The document discusses several important biogeochemical cycles in marine systems, including the nitrogen, carbon, phosphorus, and oxygen cycles. It provides details on the various chemical forms each element takes in the ocean, the processes of transformation between different forms, and the key organisms and environmental factors involved in regulating the fluxes between different reservoirs within each cycle. The marine nitrogen cycle is highlighted as being particularly complex, with nitrogen undergoing numerous chemical transformations facilitated by marine organisms as part of their metabolic processes.

1. RECENTAPPROACHES INTHE
STUDY OF MARINE
BIOGEOCHEMICALCYCLES

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. NitrogenCycle
 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. CarbonCycle
• 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)
ismade up of bicarbonate
3 3
(HCO −), carbonate (CO 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).
• DICcan also be convertedto particulate organiccarbon(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 CARBONPUMPS
• 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
• Carbonicacid 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 phosphoruscycle
• 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,
• Crucialin 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

16. Phosphorus sources andsinks
• 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).

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