Sargasso Sea
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Introduction
Deep within the Atlantic Ocean, near the Bermuda Triangle, lies a sea shrouded with mystery named the Sargasso Sea. Uncannily calm, the Sargasso Sea was believed to be the demise of sailors. With little wind, ships lay stagnant for days in this “sea of lost ships”, leading to historical accounts of crews going into the sea, but never coming out. Eventually these accounts were mutated into urban legends of whole ships disappearing or broken down vessels roaming this sea manned by skeleton ghost crews and all.(The Sargasso Sea) Realistically speaking though, the Sargasso Sea was believed to be first found by Christopher Columbus and his crew. They named this sea after the seaweed dominating the sea’s surface named Sargassum. They named the sea “Sargaco”, which means grape because Sargassum looks like grapes. (Gladnick, 2008)
Description of Niche
Where located?
More precisely, the Sargasso Sea is located in the middle of North Atlantic Ocean bounding the Great Antilles on the south, the Gulf stream on the west, and Bermuda on the north.(Teal & Teal, 1975) These currents move around the Sargasso sea in a clockwise orientation. Having a latitude between 20N and 35 N and a longitude between 30W and 70W, the Sargasso Sea is comparable to the United states in size.( Teal & Teal, 1975) Both this great size and the location of the Sargasso Sea have great implications on its physical conditions.
Physical Conditions?
What are the conditions in your niche? Temperature, pressure, pH, moisture, etc. Because it is not bounded by coastline and is surrounded by strong currents, the Sargasso Sea is considered an isolated, oligotrophic (little to sustain life) area. The surrounding currents provide a strong physical boundary separating the Sargasso sea from the more nutrient-rich waters of the North America shelf. The northern region contains warm water known as eighteen-degree water that moves outwards along the surface of the sea, allowing it to maintain that temperature year round whereas coastal waters with the same latitudes freeze in the winter. With increasing depth, water temperature declines and pressure builds up restricting much life (Teal & Teal, 1975) Also, the water in the Sargasso Sea is said to be salty and warm, maintaining a salinity around 36% and euphotic zone temperature up to 22°C. For these reasons, even though there are several species of plankton and massive amounts of seaweed floating on the water surface, the Sargasso Sea is still not nutritious enough to attract large communities of fishes. These factors of low wind, low nutrients, and high salinity help to justify why the Sargasso Sea is considered a desert of oceans with little sign of life.
As mentioned before, little nutrients are detected in the Sargasso Sea. One conformation of this belief is Phosphorous amounts. With dissolved inorganic phosphate (DIP) concentrations of 0.2 to 1.0 nanomolar on surface water, there are signs of a relatively severe phosphorus depletion in the Atlantic. This depletion can be attributed to the high ratio of DNN (dissolved nitrate plus nitrite) to DIP (dissolved inorganic phosphate). This high nutrient source N:P ratio causes available P to be depleted before N by algal growth after upward nutrient injections into the euphotic zone through advection or diffusion.( Wu et al., 2000)
Another type of nutrient found in the Sargasso Sea are slicks, natural fat and oil buildup. Not only do these nutrients contribute to the sea’s overall calm glasslike surface, bacteria can collect and grow on these leveled surfaces. They contain abundant useable organic materials; and, once the bacteria dies, the bacteria yields even more oil to these slicks. Unfortunately, one slick area does not last long and tends to fade away gradually due to various chemical and bacterial activities.(Teal & Teal, 1975)
How Research Began
To understand what is known about the Sargasso Sea, one needs to understand why we wanted to know more about it in the first place. Although most scientists thought that this expanse of sea contained no sustainable life, observations showed that oxygen and other elements were being consumed at a higher rate than theories and models could account for. This led scientists to think there must be some nutrient source fueling the blooms of phytoplankton in the Sargasso Sea.( Carlowicz, 2006, LiveScience) This discovery, in turn, led to the discovery of eddies.
Eddies and Their Influence on the Sargasso Sea
Even with such a hostile environment, there exists vast phytoplankton bloom and higher oxygen consumption rate than predicted in the surface euphotic area. The vast amount of organisms in the Sargasso Sea documented in Venter et. al is shocking in comparison to the lack of “nitrate, phosphate, trace metals, and other nutrients” present (Carlowicz, 2006). This suggests that there is internal water mixing and continually pumping up of rich nutrients from deeper water layers. This swirling water system is called eddies. (LiveScience) Eddies are episodic underwater current systems that pump nutrients up from the ocean floor. They form from “differences in ocean temperature and salinity that give water different densities. Like oil and water, water masses of different densities tend to keep separate, rather than mix. ” What causes eddies to spin and therefore mix during the summertime is the Earth’s natural rotation, also known as the Coriolis force. (Carlowicz, 2006) This brings necessary food to the phytoplankton and other microbes. Specifically, during the winter, the incoming cold water forms a “subtropical mode,” that allows phytoplankton to multiply due to increase nutrients resulting in an increase in other organisms such as zooplankton that ultimately feed the entire environmental niche. (Venter et al., 2004, LiveScience) This also fuels sargassum growth and without this, organisms living within the sargassum would not be able to survive (“Sargasso Sea Without a Coastline”). Therefore eddies, often called “the oasis of the ocean”, can be essential to many microbes living in the Sargasso Sea. (LiveScience)
Who lives there?
Several species of the brown algae belonging to the genus Sargassum are found in the world’s temperate and tropical oceans. The main species found on the Sargasso are Sargassum natans and S. fluitans – both of which are completely adapted to living in the open ocean (Coston-Clements et al., 1991).
Cyanobacteria such as Dichoothrix and Oscillatoria are primarily involved in the N2 fixation within the Sargassum community. (Coston-Clements et al., 1991).
The fact that the water evaporation rate of the Sargasso is faster at its center than at the surrounding waters generates an inward current that aggregates the countless tons of kelp in the middle of the Sargasso sea. Sargasso – or gulf weed – with its complex branching systems, allow tiny animals to hide from predators and serve as a nursery for dozens of small fishes. Their blades are also the only solid surface for hundreds of miles where sedentary animals can become attached. This community functions as a filter that sieves out nutrients and particles from the waters creating a complex ecosystem where all animals are intertwined. However, not all life forms in the Sargasso are minuscule, sea turtles, dolphins and even humpback whales find the Sargasso to be a haven – it was there that their music was first recorded (Genthe, 1998).
Although most of the life is microscopic, humpback whales, dolphins, swordfish, and sea turtles all journey far to the beautiful blue Sargasso Sea. Interestingly, this is where the humpbacks songs were first recorded (Genthe, 1998).
The sea’s food chain in which all existing animals rely on for survival, starts with the surface plants acquiring energy and sunlight. The plant tissues grow, containing various synthesized chemicals that animals can consume and pass along as they are eaten themselves. However, the process of vertical migration is very gradual and slow in the sea. Vertical migration is the downward transport of assorted animal species who have consumed other animals or phytoplankton and will be eaten themselves by the lower-dwelling animals; this process occurs in order to obtain food and gain energy. This food chain allows the sea’s animals to coexist and depend upon one another for food and energy gain (Teal & Teal, 1975).
Which microbes are present?
Prokaryotes
Silicibacter pomeroyi
One of the most interesting microbes comes from the Roseobacter clade, of α-proteobacteria. Silicibacter pomeroyi is one of the most numerous oligotrophs that inhabit the Sargasso Sea. Having evolved in an area of the world with low nutrients, it has developed a unique physiology compared to other marine oligotrophs. Some of these include; sulfonate degradation, oxidation of lignin-related compounds, and the use of hydroxylamine oxidoreductase to generate nitrite. These characteristics allow Silicibacter to cope with the under nourishing habitat. The microbe also contains genes that enable a symbiotic relationship with plankton, uptake algal produced compounds, fast growth and cell density dependent regulation (Moran et al., 2004).
SAR Family Bacteria
One of the documented most abundant microbes of the marine world, as well as the Sargasso Sea, belongs to the clade of SAR11 α – proteobacteria. The name “Sar” was given to this family of bacteria due to their discovery in the Sargasso Sea in 1990. One species that is of most interest to researchers is that of Pelagibacter ubique. P. ubique covers 30% of the surface in the Sargasso. During the summer months the population of this bacterium can cover up to 50% of the ocean’s surface weighing more than the weight of all of the fish in the all of the collective oceans. (Giovanni, et al., 2005) What make these bacteria an interest of research, is its ability to thrive in an environment low in nutrients and resources. This bacterium can surprisingly replicate very efficiently as possible in a low nutrient environment and is one of the smallest self-replicating cells found. Evolutionary genome reduction has been observed in this microbe. This is consistent with the hypothesis of “genome streamlining driven by selection acting on a very large population which resides in a very low nutrient habitat” .The belief is that the bacterium’s genome is being reduced not to expend energy on replicated DNA with no adaptive value. This saves the the organism from performing unnecessary metabolic tasks (Giovanni, et. al, 2005)
SAR86 is part of the Proteorhodopsins (PRs) family-related species. PRs are predicted to have important roles in supplying light energy for microbial metabolism. They are divided into two groups depending on two distinct absorption spectrums of light intensity: Green-absorbing (GPRs) and Blue-absorbing (BPRs). In the Sargasso Sea, the dominant light energy at surface water (until 40m depth) is in the blue range with maximal intensity near 440m. Therefore, only BPRs are discovered regardless of seasonal changes and depth of the surface water column, an indication of different light intensity. This is a unique feature of the Sargasso Sea that is different from other ocean areas, such as Mediterranean Sea, where you will find GPRs dominant in the upper most water layer because of the presence of green pigments, then followed by BPRs in the next water layers (Sabehi et al., 2007). This adaptation to spectrum changes in the environment is called spectral tuning, which requires a single amino-acid change at position 105: leucine for green and glutamate for blue (Man et al., 2003).
Cyanobacteria
Though known as the smallest photosynthetic organism (d), Prochlorococcus is still one of the major phototrophs in the ocean and greatly impacts the carbon cycle. (b,c). It is a small photosynthetic marine cyanobacterium found up to 200 meters below the sea surface throughout the euphotic zone.(b, c) Though a cyanobacteria, Prochlorococcus has a unique pigment composition. It contains divinyl derivatives of chlorophyll a and b, α -carotene, zeaxanthin (I, j) and phycoerythrin (k). Growth of Prochlorococcus is best when surface waters are devoid of major nutrients and worst during winter when Synechococcus and other eukaryotic organisms have their plankton bloom. (F, G, H) Procholorcoccus have different ratios of chlorophyll pigments based on where they live. High-light adapted Prochlorococci live in the upper 100 meters which are well lit. Low-light adapted Procholorococci are found mainly in deep euphotic zone of depth 80-200 meters, which is a less bright environment but nutrient rich. (E) Low-light adapted Prochlorococci are generally found to be able to take up NO3-. (L)
Another major contributor to the Sargasso ecosystem and closely related to Prochlorococcus is the cyanobacteria Synechococcus. Though less abundant than Prochlorococcus, Synechococcus has a larger cell size and broader lateral distribution (M) therefore giving it the ability to compete metabolically with its more numerous counterpart, Prochlorococcus. Synechoccus is restricted to living in the upper 100 meters under the sea surface. (A, N) Unlike Prochlorococcus, Synechoccus contains a pigment composition typical of cyanobacteria consisting of chlorophyll a and phycobilins. It is known to be good at obtaining trace metals and major nutrients even in oligotrophic environments such as the Sargasso Sea. (O)
Luminous Bacteria
Vibrio fischeri and Lucibacterium harveyi consist of 90% of the luminous bacteria isolated from Sargasso Sea. Photobacterium leiognathi and Photobacterium phosphoreum constitute the rest of the population. Only Vibrio fischeri and Lucibacterium harveyi are found in the upper water layer, at depths of 160m to 320m, the average temperature at 20C, which is where thermocline occurs. No luminous bacteria are found in surface microlayer. Luminous bacteria are small rod shape, gram negative bacteria. They could be free-living planktons, parasites or they can symbiont with fishes. Their major role is the production of bioluminescence which serves as specific functions for different species. It’s said that free-living life style is less likely to occur in the Sargasso sea since it’s an oligotrophic area, therefore the majority of them may symbiont with small sea animals that live within the Sargassum (Orandorff & Colwell, 1980).
Eukarya
Most of the algae in the Sargasso float in the phytoplankton. Diatoms are among the best-known groups of algae, belonging to a group called golden brown algae. The Sargasso’s diatoms are extremely small, approaching the size of bacteria. Another type of golden-brown phytoplankters, the coccolithophores, exists most commonly in the Sargasso. The coccolith refers to tiny calcium plates which form in its outer skeleton (Stingl et al., 2007).
Viruses
Viruses are the most common biological entities in the marine environment. Cyanophages and a newly discovered clade of single-stranded DNA phages dominate the Sargasso Sea. Most marine viruses are phages (bacteriophages) that kill the heterotrophic and autotrophic microbes (both Bacteria and presumably Archaea) that dominate the world’s oceans. Phages and the other major microbial predator guild nanoflagellates, and they control the numbers of marine microbes to a concentration of about; 53 X 105 cells per milliliter of surface seawater. Phages affect microbial evolution by inserting themselves into genomes as prophages. Prophages often account for most of the difference between strains of the same microbial species, and they can dramatically change the phenotype of the hosts via lysogenic conversion. Phages also affect microbial evolution by moving genes from host to host. (Angly et al., 2006)
Non-Microbes: Plants and Animals
Pelagic sargassum supports a diverse community of marine organisms including micro and macro epiphytes, fungi, more than one hundred species of invertebrates, over one hundred species of fishes, and four species of sea turtles.(Coston-Clements et al., 1991)
Among the diverse organisms living within the Sargasso Sea include sea turtles and eels. After a sea turtle hatches, it goes into the sea and migrates to an area with favorable conditions. The turtles generally spend 2-4 years here then migrate back to the North American coast. (AH) During their first year of life, turtles associate themselves with Sargassum rafts, mats of brown algae, about 7 million tons of which can be found in the Sargasso Sea.(AG) Though different species of sea turtles aquire different diets as they grow older, Sargassum provides food and shelter for the sea turtles which is desperately needed during their early years. (AG) To get to the Sargasso Sea, turtles use earth’s magnetic field as guidance. This is extremely important because although the Sargasso Sea and the rest of the North Atlantic gyre have a favorable environment for the sea turtles to grow in, going north of the gyre to lower temperatures could be detrimental to the turtle’s survival. (AH)
Unlike sea turtles, who migrate to the Sargasso Sea as juveniles, for eels the Sargasso Sea is a place to spawn. Two major species of eels are found here, the European eels Anguilla anguilla and the American eels Anguilla rostrata. (AE) Eels have a variety of diets based on age and species. For instance, American eels will eat large invertebrates as adults and phytoplankton as babies. After birth, the eels head toward Europe or North America and come back later in life to lay their own eggs. AF Sadly, there seems to be a decline in eel recruitment which scientists believe to be linked to climate changes in the Sargasso Sea.(AD)
Do the microbes that are present interact with each other?
Describe any negative (competition) or positive (symbiosis) behavior
Do the microbes change their environment?
Do they alter pH, attach to surfaces, secrete anything, etc. etc.
Do the microbes carry out any metabolism that affects their environment?
Do they ferment sugars to produce acid, break down large molecules, fix nitrogen, etc. etc.
Current Research
Genetic Analysis: A new way to explore microbial ecology and to discover new species from environmental sample collections
Scientists now believe that genetic material may originate outside its environment and that the physical processes of the Earth or carrier organisms (for example, birds) are their means of travel. A study has recently critically analyzed the Baas-Becking hypothesis, stating that everything is everywhere and the environment selects. This study on comparitive metagenomics takes two very different environmental sequencing projects, the first from Minnesota farm soil and the second from the Sargasso Sea microbes, and compares the sequencing projects to find molecular evidence of a transfer of microbes over distant environments. The theory behind this is that genetic material can be anywhere and doesn’t necessarily mean it will survive in a new environment but, it can still contribute its genome. To determine this, they measured three distinct characteristics, guanine/cytosine (GC) content, oligomer frequency patterns (OFPs), and lastly protein similarity between translated open reading frames in both data sets (Hooper et al., 2008).
Later on, by applying shot-gun sequencing and phylogenetic comparison on uncultured microbes collected from ocean samples, the scientists discovered several previously unknown species that don’t grow in laboratory condition. For example, in 2007, Not et al., showed the discovery of Picoplanktonic protists. By 18s rRNA analysis of different water columns in Sargasso Sea, species in Kingdom Chromalveolata and Rhizaria are found to be the prodominent protista. Among Chromalveolata, Stramenopiles, which exist only in surface euphotic area, and Alveolata, which are discovered from surface to deep sea area, are dominant species. Many of which contain choloroplast; thus living as autotrophs carrying out photosynthesis. Radiolaria is the major species found in Kingdom Rhizaria, they live mainly from 500m thermocline to 3000m deep sea and are heterotrophic zooplanktons. They may be important to ocean biogeochemical cycling and carbon transport (Ghedin & Claverie, 2005). (please refer to microbewiki for details in these species)
Not et al., utilized the similar method, genome sequence analysis and phylogenetic comparison analysis to unveil the existence of Mimivirus relatives in Sargasso Sea, a DNA virus of 0.1 to 0.8 microns in size. They are evolutionarily closer to Mimivirus and exist in large abundance in the collected water samples. The discovery is published in 2005 and further research is needed (Not et al., 2007).
Metabolic and Proteome Analysis – “SAR11 Bacteria and the State of the Ocean”
Dr. Stephen Giovanni and his cohorts have done extensive research on the SAR clade of bacteria. Of special interest to them is the Pelagibacter ubique organism. Currently they are trying to use P.ubique for a few different research interests. First, they are trying to predict the different organic carbon sources used by P.ubique. They are doing this through a method of metabolic reconstruction. Second, they are using mass spectrometry to understand P.ubique’s regulatory responses in regards to environmental factors. By doing this, it will give the researchers insight to the proteomic state of the organism. With this they can then use P.ubique as a proxy to report the biological state of the specific system they are in. Last, a long-term goal is to be able to model the metabolic processes of P.ubique. It is an ideal candidate because it is one of the smallest and simplest known cells. With this information, it could be possible to integrate and optimize metabolic processes to be more efficient during low nutrient periods (Monterey Bay Aquarium Research Institute).
Unique Geological Feature: Floating Plastic
Frequently discovered throughout the western regions of the Sargasso Sea are unusual amounts of broken plastic fragments afloat on the water surfaces, roughly 3500 per square kilometer. Since these plastic fragments have become apart of the sea, various groups of hydroids and diatoms are often found on them. There are concerns regarding the ever increasing plastic production along with its improper disposal methods, which can disrupt ocean life and harm animals that may ingest plasticizers such as polychlorinated biphenyls (PCB’s) (Carpenter & Smith, 1972). Researchers inspecting the Sargasso discovered on average, about 8,000 to 10,000 of floating plastic pieces per square mile (Murphy, 1986). In addition, the Sargasso Sea consists of many plastic fragments due to its slow circulation (Greenpeace International) and its northern region includes one main, circularly moving gyre in which waste and plastic pieces flows to and accumulates (Watson, 2006).
Unique Biological Feature: Antioxidant Activity
It has been found that the seaweed of the genus Sargassum yields a methanol extract with potent antioxidant and antimicrobial properties. When compared with Ampicillin, it shows a more pronounced bactericidal effect on gram-positive and gram-negative bacteria. Scientists were able to evaluate these properties against three different species of bacteria, namely, Staphylococcus aureus, Bacillus subtilis, and Eschericia coli – the last being among the most common cause of food poisoning. For this reason, it has been suggested that the methanol extract of Sargassum sp. should be utilized as a cheap and plentiful natural source of food with the benefits of having antimicrobial and antioxidant properties. Nonetheless, more studies are required due the fact that the precise mechanism of how it works against bacteria is not well understood as well as possible allergic reactions. (Patra et al., 2008).
Conclusion
Conclusion To sum it all up, the culmination of data that has arisen in the past decade after the initial study by Venter et al. has shed light into the oceans unexplored biodiversity. The study of the Sargasso Sea has brought more questions to the table that researchers are tackling currently. Questions such as: “What else in the ocean don’t we know about? Are there many more genes in marine organisms than we ever imagined? Have we missed the major players in the ocean’s energy cycles? “ have scientists baffled at the possibilities(Ruder et al., 2008).
References
Edited by [Sherry Pablo, Hugo Frazao, Patricia Tu, Asa Gardner, Cam Nguyen, Shanice Wang], students of Rachel Larsen
