A biogeochemical cycle is the circulation of an element in the Earth system. It involves various reservoirs that store the element, fluxes between reservoirs as well as the physical, chemical and biological parameters that regulate the fluxes. The oceans play a key role in the biogeochemical cycling of elements on our planet. As primary productivity is strictly limited to the photic zone and decay of organic matter is pursued in the deeper water masses of the oceanic system, the distribution of many elements exhibits a strong vertical gradient. A biogeochemical cycle refers to the cycling and transport of a chemical element or compound, usually in multiple forms and physical states, through the biotic (living) and abiotic (nonliving) components of the earth system. Some of the most commonly examined biogeochemical cycles include carbon, nitrogen, oxygen, iron and phosphorous. The marine nitrogen cycle is one of the most complicated biogeochemical cycles in the ocean. Nitrogen is a biologically limiting element and changes in its form, or concentration, can cause changes in the cycling of other elements, such as carbon and phosphorus. Marine nitrogen cycle is perhaps the most complex and therefore the most fascinating among all biogeochemical cycles in the sea. Nitrogen exists in more chemical forms than most other elements, with a myriad of chemical transformations. All these transformations are undertaken by marine organisms as part of their metabolism, either to obtain nitrogen to synthesize structural components, or to gain energy for growth. Nitrogen gas (N2) from the atmosphere dissolves into seawater at the ocean surface. Nitrogen gas is the most abundant form of nitrogen in the ocean, but is not useful to most living things. Dissolved nitrogen gas is taken up by just a few types microbes, which convert the nitrogen into a much more useable form, known as ammonium (NH4+). This process, known as “nitrogen fixation,” is vitally important. Without it, very little nitrogen would available for thousands of other organisms that live near the ocean surface. Ammonium is the form of nitrogen that is most easily consumed by microorganisms. For this reason, ammonium is consumed almost as fast as it is produced, a process called “assimilation.” The result is that the nitrogen becomes incorporated into the cells of living organisms. Some marine microbes consume nitrite and nitrate, another form of assimilation. When microbes (and other organisms) die, they decompose, releasing ammonium and tiny particles containing particulate organic nitrogen (PON), as well as dissolved organic nitrogen (DON) into the surrounding seawater. Some microbes convert ammonium to nitrite (NO2-) and then nitrite to nitrate (NO3-). This two-step process is called “nitrification.” The result of this process is that nitrate is released into the ocean. A host of organisms consume particulate organic nitrogen and dissolved organic nitrogen, converting some of the nitrogen back to a

1. RECENT APPROACHES IN THE
STUDY OF MARINE
BIOGEOCHEMICAL CYCLES
Presented By
NIZAM ASHRAF
IInd Sem MSc. Marine Microbiology
KUFOS

2. A biogeochemical cycle is the circulation of an element in the Earth
system. It involves various reservoirs that store the element, fluxes
between reservoirs as well as the physical, chemical and biological
parameters that regulate the fluxes.
The oceans play a key role in the biogeochemical cycling of
elements on our planet.
As primary productivity is strictly limited to the photic zone and
decay of organic matter is pursued in the deeper water masses of the
oceanic system, the distribution of many elements exhibits a strong
vertical gradient.

3. • A biogeochemical cycle refers to the cycling and transport of a
chemical element or compound, usually in multiple forms and
physical states, through the biotic (living) and abiotic (nonliving)
components of the earth system. Some of the most commonly
examined biogeochemical cycles include carbon, nitrogen, oxygen,
iron and phosphorous

4. Nitrogen Cycle
The marine nitrogen cycle is one of the most complicated
biogeochemical cycles in the ocean.
Nitrogen is a biologically limiting element and changes in its form,
or concentration, can cause changes in the cycling of other elements, such
as carbon and phosphorus.
Marine nitrogen cycle is perhaps the most complex and therefore
the most fascinating among all biogeochemical cycles in the sea
Nitrogen exists in more chemical forms than most other elements,
with a myriad of chemical transformations
All these transformations are undertaken by marine organisms as
part of their metabolism, either to obtain nitrogen to synthesize structural
components, or to gain energy for growth

5. • Nitrogen gas (N2) from the atmosphere dissolves into seawater at the
ocean surface. Nitrogen gas is the most abundant form of nitrogen in
the ocean, but is not useful to most living things.
• Dissolved nitrogen gas is taken up by just a few types microbes, which
convert the nitrogen into a much more useable form, known as
ammonium (NH4+). This process, known as “nitrogen fixation,” is
vitally important. Without it, very little nitrogen would available for
thousands of other organisms that live near the ocean surface.
• Ammonium is the form of nitrogen that is most easily consumed by
microorganisms. For this reason, ammonium is consumed almost as
fast as it is produced, a process called “assimilation.” The result is that
the nitrogen becomes incorporated into the cells of living organisms.
• Some marine microbes consume nitrite and nitrate, another form of
assimilation.

6. • When microbes (and other organisms) die, they decompose, releasing
ammonium and tiny particles containing particulate organic nitrogen
(PON), as well as dissolved organic nitrogen (DON) into the surrounding
seawater.
• Some microbes convert ammonium to nitrite (NO2-) and then nitrite to
nitrate (NO3-). This two-step process is called “nitrification.” The result
of this process is that nitrate is released into the ocean.
• A host of organisms consume particulate organic nitrogen and dissolved
organic nitrogen, converting some of the nitrogen back to ammonium.
This process is called “remineralisation.”
• To complete this complex cycle, some microbes convert nitrate and
nitrite back to nitrogen gas through a process called “denitrification.”

9. Carbon Cycle
• Carbon compounds can be distinguished as either organic or inorganic, and
dissolved or particulate, depending on their composition.
• Organic carbon forms the backbone of key component of organic compounds such
as - proteins, lipids, carbohydrates, and nucleic acids.
• Inorganic carbon is found primarily in simple compounds such as carbon dioxide,
carbonic acid, bicarbonate, and carbonate (CO2, H2CO3, HCO3
−, CO3
2− respectively).
• Dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.

10. • There are two main types of inorganic carbon that are found in the
oceans. Dissolved inorganic carbon (DIC) is made up of bicarbonate
(HCO3
−), carbonate (CO3
2−) and carbon dioxide (including both dissolved
CO2 and carbonic acid H2CO3).
• DIC can be converted to particulate inorganic carbon (PIC) through
precipitation of CaCO3 (biologically or abiotically).
• DIC can also be converted to particulate organic carbon (POC) through
photosynthesis and chemoautotrophy (ie; primary production).
• DIC increases with depth as organic carbon particles sink and are
respired.
• Particulate inorganic carbon (PIC) is the other form of inorganic carbon
found in the ocean. Most PIC is the CaCO3 that makes up shells of various
marine organisms, but can also form in whiting events. Marine fish also
excrete calcium carbonate during osmoregulation.

11. MARINE CARBON PUMPS
• SOLUBILITY PUMP
Oceans store the largest pool of reactive carbon on the planet as DIC, which is introduced as
a result of the dissolution of atmospheric carbon dioxide into seawater - the solubility pump
• (1) CO2(aq) + H2O -> H2CO3
• Carbonic acid rapidly dissociates into free hydrogen ion (technically, hydronium) and
bicarbonate.
• (2) H2CO3 -> H+ + HCO3^-
• The free hydrogen ion meets carbonate, already present in the water from the dissolution
of CaCO3, and reacts to form more bicarbonate ion.
• (3) H+ + CO3^2- -> HCO3^-

12. CARBONATE PUMP
• Ca^2+ + 2HCO3^- <=> CaCO3 + CO2 + H2O
• Coccolithophores, a nearly ubiquitous group of phytoplankton that produce shells
of calcium carbonate, are the dominant contributors to the carbonate
pump. they provide a large mechanism for the downward transport of CaCO3

13. BIOLOGICAL PUMP
• Particulate organic carbon, created through biological production, can be exported from
the upper ocean in a flux commonly termed the biological pump, or respired back into
inorganic carbon. In the former, dissolved inorganic carbon is biologically converted
into organic matter by photosynthesis and other forms of autotrophy that then sinks and
is, in part or whole, digested by heterotrophs
• 6 CO2 + 6 H2O -> C6H12O6 + 6 O2
• carbon dioxide + water + light energy → carbohydrate + oxygen
• C6H12O6 + 6 O2 -> 6 CO2 + 6 H_2O + heat
• carbohydrate + oxygen → carbon dioxide + water + heat

15. Marine phosphorus cycle
• Phosphorus (P) is an essential element to all life, being a
structural and functional component of all organisms.
• Provides the phosphate-ester backbone of DNA and RNA,
• Crucial in the transmission of chemical energy through the
ATP molecule
• Phosphorus cycle is alos called mineral cycle
• Slowest cycle
• In some marine and estuarine environments, P availability is
considered the proximal macronutrient that limits primary
production.
• Thus, the availability of P in marine systems can strongly
influence the marine carbon cycle

16. Phosphorus sources and sinks
• Phosphorus is primarily delivered to the ocean via continental
weathering. This P is transported to the ocean primarily in the
dissolved and particulate phases.
• However, atmospheric deposition through aerosols, volcanic ash,
and mineral dust is also important
• Continental shelves and is thus not important for open ocean
processes
• The dominant sink for oceanic P is deposition and burial in marine
sediment
• A minor sink for P is uptake through seawater-oceanic crust
interactions associated with hydrothermal activity on the ocean’s
floor

17. Cycle
• Phosphorus in the ocean exists in both dissolved (DOP) and particulate
forms (POP)throughout the water column
• The dissolved fraction (which passes through the filter) includes inorganic
phosphorus (generally in the soluble orthophosphate form), organic
phosphorus compounds, and macromolecular colloidal phosphorus.
Particulate P (retained on the filter) includes living and dead plankton,
precipitates of phosphorus minerals, phosphorus adsorbed to
particulates, and amorphous phosphorus phases.
• P can be in the form of inorganic (orthophosphate, pyrophosphate,
polyphosphate, and phosphate containing minerals) or organic (P-esters,
P-diesters, phosphonates) compounds

18. • The organic and inorganic particulate and dissolved forms of
phosphorus undergo continuous transformations.
• The dissolved inorganic phosphorus (usually as orthophosphate) is
assimilated by phytoplankton and altered to organic phosphorus
compounds.
• The phytoplankton are then ingested by detritivores or zooplankton. A
large fraction of the organic phosphorus taken up by zooplankton is
excreted as dissolved inorganic and organic P.
• Phytoplankton cell lysis also releases cellular dissolved inorganic and
organic P to seawater.

19. • Continuing the cycle, the inorganic P is rapidly assimilated by
phytoplankton while some of the organic P compounds can be
hydrolyzed by enzymes synthesized by bacteria and phytoplankton
and subsequently assimilated.
• Dissolved inorganic and organic P is also adsorbed onto and desorbed
from particulate matter sinking in the water column moving between
the dissolved and the particulate fractions.
• Much of this cycling and these transformations occur in the upper
water column, although all of these processes, with the exception of
phytoplankton assimilation, also occur at depth, throughout the
water column.

21. Oxygen Cycle
• The oxygen cycle is the biogeochemical cycle that describes the
movement of oxygen within its three main reservoirs: the atmosphere
the total content of biological matter within the biosphere,
hydrosphere and the lithosphere.
• Photosynthesis derived O2 not only transformed our atmosphere but
oxidized large pools of reduced minerals such as ferrous iron and
sulfides.
• O2 deposited in ferric iron and in dissolved and sedimentary sulfates
exceeds the O2 in the atmosphere by several fold.
• These mineral reservoirs of O2, including the carbonates, participate to
some extend in the O2 cycle.
• Nitrate is a small, rapidly cycled O2 reservoir.

23. • The main driver of the cycle is the production of O2 by photosynthesis
and its use in the respiration, decomposition and combustion
(oxidation) of organic matter.
• Another cycle takes place in the upper atmosphere, where ultraviolet
radiation from the Sun constantly transforms molecules of O2 into
ozone (O3) and molecules of O3 into O2.

24. • Two oxygen cycles are presented here: the first describes the
circulation of this element between the atmosphere, the biosphere
and the lithosphere and the second shows reactions that take place in
the upper atmosphere.
• The biosphere produces, through photosynthesis, almost all of the O2
in the atmosphere by splitting the H2O molecule.
• During that process, photosynthetic organisms separate the hydrogen
and oxygen atoms contained in the water molecule, they use the
hydrogen atoms to produce organic matter and release O2.
• At least half of this O2 is produced in the ocean.

25. • In addition to photosynthesis, a small amount of O2 is produced by the
photolysis (i.e. decomposition) of water molecules (H2O) and nitrogen
oxide (N2O) in the atmosphere by the ultraviolet radiation from the
Sun.
• The oxygen cycle is coupled with both the carbon and water cycles
• Part of the fossil organic matter that accumulates in the marine
sediments is transported toward continents by tectonic plate
movements.
• Magmatic rocks form the substrate on which marine sediments
deposit (oceanic plates).

26. • In the ocean, the concentration of O2 dissolved in the surface layer is
usually high due to exchanges with the atmosphere (which occur in
both directions) and phytoplankton photosynthesis.
• This is also the case at depth due to the thermohaline circulation that
transports O2 from surface waters to ocean depths.
• Between the surface and deep waters, there is often a minimum in
oxygen concentration, at least at low and mid latitudes. This minimum
is located at depths where the respiration and decomposition of
organic matter consume O2 faster than its replenishment from the
surface.
• In some cases, the concentration of O2 at intermediate depths is very
low, in which case oceanographers use the expression oxygen
minimum zone.

27. • When the concentration of O2 is low, some anaerobic bacteria use the
oxygen contained in nitrate (NO3 - ), which transforms this inorganic
nitrogenous nutrient into a dissolved gas (N2) by a process called
denitrification.
• When the concentration of O2 is null, other bacteria, which are strictly
anaerobic, use the oxygen contained in sulfate (SO4 2 -), a process
called sulfate reduction (or respiration), which produces hydrogen
sulfide (H2S), a toxic gas with the characteristic foul odor of rotten
eggs.
• In the sea, the production of H2S primarily takes place in sediments or
in deep anoxic zones (i.e. zones without oxygen).

30. Thank You