🐠Marine Biology Unit 4 – Marine Microbes: Primary Producers

What's the Deal with Marine Microbes?

• Marine microbes are microscopic organisms that live in the ocean and play a crucial role in marine ecosystems
• Consist of a diverse group of organisms including bacteria, archaea, protists, and viruses
• Despite their small size, marine microbes are the most abundant organisms in the ocean and make up a significant portion of the ocean's biomass
• Responsible for driving many of the ocean's biogeochemical cycles and are the foundation of marine food webs
• Capable of thriving in a wide range of environments from the surface waters to the deep sea and from polar regions to tropical latitudes
• Adapt to various environmental conditions such as temperature, salinity, and nutrient availability
• Some marine microbes are primary producers and form the base of the marine food web by converting sunlight into organic matter through photosynthesis
• Others are decomposers and break down dead organic matter, recycling nutrients back into the ecosystem

Meet the Tiny Titans: Types of Marine Primary Producers

• Phytoplankton are microscopic algae that drift in the water column and are the main primary producers in the ocean
  • Include diatoms, dinoflagellates, and coccolithophores
  • Diatoms are unicellular algae with silica cell walls and are the most abundant type of phytoplankton (genera Chaetoceros, Thalassiosira)
  • Dinoflagellates are unicellular algae with two flagella and are known for forming harmful algal blooms (genus Alexandrium)
• Cyanobacteria are photosynthetic bacteria that are also important primary producers in the ocean
  • Include Prochlorococcus and Synechococcus, which are the most abundant photosynthetic organisms on Earth
• Macroalgae are multicellular algae that are attached to the seafloor and are important primary producers in coastal ecosystems (kelp forests, seagrass beds)
• Symbiotic algae live within the tissues of other organisms such as corals and provide them with nutrients through photosynthesis (zooxanthellae in reef-building corals)
• Chemoautotrophic bacteria obtain energy from chemical reactions and are primary producers in deep-sea hydrothermal vent communities

How These Microbes Make Their Own Food

• Primary producers use photosynthesis to convert sunlight, carbon dioxide, and water into organic matter and oxygen
  • Photosynthesis occurs in specialized organelles called chloroplasts that contain chlorophyll pigments
  • The general equation for photosynthesis is: $6CO_2 + 6H_2O + light energy → C_6H_12O_6 + 6O_2$
• Photosynthesis consists of two stages: the light-dependent reactions and the light-independent reactions (Calvin cycle)
  • In the light-dependent reactions, light energy is captured by chlorophyll and used to split water molecules, releasing oxygen and generating ATP and NADPH
  • In the light-independent reactions, the ATP and NADPH are used to convert carbon dioxide into glucose through a series of enzymatic reactions
• Some marine microbes use alternative pathways for carbon fixation such as the reverse tricarboxylic acid cycle (rTCA) and the Wood-Ljungdahl pathway
• Chemoautotrophic bacteria use chemical energy from the oxidation of reduced compounds (hydrogen sulfide, methane) to fix carbon dioxide into organic matter
• Marine primary producers have adaptations to optimize photosynthesis in the ocean environment
  • Phytoplankton can adjust their buoyancy to stay in the photic zone where light is available
  • Some phytoplankton have specialized pigments (phycobilins) that allow them to capture different wavelengths of light at different depths

Where They Hang Out: Habitats and Distribution

• Marine microbes are found throughout the ocean from the surface waters to the deep sea
• Phytoplankton are most abundant in the photic zone where sunlight penetrates (usually the top 200 meters of the water column)
  • Their distribution is influenced by factors such as nutrient availability, water temperature, and ocean circulation patterns
  • Highest abundances are found in upwelling regions where nutrient-rich deep waters are brought to the surface (coastal upwelling zones, equatorial upwelling)
• Cyanobacteria have a wider vertical distribution and can be found in the photic zone as well as in the mesopelagic zone (200-1000 meters depth)
  • Prochlorococcus is adapted to low-light conditions and is abundant in the oligotrophic open ocean
  • Synechococcus is more abundant in coastal waters and upwelling regions
• Macroalgae and symbiotic algae are found in shallow coastal waters where they can attach to the seafloor or live within host organisms
  • Kelp forests are found in temperate coastal waters and provide habitat for a diverse community of organisms
  • Coral reefs are found in tropical waters and are built by corals that have symbiotic algae living within their tissues
• Chemoautotrophic bacteria are found in deep-sea hydrothermal vent communities where they form the base of the food web
  • They are adapted to the extreme conditions of high pressure, high temperature, and toxic chemicals found in these environments

Microbes' Impact on Ocean Chemistry and Nutrient Cycles

• Marine microbes play a crucial role in the ocean's biogeochemical cycles and have a significant impact on ocean chemistry
• Primary producers are responsible for the biological pump which transfers carbon from the atmosphere into the deep ocean
  • Phytoplankton take up carbon dioxide during photosynthesis and convert it into organic matter
  • When phytoplankton die or are consumed by zooplankton, the carbon is exported to the deep ocean where it can be sequestered for hundreds to thousands of years
• Marine microbes are also involved in the nitrogen cycle and can fix atmospheric nitrogen into a biologically available form
  • Cyanobacteria such as Trichodesmium are important nitrogen fixers in the open ocean
  • Other marine microbes such as denitrifying bacteria convert nitrate back into atmospheric nitrogen gas
• Marine microbes also play a role in the sulfur cycle and can produce dimethylsulfide (DMS) which is a climatically active gas
  • DMS is produced by phytoplankton and is released into the atmosphere where it can form cloud condensation nuclei and affect the Earth's climate
• Marine microbes are important for the recycling of nutrients in the ocean and can regenerate nutrients such as nitrogen and phosphorus from dead organic matter
  • Decomposer bacteria break down dead phytoplankton and zooplankton and release the nutrients back into the water column where they can be taken up by new phytoplankton
• The activities of marine microbes can also affect the ocean's pH and contribute to ocean acidification
  • The uptake of carbon dioxide by phytoplankton during photosynthesis can increase the pH of the surrounding water
  • The respiration of organic matter by heterotrophic microbes can decrease the pH of the water by releasing carbon dioxide

The Good, the Bad, and the Ugly: Ecological Roles

• Marine microbes play diverse ecological roles in the ocean and can have both positive and negative impacts on marine ecosystems
• Primary producers form the base of marine food webs and support the growth and reproduction of higher trophic levels
  • Phytoplankton are consumed by zooplankton which in turn are consumed by larger organisms such as fish and whales
  • Macroalgae and seagrasses provide habitat and food for a diverse community of organisms in coastal ecosystems
• Some marine microbes form symbiotic relationships with other organisms and provide them with nutrients and energy
  • Zooxanthellae live within the tissues of corals and provide them with glucose produced through photosynthesis
  • Chemosynthetic bacteria form symbiotic relationships with tubeworms and clams in deep-sea hydrothermal vent communities
• Marine microbes are also important for the biodegradation of pollutants and the cleanup of oil spills
  • Some bacteria can break down hydrocarbons and use them as a carbon and energy source
• However, some marine microbes can also have negative impacts on marine ecosystems and human health
  • Harmful algal blooms (HABs) can produce toxins that can kill fish and marine mammals and cause illness in humans who consume contaminated seafood
  • Pathogenic bacteria such as Vibrio can cause diseases in marine organisms and humans (cholera, flesh-eating bacteria infections)
• The activities of marine microbes can also contribute to the formation of dead zones in coastal waters
  • Excess nutrients from agricultural runoff can stimulate the growth of phytoplankton blooms which can lead to hypoxia (low oxygen) when they die and decompose

Climate Change and Marine Microbes: It's Complicated

• Climate change is having significant impacts on marine ecosystems and the microbes that inhabit them
• Warming ocean temperatures can affect the growth and distribution of phytoplankton and other marine microbes
  • Some species may shift their ranges poleward or to deeper waters to stay within their optimal temperature range
  • Warming can also lead to stratification of the water column which can reduce the upwelling of nutrients and limit phytoplankton growth
• Ocean acidification caused by the absorption of atmospheric carbon dioxide can affect the ability of some phytoplankton to build their calcium carbonate shells
  • Coccolithophores and foraminifera may be particularly vulnerable to ocean acidification
  • Reduced calcification can affect the ocean's carbon cycle and the export of carbon to the deep sea
• Changes in ocean circulation patterns caused by climate change can affect the distribution and productivity of marine microbes
  • Reduced sea ice cover in the Arctic can lead to increased primary production by phytoplankton as more light becomes available
  • Changes in the timing and intensity of upwelling events can affect the supply of nutrients to surface waters and the timing of phytoplankton blooms
• Climate change can also affect the interactions between marine microbes and other organisms in the ecosystem
  • Warmer temperatures can increase the growth and virulence of pathogenic bacteria and harmful algal blooms
  • Changes in the timing of phytoplankton blooms can lead to mismatches with the life cycles of zooplankton and other consumers
• Marine microbes can also play a role in mitigating the impacts of climate change through their involvement in the biological pump and other biogeochemical cycles
  • Increased primary production by phytoplankton in some regions could lead to increased carbon sequestration in the deep ocean
  • Changes in the microbial community composition could affect the cycling of nutrients and the production of climatically active gases such as DMS

Why Should We Care? Real-World Applications

• Marine microbes are not only important for the functioning of marine ecosystems but also have many real-world applications for human society
• Marine microbes are a valuable source of natural products and bioactive compounds that can be used in pharmaceuticals, biotechnology, and other industries
  • Some marine bacteria produce antibiotics that can be used to treat infections in humans and animals (e.g. the antibiotic bryostatin from the bacterium Bugula neritina)
  • Marine microalgae are used in the production of biofuels, cosmetics, and nutritional supplements (omega-3 fatty acids, carotenoids)
• Marine microbes are also important for the development of new technologies and materials
  • The silica cell walls of diatoms have unique properties that can be used in the development of new nanomaterials and optical devices
  • The bioluminescent proteins produced by some marine bacteria are used in biomedical research as reporters of gene expression and cell activity
• Marine microbes are used in bioremediation to clean up polluted environments and degrade toxic substances
  • Oil-degrading bacteria such as Alcanivorax borkumensis are used to clean up oil spills in marine environments
  • Marine cyanobacteria are used to remove heavy metals and other pollutants from wastewater and industrial effluents
• Marine microbes are important indicators of ecosystem health and can be used in environmental monitoring and assessment
  • Changes in the abundance and diversity of phytoplankton can indicate changes in water quality and the impacts of pollution or climate change
  • The presence of certain indicator species can be used to assess the ecological status of marine habitats and to guide conservation and management efforts
• Understanding the biology and ecology of marine microbes is crucial for predicting and mitigating the impacts of human activities on marine ecosystems
  • Knowledge of the factors that control the growth and distribution of harmful algal blooms can help in the development of early warning systems and management strategies
  • Understanding the role of marine microbes in biogeochemical cycles can inform models of climate change and the impacts of ocean acidification on marine ecosystems