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What Is A Biogeochemical Cycle?
First, some clarification on what precisely we're talking about here. Biogeochemical cycles refer to the movement of various chemical elements between living (biotic) and nonliving (abiotic) systems within the environment, here marine. While virtually every element found on this planet are cycled in one way or another, there are four main elements that we need to be concerned about: carbon, nitrogen, phosphorus, and sulfur, all necessary to life here on Earth and critical to broader processes such as climate change and evolution.
These various elements are not found in their 'pure' form, but rather in compounds with other elements, which may vary by where they are found in the cycle. Carbon and oxygen, for instance, form carbon dioxide, or carbonate ions, or even form sugars and proteins within living organisms. All of these elements have forms that are found in the atmosphere, in rocks, in the soil, in water, and in the living body. Each of these locations may be thought of as a “pool" in which these elements are collected and held for a period of time before being passed on to the next portion of the cycle. We will be primarily concerned in this article in the role that bacteria plays as a living being in the cycle as it occurs in marine environments.
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Sulfur and Carbon Cycles
Seawater contains a lot of sulfur, especially compared to most soils and to the atmosphere. In these marine environments, some types of bacteria act to reduce sulfur primarily in the form of sulphate, often in a chemosynthetic reaction in anaerobic conditions, substituting sulfur for oxygen to generate the electrons required, to produce the carbon-based compounds they need to survive.
To put it in slightly less technical terms, certain types of bacteria can, in conditions without either light nor oxygen as is found in the depths of the ocean, create the energy they require through a series of chemical reactions using sulfur. This is very different from life here on the surface, where life forms require both light and oxygen to survive. Granted a majority of life forms depend on photosynthetic and not chemosynthetic reactions to power themselves anyway, as chemosynthetic-based organisms are only really common around hydrothermal deep sea vents, but it's still a fascinating evolutionary adaptation that enables populations of animals where we would otherwise think nothing could survive.
There are also some bacteria that have photosynthetic yet anaerobic processes, primarily in estuaries. Here, it is most useful to aid in decomposition of (carbon-based) organic materials, which otherwise typically requires oxygen which is not available in certain parts of estuaries due to poor water mixing.
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Bacteria also plays an important role in nitrogen cycles. In a marine environment, cyanobacteria (blue green algae) works to “fix" nitrogen, that is, to put it in a form that is then usable to other life forms. From there, the fixed nitrogen can follow one of two pathways, nitrification which results in the creation of energy for the organism, or ammonia assimilation, in which the nitrogen is added to the biomass of the organism. It should be noted that it was cyanobacteria that created most of the oygen in our atmosphere today.
There are also bacteria that work to “de-fix" the nitrogen found in dead organisms, a decomposition process known as denitrifying that returns nitrogen back to the environment.
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For More Information
There really isn't much literature on this topic out there that is accessible to absolute laymen, but with knowledge of some basic biochemistry, here are a number of mildly technical reads for more on the subject:
Interactions between the Carbon and Sulphur Cycles in the Marine Environment (from The Major Biogeochemical Cycles and their Interactions resource.)