The oxidized iron precipitates as Fe(OH)₃. Electrons released from inorganic donors generate a proton motive force across the membrane, which is used to drive the reduction of NAD⁺ to NADH.

Not all inorganic donors can supply electrons at a sufficiently low redox potential to reduce NAD⁺ directly. In such cases, reverse electron transport, driven by the electrochemical gradient, is required to generate NADH.

Water Contamination and Microbial Growth

The presence of organic contamination in water can increase the number of bacteria. Monitoring microbial populations is therefore important for assessing water quality.

Predatory and Halophilic Bacteria

• Bdellovibrio: a predatory, Gram-negative, aerobic bacterium that controls other microbial populations.
• Oceanospirillum and Photobacterium: Gram-negative halophiles.
• Haloferax: a halophilic archaeon adapted to extreme salinity.
• Marinococcus: a halotolerant Gram-positive bacterium.
• Natrobacterium: a halophilic, alkaliphilic microorganism.

Adaptations to Extreme Environments

Microorganisms living in extreme conditions display distinctive structural features:

• Cell walls lacking peptidoglycan, such as pseudopeptidoglycan in Methanosarcina, or S-layers formed by proteins or glycoproteins in Halococcus.
• Membranes containing isoprenoid chains linked to glycerol through ether bonds, rather than fatty acids linked by ester bonds, which increases stability at high temperatures.
• Complex RNA polymerases adapted to extreme conditions.

Metabolic Diversity

• Nitrobacter uses O₂ and CO₂, oxidizing nitrite to nitrate.
• Desulfuromonas is a strict anaerobe that does not use O₂, respiring organic acids and alcohols with sulfur compounds.
• Pseudomonas can use O₂, sugars, or nitrate under anaerobic conditions.
• Hyphomicrobium and Methylococcus are methylotrophs that grow on CH₄ and CO.
• Thiobacillus is a chemolithotrophic sulfur oxidizer that fixes CO₂ and oxidizes H₂S and S₂O₃^2-.

Membrane Structure and Stability

At high levels of membrane saturation, long-chain lipids pack tightly, reducing fluidity. Membrane proteins interact through covalent and non-covalent bonds, and hydrophobic regions may pose structural challenges.

Hydrothermal Vent Symbiosis

Riftia pachyptila lacks a mouth and digestive system. Instead, it depends nutritionally on bacteria housed in a specialized organ called the trophosome. The tubeworm absorbs H₂S and CO₂ from hydrothermal fluids and transports them through its blood to the trophosome, where chemolithotrophic bacteria use H₂S as an energy source and CO₂ as a carbon source, producing organic compounds the animal uses. This is a mutualistic relationship in which both partners benefit.

Thermophilic and Hyperthermophilic Archaea

• Haloferax volcanii: lives in the Dead Sea and requires 20–30% salt.
• Methanopyrus: a hyperthermophile living near hydrothermal vents, growing at temperatures up to 110 °C.
• Thermococcus: a hyperthermophile with an optimal growth temperature of 85 °C, which reduces sulfur.
• Thermotoga: a primary producer found in hydrothermal sources, forming filamentous structures.

Colonization and Biofilms

Some bacteria, such as Flavobacterium and Cytophaga, colonize algae, while others live freely in the water. Biofilms form on solid surfaces, and their matrix is composed mainly of mucopolysaccharides. Water can enter the biofilm, allowing the exchange of organic and inorganic solutes with the external environment through diffusion.

Indicators of Contamination

The presence of certain organic substances may indicate contamination. Stagnant areas with low oxygen favor the growth of sulfate-reducing bacteria, which produce putrefactive H₂S. Coliform bacteria, found in the intestinal tract, are commonly used as indicators of fecal contamination. Their survival in seawater is limited.

Environmental Conditions for Marine Microbes

• Marine bacteria typically grow at temperatures between 5 °C and 15 °C, and in deep-sea environments between -5 °C and +100 °C depending on depth and location.
• Optimal growth for many marine species occurs between 10 °C and 55 °C.
• Arqueobacteria require at least 2% salt in culture media.
• Many marine microorganisms are oligotrophic, adapted to low nutrient concentrations.
• Chemolithoautotrophs fix CO₂ using inorganic energy sources to build cellular organic matter.

Remineralization and Nutrient Cycling

Microbial metabolism transforms organic matter into carbon dioxide, water, and inorganic salts, a process known as remineralization. This recycling returns essential nutrients to the environment and sustains marine food webs.

Bioremediation and Toxicity Testing

Microorganisms are used in trials that assess the toxicity of industrial effluents, helping to evaluate environmental risks and guide wastewater treatment strategies.