Marine Biology Flashcards
(8 cards)
Biogeochemical cycling
the processes which drive the flow of elements
between soils, sediments, water, life, and the atmosphere
Redfield Ratio
the ratio of carbon to nitrogen to
phosphorus atoms in marine plankton
106 C:16 N:1 P
Marine Carbon cycle
Autotrophic microbes absorb dissolved CO2 and convert it to
organic carbon molecules
Organic carbon: “naturally produced” carbon molecules, often
with carbon chains. Includes DNA, lipids, carbohydrates, etc.
Microbes are extremely important for carbon fixation in the ocean
which produces organic carbon, in contrast to land where it is
mostly done by plants
Remember, half of all carbon fixation takes place in the ocean
Sunlight will only penetrate the top 1-200m of the
ocean: this is called the (eu)photic zone. Both
autotrophy and heterotrophy occur at these
depths
Deeper into the water there is less light, so
photosynthetic microbes need more light
harvesting complexes to survive. This creates a
peak of chlorophyll called the deep chlorophyll
maximum
Below the photic zone there isn’t enough light for
photosynthesis, so heterotrophs dominate
Photosynthetic plankton fix CO2 which is dissolved in the water
The fixed microbial carbon can then go to many places:
The plankton are eaten by another organism - heterotrophy
Released as a gas, often CO2 through respiration
Released into the water in dissolved or particulate forms
These are generally defined by size: particulates are caught by a 0.22 μm filter, dissolved
carbon isn’t
Particulate organic carbon (POC) can include dead cells, faecal pellets, flocs and
agglomerates
Dissolved organic carbon (DOC) will include proteins, carbohydrates, DNA, excreted
compounds etc.
Sinking particles are called marine snow because of their
appearance
It is colonised by microbes which break it down, and so marine
snow is an important nutrient source in the deep oceans.
This metabolism returns nutrients to the water where they will feed
other cells.
Remaining marine snow continues downwards
transport until it reaches the deep ocean, where it
forms new sediment
The process of microbes fixing C from the air and
then transporting it to the deep via marine snow
is often called the microbial carbon pump.
It removes billions of tons of carbon from the
atmosphere each year1, and so is a critical stage
in regulating the global carbon cycle.
Snow which does reach the sea floor forms new
sediment. This can remove it from biological C
cycling for billions of years
Because microbes are the “engine” of the microbial
carbon pump they are the focus of a lot of research
The Carbonate Buffer System
The microbial pump and viral shunt are essential for removing CO2
from the atmosphere
This is possible because CO2 from the air dissolves into the ocean
This forms carbonic acid (H2O + CO2 = H2CO3)
The H2CO3 can break down to bicarbonate (HCO3-) and carbonate
(CO32-) by releasing protons (H+), and then the CO32- can precipitate as
calcium carbonate mineral
This creates a non-biological route to carbon removal
The interchange of each of these forms of carbon creates a pH buffering
system, leaving the ocean mildly alkaline (pH 7.6-8.2)
Marine Nitrogen Cycle
Like carbon fixation, marine nitrogen
fixation is dominated by just a few
microbes
In particular Trichodesmium spp.
probably carry out more than half of N
fixation
Trichodesmium is a genus of
cyanobacteria common to (sub)tropical
oceans which forms filamentous
colonies
Trichodesmium spp. are also capable of
photosynthesis, making them extremely adapted
to oligotrophic environments
The nitrogenase enzymes which fix nitrogen are
permanently inhibited by oxygen gas
Normally photosynthesis would inhibit N2 fixation,
but Trichodesmium spp. have evolved some
unique ways around this
Within chains of Trichodesmium
cells some halt photosynthesis
and specialise in N fixation,
keeping the processes separate
This requires cooperation
between carbon fixing and
nitrogen fixing cells
Alternatively, cells fix nitrogen at
night when photosynthesis is
halted by a lack of sunlight
These N-fixing microbes fertilise the ocean, allow it to be as rich
and biodiverse as it is
The enzymes for nitrogen fixation require iron to function
Soluble and available iron is extremely scarce in the marine
environment
Low iron concentrations can limit nitrogen fixation, making the
entire area nitrogen and iron co-limited
Co-limitation: the limitation of growth by more than one substance
Microbes produce siderophores to get iron
Marine Phosphorus Cycle
Phosphorus is one of the few macronutrients which doesn’t have a
gaseous phase
Unlike C,H,O,N,S
All life prefers using PO42- as a source of P, because it can be used
in reactions without any further processing. However, this is quite
scarce in the oceans
Phosphorus is mostly found in ground deposits
These erode and phosphate particles are eventually carried to the sea
by rivers, but this is a very slow process
These particles dissolve in coastal waters and then begin the slow
process of diffusing to more remote ocean areas
The only source of
P to the oceans is
what erodes off
land masses and is
carried down to the
coasts, often by
rivers. There is no
gas phase of
phosphorus.
Microbes rely on scavenging
and recycling P, particularly
from organic phosphorus
“Organic P” means organic
molecules with a phosphorus
group, often fragments of dead
cells or excreted compounds
(e.g. DNA, lipids)
Microbes make a wide range of
enzymes to break down
organic P, e.g. phosphatases
They can also stockpile P and reduce P use:
Some replace part of their phospholipids
with sulfolipids. One Pelagibacter sp. can
save enough P to make 56%+ of their
genome this way1
When extra P is available many will
accumulate it as polyphosphate granules
inside the cell to store it for later
Hydrothermal vents
Hydrothermal vents are a rare exception to the idea that ecosystems are
driven by energy from the sun: the biological community derives its
energy from the chemicals released by the vent
It is microbes which harvest nutrients from the vents, and then they
become the base of the entire food web in this community
This leads to areas of the deep ocean which are surprisingly full of life
- Hydrothermal vents sustain
deep sea communities - Seawater percolates through
the Earth’s crust, where it is
heated by magma. - The heated water expands
and rises back to the seabed,
dissolving minerals from the
crust. - This brings compounds such
as reduced iron, H2S, CH4,
H2, MnS, NH4, CO2 to the
deep sea
The bacteria in and around these vents harvest chemicals
(often H2S) for energy and electrons – chemolithotrophy
The microbes can fix carbon dioxide (autotrophy) or use
methane dissolved in the water (heterotrophy) for carbon
This leads to the growth of thick, fuzzy microbial mats on
and around the hydrothermal vent
The can be grazed and consumed by other species as a
food source, and so form the base of the local food web
The red plumes capture H2S and O2 gases dissolved in
the vent water using modified haemoglobin, and pass
this and CO2 to trophosomes. These contain cells
called bacteriocytes where symbiotic bacteria are kept
The bacteria oxidise the H2S with the O2 for energy and
electrons, and also fix the CO2: chemolithoautotrophy
Some of these nutrients are passed to the host.
However, the bacteriocytes will eventually be digested
by the host to harvest more nutrients and prevent the
bacteria from growing out of control
Cold seeps
These “chemosynthetic” communities also exist beyond
hydrothermal vents
Underwater hydrocarbon deposits, particularly natural gas,
are rich in methane and H2S
Sometimes these begin to seep out of the crust and into
the ocean, where microbial communities again form the
basis of ecosystems
In these communities chemicals like H2S are again the
energy and electron sources, but the methane is the
carbon source (chemolithoheterotrophs)
Beggiatoa spp. are common in these mats
