Marine Microorganisms: Viruses, Bacteria, Fungi, Protozoans, and Microalgae in Ocean Ecosystems

Table of Contents

• Introduction to Marine Microorganisms
• Marine Viruses
• Archaebacteria
• Eubacteria
• Marine Fungi
• Marine Protozoan
• Marine Microalgae
• References

Introduction to Marine Microorganisms

• Microorganisms are widespread and inhabit diverse terrestrial environments such as clouds, land, hot springs, and the gut microbiome.
• Marine microorganisms reside in aquatic environments including seas, oceans, estuaries, and other marine habitats.
• These marine microorganisms contribute to approximately 90% of the total biomass found in marine ecosystems.
• The first observation of microorganisms was made by Antonie van Leeuwenhoek in the 17th century, referring to them as "animalcules," which were derived from aquatic environments.
• In the 1800s, German physiologist Christian Andreas Victor Hensen laid foundational work in biological oceanography and introduced the term “plankton” to describe microorganisms floating in seawater.
• By the mid-20th century, marine microbiology advanced significantly with contributions from several microbiologists, especially Claude Zobell.
• Claude Zobell, often regarded as the father of marine microbiology, demonstrated how marine bacteria decompose organic matter and recycle nutrients within the marine ecosystem.
• He also conducted extensive research on biofilms in seawater and authored several foundational textbooks on marine microbiology.
• Marine microorganisms encompass a variety of forms including viruses, archaea, eubacteria, fungi, phytoplankton, and protozoa.
• These organisms are capable of surviving and thriving across various marine habitats, from surface waters to deep-sea hydrothermal vents, due to their unique physiological adaptations.

Marine Viruses

• Marine viruses are biological entities found in the ocean, composed of nucleic acid enclosed within a protein coat, and are only capable of replicating inside a living host cell.
• They are considered the most abundant lifeforms in the ocean, with an estimated 4×10³⁰ viral particles present in the marine environment.
• Marine viruses vary in size from 25 to 300 nanometers.
• The first observations of marine viruses were made using transmission electron microscopy, which revealed approximately 10⁷ viruses per milliliter of seawater; their abundance decreases with ocean depth and distance from the shore.
• Marine sediments show a higher concentration, ranging from 10⁸ to 10⁹ viruses per cubic centimeter.
• Enumeration of marine viruses can be carried out using various methods:
• Indirect viable counts, which rely on the virus’s ability to lyse cultured host cells.
• Direct counts, which involve counting viruses directly from environmental samples using techniques like transmission electron microscopy, flow cytometry, and epifluorescent microscopy.
• Epifluorescence microscopy is the most preferred method for counting marine viruses due to its high level of accuracy.
• Marine viruses are known to infect a diverse range of marine organisms, including bacteria, archaea, protists, algae, and marine animals.
• The abundance of marine viruses is dynamic and often fluctuates due to consumption by other microorganisms and inactivation by solar radiation.
• Most marine viruses possess double-stranded DNA and belong to one of three families:
• Myoviridae, the most common phage, characterized by long, contractile tails and broad host ranges.
• Podoviridae, which have curved, non-contractile tails and narrow host ranges.
• Siphoviridae, known for short, non-contractile tails, commonly isolated from freshwater environments.
• Based on the type of host they infect and their interaction, marine viruses can be classified as:
• Bacteriophages, the most abundant marine viruses, which regulate bacterial populations and support horizontal gene transfer. Examples include Myovirus and Siphonivirus infecting Vibrio species, Actinophages targeting marine Actinobacteria, and Coliphages infecting Escherichia coli.
• Cyanophages, viruses that infect cyanobacteria, impact primary producers of the ocean, and play a role in the ocean’s carbon cycle. Examples include those infecting Prochlorococcus and Synechococcus.
• Algal marine viruses, primarily from the Phycodnaviridae family, are double-stranded DNA viruses infecting marine algae, which are key oceanic primary producers.
• Fungal marine viruses, such as a virus related to Bunyavirus, infect marine fungal-like organisms like Halophytopthora species, an oomycete.
• Fish viruses, which pose significant risks to aquaculture and fisheries. These include Rhabdovirus, Togarividae, Birvaviridae, Alloherpesvirus, and Betanodavirus, all known for causing diseases in fish.
• Mammal viruses, which infect marine mammals. Notable examples include Morbillivirus (dolphins, seals), Astrovirus (sea lions), Influenza virus (cetaceans and pinnipeds, including some seals).

Marine Prokaryotes

• Prokaryotes are unicellular organisms that are considered among the earliest forms of life on Earth.
• They are capable of inhabiting marine environments through specialized physiological mechanisms.
• Marine prokaryotes play a vital role in driving biogeochemical cycles within the ocean ecosystem.
• These marine prokaryotic organisms include two major groups: Archaebacteria and Eubacteria.

Archaebacteria

• Archaebacteria are ancient prokaryotic microorganisms lacking a true nucleus and are considered the oldest living organisms.
• They are capable of inhabiting extreme marine environments such as deep-sea hydrothermal vents.
• Their cell walls do not contain peptidoglycan, and their cell membranes are composed of fatty acids linked by ether bonds.
• Archaebacteria are currently classified into five major phyla: Crenarchaeota, Euryarchaeota, Korarchaeota, Nanoarchaeota, and Thaumarchaeota.
• These archaea are found in diverse marine environments, including surface waters, marine sediments, and deep-sea hydrothermal vents.
• Crenarchaeota and Euryarchaeota form a significant portion of the picoplanktonic population in the world's oceans.
• The genome of Nanoarchaeota equitans, a hyperthermophilic obligate archaeon that grows in association with the crenarchaeote Ignicoccus, is the smallest sequenced genome to date, measuring 0.49 Mbp.
• Thermoarchaeota, found in oceanic plankton, contribute to the nitrogen cycle as ammonia oxidizers, with examples such as Nitrososphaerota.
• Other marine archaebacteria include various hyperthermophiles and extremophiles such as Pyrolobus fumarii, Pyrococcus, Methanocaldococcus, Methanopyrus, and Methanogens.

Eubacteria

• Eubacteria, also known as “true bacteria,” are prokaryotic microorganisms with a rigid cell wall composed of peptidoglycan.
• They have a lipid bilayer membrane linked via ester bonds.
• In marine environments, eubacteria are highly diverse and play both beneficial and harmful roles.

Beneficial Roles:

• Initiation of photosynthesis.
• Contribution to nutrient cycling.
• Participation in decomposition.
• Establishment of symbiotic relationships.

Harmful Roles:

• Production of toxins.
• Causing diseases in plants and animals.

Major Groups of Marine Eubacteria

Cyanobacteria

• A diverse group of Gram-negative organisms, also known as blue-green algae.
• Conduct oxygenic photosynthesis using chlorophyll and phycobiliprotein.
• Stromatolites, ancient layered sedimentary formations, were formed by cyanobacteria around 3.5 billion years ago.

Examples:

• Unicellular Cyanobacteria: Microcystis sp., Synechococcus sp., Synechocystis sp., Hyella caespitosa, Prochlorococcus
• Filamentous Cyanobacteria: Lyngbya majuscula, Oscillatoria sp., Nostoc sp., Anabaena sp., Fischerella muscicola, Trichodesmium
• In nutrient-rich coastal waters (due to runoff), cyanobacteria may rapidly grow, causing eutrophication.
• This leads to decay of organic matter and toxin production, a phenomenon known as harmful algal bloom (HAB), impacting both marine life and humans.

Proteobacteria

• A group of Gram-negative bacteria, including both autotrophic and heterotrophic species.
• Play significant roles in:
• Sulfur, methane, and hydrogen oxidation.
• Sulfate and nitrate reduction.

Subgroups:

• Alpha-proteobacteria:
• Pelagibacter ubique (SAR11): Most abundant marine bacterium, thrives in nutrient-poor open ocean environments.
• Gamma-proteobacteria:
• Common in marine sediments.
• Includes:
• Aerobic anoxygenic phototrophs: Erythrobacter, Roseobacter.
• Methanotrophs: Methylococcaceae.
• Purple sulfur bacteria: Marichromatium, Halochromatium, Thiorhodococcus, Allochromatium, Thiocapsa, Thiorhodovibrio, Thiohalocapsa.
• Pathogenic species: Vibrio sp. – harmful to marine organisms and humans.
• Decomposers: Alteromonas – involved in degradation of organic material.

Firmicutes

• Gram-positive eubacteria with thick peptidoglycan cell walls.
• Many can form spores to survive in extreme conditions.
• Prominent marine member: Phylum Bacillota, known for producing novel bioactive compounds.

Actinobacteria

• Aerobic, Gram-positive bacteria with branching filamentous morphology, resembling fungi.
• Commonly found in marine sediments and water columns.
• Produce abundant antimicrobial compounds and other bioactive natural products.

Marine genera include:

• Dietzia, Rhodococcus, Streptomyces, Salinispora, Micromonospora

Bacteroidetes

• Strictly anaerobic, non-spore-forming, Gram-negative bacteria.
• Often act as obligate parasites in host organisms.
• Capable of degrading complex carbohydrates, such as pectin and chitin.

Important marine species include:

• Polaribacter sp. MED 152
• Dokdonia sp. MED 134
• Leeuwenhoekiella blandensis MED 217
• Gramella forsetti KT0803
• Some Cytophaga species prey on marine phytoplankton like diatoms and dinoflagellates.

Marine Fungi

• Fungi are chlorophyll-less, eukaryotic, non-vascular organisms.
• They reproduce by sexual and asexual means and have a filamentous branched somatic structure.
• Their cell walls are composed of cellulose, chitin, or both.
• Fungi are heterotrophic and exist in two major forms:
• Yeast: Unicellular fungi
• Molds: Multicellular fungi with filaments called hyphae, which form a network known as mycelium.
• They are capable of forming both sexual and asexual spores.
• In marine ecosystems, fungi play a vital role in energy flow by processing dissolved organic matter (DOM).
• Marine fungi are a source of natural secondary metabolites, including:
• Terpenes
• Steroids
• Alkaloids
• These metabolites exhibit antimicrobial, anticancer, and antiviral activities.
• Due to these properties, marine fungi hold significant potential in biotechnology and pharmaceuticals.

Ascomycota

• The majority of fungal species in marine environments belong to the phylum Ascomycota, also known as sac fungi.
• These fungi are septate, meaning their filaments (hyphae) are divided by septa (cross-walls).
• They reproduce sexually by forming spores called ascospores.

Common marine Ascomycota genera include:

• Mycosphaerella.
• Ramichloridium.
• Sphaerulina.
• Pharcidia.
• Rhabdospora.
• Cladosporium.
• Metulocladosporiella.
• Amarenomyces.
• Passeriniella.
• Floricola.
• Alternaria.
• Aspergillus.

Marine yeasts from Ascomycota that inhabit ocean environments include:

• Cryptococcus.
• Debaryomyces.
• Pichia.
• Hansenula.
• Rhodotorula.
• Saccharomyces.
• Trichosporon.
• Torulopsis.

Basidiomycota

• Filamentous fungi known as club fungi due to the club-shaped structure called basidia, where sexual spores are produced.
• Common marine Basidiomycota genera include:
• Flamingomyces
• Parvulago
• Halocyphina villosa

Chytridiomycota

• Aerobic, zoosporic fungi predominantly found in aquatic habitats.
• Exhibit a saprotrophic mode of nutrition, using organic debris as a food source.
• Some species are pathogenic to marine organisms.
• Marine Chytridiomycota genera include:
• Rhizidium.
• Phlyctochytrium.
• Chytridium.
• Catenochytridium.

Blastochytridiomycota

• Lesser-known group of fungi, classified among the heterokonts (organisms with two flagella, where the immature flagellum has a tripartite flagellum).
• Have a closer resemblance to brown algae than to traditional fungi.
• Inhabit aquatic habitats and play a role in the decomposition of organic matter.
• Exhibit a saprophytic mode of nutrition (feeding on dead or decaying organic material).
• Example genus: Catenaria

Marine Protozoan

• Unicellular, eukaryotic organisms that can be either free-living or parasitic.
• Have varied morphology, feeding strategies, and play a crucial ecological role.
• Found in coastal waters to deep-sea sediments.
• Size range: from several micrometers to millimeters.

Major Groups of Marine Protozoans

Foraminifera

• Microscopic, heterotrophic, single-celled organisms.
• Locomotion through pseudopodia.
• Construct a calcium carbonate test around themselves.
• Used as bioindicators for paleoclimatic studies.
• Fossilize along with marine sediments to form deep-sea sediments.
• Examples: Xenophyophores, Globigerina bulloides.

Radiolaria

• Heterotrophic, unicellular eukaryotes from the group Rhizaria.
• Capable of building silica-based skeletons (SiO2).
• Possess radial symmetry.
• Inhabit the open ocean as part of plankton.
• Examples: Spumellaria, Nassellaria.

Ciliates

• Microorganisms with numerous short hair-like appendages called cilia for locomotion and feeding.
• Some use cilia to filter feed into their oral groove.
• Examples: Tintinnids, Vorticella, Paramecium, Coleps, Colpoda, Balantidium, Didinium, Stentor.

Dinoflagellates

• Protists with two flagella for movement.
• Possess a protective armor made of cellulose.
• Can be photosynthetic or heterotrophic.
• Photosynthetic species (e.g., Zooxanthellae) are symbiotic with corals.
• Some species (e.g., Prorocentrum cordatum) can proliferate in warm, nutrient-rich habitats leading to harmful algal blooms.

Amoeboid Protozoans

• Variable morphology, move by pseudopodia.
• Phagotropic mode of nutrition: engulf their food.
• Examples: Pelomyxa, Amoeba proteus.

Marine Microalgae

• Microalgae are a diverse and abundant group of photosynthetic microorganisms found in marine habitats.
• They play a crucial role in the marine food web as primary producers, capable of photosynthesis.

Major Groups of Marine Microalgae:

Bacillariophyta (Diatoms)

• Have a silica-based cell wall called a frustule, consisting of two overlapping halves.
• Examples: Thalassiosira, Chaetoceros, Pseudonitzschia.

Dinophyta (Dinoflagellates)

• Possess two flagella: one for propulsion and the other for steering.
• Have a protective outer layer called the theca, made of cellulose plates.
• Examples: Alexandium, Ceratium, Karenia.

Haptophyta (Coccolithophores)

• Single-celled, golden-brown algae.
• Possess calcium carbonate plates (coccoliths) as an external covering, serving as armor.
• Example: Emiliania huxleyi.

Chlorophyta (Green Algae)

• Contain chlorophyll a and b.
• Less dominant in marine environments.
• Examples: Dunaliella, Tetraselmis.

Cyanophyta (Cyanobacteria)

• Often called blue-green algae, but lack a nucleus, hence referred to as Cyanobacteria.
• Play a crucial role in nitrogen fixation and can form blooms in nutrient-rich environments.
• Examples: Prochlorococcus, Synechococcus.

Dictyochophyta (Silicoflagellates)

• Single-celled algae with silica-based internal skeletons.
• Found in both cold and warm oceans and contribute to siliceous ooze in marine sediments.
• Example: Dictyocha.

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