4.1 Marine bacteria and archaea

Marine Bacteria and Archaea

The ocean is the largest microbial habitat on Earth. Bacteria and archaea are found in every marine environment, from sunlit surface waters to the deepest trenches, and they drive the biogeochemical cycles that keep ocean ecosystems functioning. Understanding these organisms is central to understanding how the ocean processes carbon, nitrogen, and other essential elements.

Diversity of Marine Microorganisms

The sheer number of prokaryotes in the ocean is staggering: an estimated $10^{28}$ bacterial and archaeal cells. That's more cells than there are stars in the observable universe. Their distribution and diversity shift dramatically depending on where you look.

• Epipelagic zone (surface waters): This is where abundance and diversity peak. Sunlight fuels high primary productivity, which generates organic matter that heterotrophic bacteria feed on.
• Mesopelagic and bathypelagic zones (deep ocean): Cell counts drop, but diverse communities still persist. These organisms are adapted to low nutrient concentrations, high pressure, and permanent darkness.
• Coastal and estuarine environments: Rivers and runoff deliver pulses of nutrients and organic matter from land, supporting especially high bacterial and archaeal diversity and abundance.
• Hydrothermal vents and cold seeps: These extreme environments host unique communities of chemosynthetic organisms that derive energy from chemical reactions rather than sunlight. Temperatures at vents can exceed 300°C, and the fluids carry toxic compounds like hydrogen sulfide.

Ecological Roles of Bacteria and Archaea

Marine bacteria and archaea aren't just present in the ocean; they actively shape its chemistry. Their roles in biogeochemical cycling are what make them so ecologically important.

Carbon cycling

• Heterotrophic bacteria decompose organic matter (dead phytoplankton, fecal pellets, dissolved organics) and remineralize it, releasing nutrients like nitrogen and phosphorus back into the water column.
• Chemoautotrophic bacteria and archaea fix inorganic carbon ($CO_2$) into organic molecules without sunlight. This is a significant source of primary production in deep-sea environments like hydrothermal vents.

Nitrogen cycling

Nitrogen cycling involves several distinct microbial processes, each carried out by different groups:

1. Nitrogen fixation: Nitrogen-fixing bacteria (e.g., Trichodesmium) convert atmospheric $N_2$ into ammonia ($NH_3$), making it biologically available.
2. Nitrification: Nitrifying bacteria and archaea oxidize ammonia to nitrite ($NO_2^-$) and then to nitrate ($NO_3^-$).
3. Denitrification: Denitrifying bacteria reduce nitrate back to $N_2$ gas, effectively removing bioavailable nitrogen from the ocean.

These processes together regulate how much usable nitrogen is available to marine life.

Sulfur cycling

• Sulfate-reducing bacteria convert sulfate ($SO_4^{2-}$) to hydrogen sulfide ($H_2S$) in anoxic (oxygen-free) environments like deep sediments and oxygen minimum zones.
• Sulfur-oxidizing bacteria reverse this process, oxidizing $H_2S$ back to sulfate and often coupling that reaction to carbon fixation. This is the metabolic basis for life at hydrothermal vents.

Other roles

• Degradation of complex organic compounds, including hydrocarbons and pollutants (some marine bacteria can break down oil after spills)
• Transformation and cycling of trace metals like iron and manganese, which are essential micronutrients for phytoplankton

Adaptations to Ocean Environments

Marine prokaryotes occupy an enormous range of habitats, and they've evolved specific adaptations to match.

• Temperature:
  • Psychrophiles thrive in cold waters (polar seas, deep ocean) with optimal growth often below 15°C.
  • Thermophiles thrive at high temperatures, such as those found at hydrothermal vents (some grow above 80°C).
• Pressure:
  • Piezophiles are adapted to the crushing pressures of the deep ocean. They possess specialized membrane lipids and proteins that maintain cell structure and enzyme function under conditions that would destroy surface-dwelling organisms.
• Nutrients:
  • Oligotrophs are adapted to the nutrient-poor open ocean. They tend to have small cell sizes (high surface area-to-volume ratio) and highly efficient nutrient uptake systems, allowing them to scavenge scarce resources.
• Light:
  • Photoheterotrophs use light energy to supplement their metabolism but still require organic carbon for growth.
  • Some bacteria and archaea contain rhodopsins, light-driven proton pumps that generate energy from sunlight without photosynthesis. This is widespread in surface ocean communities.
• Symbiosis:
  • Many bacteria and archaea form symbiotic relationships with larger marine organisms like sponges, corals, and tubeworms. They may provide nutrients or chemical defenses to their host in exchange for a stable habitat and carbon sources.

Methods for Studying Marine Microbes

A major challenge in marine microbiology is that the vast majority of ocean bacteria and archaea cannot be grown in the lab. Estimates suggest fewer than 1% of marine prokaryotes are culturable using standard techniques. This has pushed the field toward molecular, culture-independent approaches.

Culture-dependent methods

• Isolation and growth of bacteria/archaea on selective media in the lab
• Allows detailed physiological and biochemical characterization of individual strains
• Limited because most marine microbes simply won't grow under laboratory conditions

Culture-independent methods

These molecular techniques bypass the need to grow organisms in culture:

1. 16S rRNA gene sequencing: Targets a conserved gene found in all bacteria and archaea. By sequencing this gene from an environmental sample, you can identify which organisms are present and how they're related to each other. This is the standard tool for surveying community composition and diversity.
2. Metagenomics: Sequences all the DNA extracted from an environmental sample, not just one gene. This reveals the full metabolic potential of a microbial community, showing what functions the organisms could perform.
3. Single-cell genomics: Isolates individual cells and sequences their entire genomes. This is especially powerful for studying uncultured organisms, since you can learn about their biology without ever growing them.
4. Other approaches: Metatranscriptomics (what genes are actively being expressed), metaproteomics (what proteins are being produced), and stable isotope probing (which organisms are metabolizing specific substrates) add further layers of functional information.