Just like microorganisms are found everywhere inhabiting terrestrial environments such as clouds, lands, hot springs, and the gut microbiome. Marine microorganisms live in an aquatic environment that is the sea, ocean, estuary, and other marine habitats. Marine microorganisms make up around 90% of the total biomass in the marine habitat.
The earliest observation of microorganisms by Antonie van Leeuwenhoek during the 17th century as “animalcule” was derived from an aquatic habitat. Moving forward to the 1800s, A German physiologist, Christian Andreas Victor Hensen established a groundwork in biological oceanography introducing the term “plankton” for microorganisms that float on seawater. By the mid-20th century, several microbiologists contributed to the field of marine microbiology by explaining their role in the food chain and nutrient cycling like Claude Zobell. He is often referred to as the father of marine microbiology. He explained how bacteria broke down organic matter, recycled nutrients in the marine ecosystem, studied biofilms in seawater, and also, published various foundational textbooks in marine microbiology.
Marine microorganisms include viruses, archaea, eubacteria, fungi, phytoplankton, and protozoa able to thrive in their respective marine habitats with their physiological adaptation, from the surface water to the deep sea hydrothermal vents.
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Marine Viruses
Marine viruses are biological entities in the ocean consisting of nucleic acid surrounded by a protein coat that can only replicate inside a living cell called the host. They represent the most abundant lifeforms in the ocean. In estimation, 4×1030 viral particles are present in the marine environment ranging in size from 25 to 300 nm.
- The first observation of marine viruses was done by transmission electron microscopy indicating ~ 107 viruses/ml, and the abundance decreased with depth and shore of the ocean. Furthermore, marine sediments possessed a higher abundance of 108-109 viruses/cm.
Different ways to enumerate marine viruses include:
- Indirect viable counts (rely on the ability of viruses to cause lysis in its culture)
- Direct counts (rely on counting viruses in the environmental sample through transmission microscopy, flow cytometry, and epifluorescent microscopy)
- Epifluorescence microscopy is considered the most preferred method due to its high accuracy.
- They are known to infect a wide range of organisms, including bacteria, archaea, protists, algae, and even marine animals.
- The number of marine viruses remains variable as they are often consumed by microorganisms or deactivated by solar radiation.
- Most marine viruses possess a double-stranded DNA belonging to one of three families:
- Myoviridae are the most common phage characterized by a long contracting tail, a broad host ranges.
- Podoviruses are viruses with curved, non-contractile tails and very narrow host ranges.
- Siphoviruses are those viruses with short, non-contractile tails commonly isolated from freshwater.
Viruses infect different types of hosts, and based on their host interaction they can be of the following types.
- Bacteriophages: Marine bacteriophages are the most abundant viruses. They play a significant role in maintaining bacterial populations and evolutionary mechanisms through horizontal gene transfer. Examples include Myovirus and Siphonivirus infecting Vibrio species, Actinophages infecting marine Actinobacteria, and Coliphage infecting Escherichia coli.
- Cyanophages: Cyanophages are those viruses specifically infecting cyanobacteria. As these viruses infect primary producers of the ocean, they affect the carbon cycle in the ocean. Examples include phages infecting Prochlorococcus and Synechococcus.
- Algal marine virus: A main known virus infecting algae belongs to the Phycodnaviridae family, They are double-stranded DNA viruses infecting the marine primary producers.
- Fungal marine virus: An example of a marine virus infecting a fungal host is a virus showing relation to Bunyavirus known for infecting fungi like Halophytopthora sp., a fungal-like oomycete.
- Fish virus: Fish can be affected by a variety of viruses, causing significant complications in the aquaculture and fishing industry. Some major fish viruses include Rhabdovirus, Togarividae, Birvaviridae, Alloherpesvirus, and Betanodavirus. These viruses cause significant threats to fish causing a variety of diseases.
- Mammal virus: The marine mammal virus is known to infect mammals in the ocean. Examples include Morbillivirus affecting dolphins and seals, Astrovirus affecting sea lions, Influenza virus affecting cetaceans and pinnipeds, and a few species of seals.
Marine Prokaryotes
Prokaryotes are unicellular organisms known to have originated the earliest on the Earth. They even inhabit marine habitats with physiological mechanisms and contribute crucially to biogeochemical cycles.
These marine prokaryotic organisms include Archaebacteria and Eubacteria.
Archaebacteria
These prokaryotic microorganisms are known to be the oldest living organisms without a true nucleus present. These bacteria can inhabit extreme conditions such as deep-sea hydrothermal vents. Their cell wall lacks peptidoglycan, and the cell membrane is composed of fatty acids connected by an ether linkage.
- Archaebacteria are currently divided into five major phyla: Crenarchaeota, Euryarchaeaota, Korarchaeota, Nanoarchaeota, and Thaumarchaeota.
- Archaea inhabits the surface, marine sediments, and deep-sea hydrothermal vents. Crenarchaeota and Euryarchaeota account for a major population of picoplanktonic living organisms in the world’s oceans. The genome of hyperthermophilic obligate Nanoarchaeota equitans that grow along with crenarchaeote, Ignicoccus is the smallest genome to be sequenced to date i.e., 0.49 Mbp. Thermoarchaeota also resides in the oceanic plankton contributing to the nitrogen cycle as ammonia-oxidizers such as Nitrososphaerota.
- Other marine Archaeobacteria include Hyperthermophiles such as Pyrolobus fumarii, Pyrococcus, Methanocaldococcus, Methanopyrus, Methanogens, and other extremophiles.
Eubacteria
The prokaryotic microorganisms are known to be “true bacteria” as they possess a rigid cell wall made of peptidoglycan. They have a lipid bilayer membrane connected by ester-linkage. In the ocean. Eubacteria displays an incredible diversity playing a crucial role in marine ecosystems in various beneficial or harmful ways. Its beneficial role includes the initiation of processes in photosynthesis, nutrient cycling, decomposition, and symbiosis. Furthermore, its harmful role includes the production of toxins, causing diseases to plants and animals.
Various groups of Eubacteria include:
Cyanobacteria
They are a diverse group of gram-negative organisms also known as blue-green algae. They are responsible for oxygenic photosynthesis through the use of chlorophyll and phycobiliprotein. A remnant of Earth’s oldest lifeform, the Stromatolites are layered sedimentary formations by cyanobacteria dating 3.5 billion years ago.
Examples:
Unicellular cyanobacteria include Microcystis sp., Synechococcus sp., Synechocystis sp., Hyella caespitosa, and Prochlorococcus.
Filamentous cyanobacteria includes Lyngbya majuscule, Oscillatoria sp., Nostoc sp., Anabaena sp., Fischerella muscicola, Trichodesmium.
In conditions where nutrients run from land. Cyanobacteria grow rapidly due to excess nutrients leading to eutrophication in coastal waters. This leads to the decay of organic matter, and the production of harmful toxins affecting marine life and human health. This phenomenon often observed in the lake, river, or ocean is termed “harmful algal bloom”.
Proteobacteria
Proteobacteria is a group of gram-negative bacteria with a diverse presence in marine environments. This group includes bacteria both autotrophic and heterotrophic. They have a significant role in the oxidation of sulfur, methane, and hydrogen, and the reduction of sulfate and nitrate.
Alpha-proteobacteria:
Pelagibacter ubique (SAR11) is the most abundant bacteria adapted in nutrient-poor marine environments such as the open ocean.
Gamma-proteobacteria:
Important genera of bacteria that can be found abundantly in the marine sediments.
These include Aerobic anoxygenic phototrophic bacteria (Erythrobacter and Roseobacter), Methanotrophs(Methylococcaceae), Purple sulfur bacteria(Marichromatium, Halochromatium, Thiorhodococcus, Allochromatium, Thiocapsa, Thiorhodovibrio, Thiohalocapsa).
Vibrio sp. are known to be pathogenic to other marine organisms and humans.
Moreover, free-living Alteromonas have a role in the degradation of organic material.
Firmicutes
Firmicutes are eubacteria with gram-positive cell walls i.e., thick peptidoglycan layer. Moreover, these organisms can have the ability to form spores and resist extreme conditions.
A prominent member of Firmicutes in the ocean involves the phylum Bacillota known to produce chemical compounds as a novel biologically active molecule.
Actinobacteria
Actinobacteria are aerobic, Gram-positive bacteria with a resemblance to fungi due to branching filamentous morphology.
They often inhabit marine sediments and water columns. This family of Prokaryotic bacteria is known to produce abundant antimicrobial compounds and has been known to be an important source of bioactive natural products. Some genera of marine Actinobacteria include Dietzia, Rhodococcus, Streptomyces, Salinispora, and Micromonospora.
Bacteroidetes
Bacteroidetes are strictly anaerobic, non-spore-forming Gram-negative bacteria. Many species of Bacteroidetes live within its host as obligate parasites.
These organisms are capable of degrading complex carbohydrates including pectin and chitin.
Some important marine Bacteroidetes include Polaribacter sp. MED 152, Dokdonia sp. MED 134, Leeuwenhoekiella blandensis MED 217, Gramella forsetti KT0803. Some Bacteroidetes like Cytophaga species are known to prey on marine phytoplankton (Diatoms, Dinoflagellates)
Marine Eukaryotes
Marine Fungi
Fungi include chlorophyll-less, eukaryotic, non-vascular organisms reproducing by sexual and filamentous branched somatic structure.
They have a morphology of cell walls made of cellulose, chitin, or both. Furthermore, fungi are heterotrophic organisms that can be yeast or mold. Yeast is a unicellular fungus where molds have multicellular filaments called hyphae forming mycelium. Fungi can form sexual as well as asexual spores.
Marine fungi play an essential role in energy flow by mediating dissolved organic matter in the marine environment. Due to their ability to produce natural secondary metabolites such as terpenes, steroids, and alkaloids which have antimicrobial, anticancer, and antiviral activities, they have promising applications.
Ascomycota
Most fungal species found in marine environments belong to Ascomycota or Sac fungi. These are septate fungi with filaments divided by divisions of septa. They form sexual spores called ascospores. The marine Ascomycota include Mycosphaerella, Ramichloridium, Sphaerulina, Pharcidia, Rhabdospora, Cladosporium, Metulocladosporiella, Amarenomyces, Passeriniella, Floricola, Alternaria, Aspergillus, etc. Marine yeasts inhabiting marine environments include Cryptococcus, Debaryomyces, Pichia, Hansenula, Rhodotorula, Saccharomyces, Trichosporon, Torulopsis, etc.
Basidiomycota
These filamentous fungi are also known as club fungi due to their club-shaped resemblance of structure called basidia that produce sexual spores. Marine Basidiomycota includes Flamingomyces and Parvulago. Halocyphina villosa, etc.
Chytridiomycota
These aerobic zoosporic fungi are pathogenic and have a saprotrophic mode of nutrition (organic debris as a source of nutrition). They are predominantly aquatic and they include genera such as Rhizidium, Phlyctochytrium, Chytridium, and Catenochytridium.
Blastochytridiomycota
They are a lesser-known group of fungi placed among the heterokont (presence of 2 flagella where immature one bears tripartite flagella). They have a closer resemblance to brown algae than to fungi. Furthermore, they inhabit aquatic habitats contributing to the decomposition of organic matter, and have a saprophytic mode of nutrition.
Examples include: Catenaria,
Marine Protozoan
Protozoans are unicellular, eukaryotic organisms either free-living or parasitic.
These organisms have a varied morphology, feeding strategy, and crucial ecological role.
They are found in coastal waters to deep-sea sediments in size ranging from several millimeters.
Major groups of Marine protozoans include:
Foraminifera
Foraminifera are microscopic, heterotrophic, single-celled organisms locomoting by pseudopodia. A foram is a single cell that builds a layer of calcium carbonate around it called a test. They are used as bioindicators for paleoclimatic studies as they fossilize themselves along with marine sediments to form deep-sea sediments. Examples include Xenophyophores and Globigerina bulloides.
Radiolaria
Radiolarians are heterotrophic, unicellular eukaryotes of group Rhizaria with the ability to build silica-based skeletons(SiO2) possessing radial symmetry. They inhabit the open ocean as a part of plankton. Example: Spumellaria and Nassellaria.
Ciliate
Ciliates are microorganisms bearing numerous, short, hair-like appendages called cilia for locomotion and feeding. Some organisms possess cilia as a mode to filter feed their food into their oral groove. Ciliates include Tintinnids, Vorticella, Paramecium, Coleps, Colpoda, Balantidium, Didinium, Stentor, etc.
Dinoflagellate
Dinoflagellates are protists with two flagella for movement, and they possess a protective armor made of cellulose. They can be either photosynthetic or heterotrophic. Photosynthetic species like Zooxanthellae are symbiotic with corals for beneficial outcomes whereas, other species including Prorocentrum cordatum can proliferate in warm, nutrient-rich habitats as harmful algal blooms.
Amoeboid
Amoeboid protozoans have variable morphology and are located by pseudopodia. They have a phagotropic mode of nutrition meaning they engulf their food. These organisms include Pelomyxa and Amoeba proteus.
Marine Microalgae
Microalgae are diverse and abundant group of photosynthetic microorganism found in marine habitats. They form a crucial part of the marine food web as primary producers as they are capable of photosynthesis.
Majors groups of Marine Microalgae include:
Bacillariophyta
This phylum includes Diatoms having a silica-based cell wall called frustule. It is in two-overlapping halve form. Examples: Thalassiosia, Chaeotoceros, and Pseudonitzschia.
Dinophyta
This phylum includes Dinoflagellate possessing two flagella, where one is for propulsion and the other for steering. It has a protective outer layer of theca within cellulose plates. Examples: Alexandium, Ceratium, and Karenia.
Haptophyta
This phylum includes single-celled, golden-brown algae of Coccolithophore. They possess calcium carbonate plates as external covering called coccoliths as armor. Examples include Emiliania huxleyi.
Chlorophyta
This microorganism includes Green algae that contain chlorophyll a and b. They are less dominant in marine environments. Examples include Dunaliella and Tetraselmis.
Cyanophyta
Despite being blue-green algae, this organism lacks a nucleus due to which it is often referred to as “Cyanobacteria”. They have a crucial role in nitrogen fixation and form blooms in nutrient-rich environments. Two important examples include Prochlorococcus and Synechococcus.
Dictyochophyta
This phylum includes Silicoflagellates which are single-celled algae with silica-based internal skeletons. They can be found in cold as well as warm oceans contributing to siliceous ooze in marine sediments. Examples include Dictyocha.
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