Source: FIU Intro to Marine Biology lecture slides
Difficulty: Introductory | Prerequisites: Part 1 of these notes (water structure, hydrogen bonds, solvent properties)
This second set of notes picks up where Part 1 left off, moving from the molecular chemistry of water to the properties that define the ocean as an environment for life. Salinity, osmosis, dissolved gases, light penetration, and pressure are the five factors that determine where and how marine organisms can survive. If you skipped Part 1, go back: the concepts of polarity, hydrogen bonds, and dissolution are assumed knowledge here. This material connects directly to later topics on ocean zonation, marine adaptations, and global ocean circulation.
Seawater averages about 35 parts per thousand (ppt) of dissolved salts, and marine organisms must constantly manage water and ion balance through osmosis, diffusion, and energy-expensive active transport. The ocean also dissolves gases (especially CO₂), transmits blue light best (hence the blue colour), and exerts crushing pressure at depth, with 1 extra atmosphere for every 10 metres.
Salinity
The total amount of dissolved salt in seawater, usually expressed as grams of salt per kilogram of seawater. Average ocean salinity is about 35 ppt. Think of it as a measure of how salty the water is.
Parts per thousand (ppt) / Practical salinity unit (psu)
The standard units for expressing salinity. 35 ppt means 35 grams of salt in 1 kilogram of seawater. In simple terms, for every kilogram of ocean water, about 35 grams of it is dissolved salt.
Cell membrane (semi-permeable membrane)
A biological barrier around cells that allows some substances (water, ions, small molecules) to pass through while blocking others (proteins, ATP). Think of it as a selective gatekeeper.
Osmosis
The passive movement of water across a semi-permeable membrane, from a region of lower solute concentration to higher solute concentration. In simple terms, water moves towards the saltier side, trying to balance things out.
Diffusion
The passive movement of ions (solutes) from an area of higher concentration to an area of lower concentration across a membrane. Think of it as ions spreading out to even themselves up.
Active transport
The energy-requiring process by which membrane proteins pump substances against their concentration gradient. In simple terms, the cell spends energy to move things in the "wrong" direction, against the natural flow.
Gas exchange
The process by which gases dissolve into seawater or are released from it at the sea surface. Think of it as the ocean breathing.
Atmosphere (unit of pressure)
A unit of pressure equal to the weight of the Earth's atmosphere at sea level. In the ocean, pressure increases by 1 atmosphere for every 10 metres of depth. In simple terms, at 10 m depth you feel twice the pressure you feel on the surface.
Swim bladder
An internal gas-filled sac in fish that controls buoyancy by inflating or deflating. Think of it as a built-in float that the fish adjusts to rise or sink in the water column.
Hyperbaric chamber
A sealed room used to treat decompression sickness by re-pressurising the patient and then slowly reducing pressure back to normal. In simple terms, it simulates the pressure of being underwater so gas bubbles in the blood and tissues can be safely reabsorbed.
Salinity is the total dissolved salt content, measured in parts per thousand (ppt) or practical salinity units (psu)
Average ocean salinity: approximately 35 ppt (35 grams of salt per kilogram of seawater)
When seawater evaporates, water molecules leave but ions remain behind, combining to form various salts
Salinity varies by environment:
Rainfall decreases salinity locally
Near river mouths (estuaries), salinity drops because fresh water dilutes the seawater
Deeper water tends to have higher salinity
Understanding salinity is critical because marine organisms live in a solute-rich environment and must constantly manage their internal water and ion balance
Cell membranes are semi-permeable: they allow water, ions, and small molecules through, but block larger molecules like proteins and ATP
Osmosis (water movement, passive, no energy required):
Water moves from low-salinity areas to high-salinity areas across the membrane
If internal salinity is higher than external: water flows into the cell
If internal and external salinity are equal: water moves in and out equally (balance)
If external salinity is higher: the cell loses water and shrinks
Diffusion (ion movement, passive, no energy required):
Ions move from high concentration to low concentration across the membrane
Key distinction: osmosis moves water, diffusion moves ions
If only osmosis and diffusion operated, unchecked water loss or solute gain would eventually kill cells
Active transport (energy required):
Membrane proteins pump substances against their natural concentration gradient
Example: a cell pumps excess sodium out, even though seawater outside is already more sodium-concentrated
Example: a cell takes up sugars and amino acids from surrounding water, even though these are already more concentrated inside the cell
Active transport can consume more than one third of a cell's total energy expenditure
Niskin bottles: cylindrical sampling bottles lowered to different depths in the water column to trap water samples
Samples are brought back to the lab for salinity analysis
Thermometers on the bottles freeze the temperature reading at sampling depth
Rosette: a modern frame mounting multiple Niskin bottles on a single rack
Uses electronic instruments to measure temperature, salinity, light, and water clarity in real time
Also uses pumps to filter water and collect phytoplankton
More flexible and efficient than individual bottle deployments
Seawater dissolves gases as well as solids, including oxygen, carbon dioxide, and nitrogen
Gas exchange occurs at the sea surface, where gases dissolve into or are released from the water
Dissolved oxygen:
Primarily produced in the ocean by algae through photosynthesis, not absorbed from the atmosphere
Oxygen is not very soluble: 1 litre of seawater holds only 0 to 8 millilitres of dissolved oxygen
Excess oxygen produced by algae is released to the atmosphere
Dissolved carbon dioxide:
Much more soluble than oxygen because it reacts chemically when it dissolves
Makes up more than 80% of dissolved gas in the ocean
Temperature effect on gas solubility: colder water dissolves more gas than warm water
Polar waters hold more dissolved gas than tropical waters
Seawater is relatively transparent, allowing sunlight to penetrate and support photosynthesis by algae and coral
Not all wavelengths (colours) penetrate equally:
Clear ocean water is most transparent to blue light, which is why the ocean appears blue
Red, orange, and yellow light are absorbed more quickly at shallow depths
Minerals and algae particles can scatter light and give water a greenish or yellowish tint
With increasing depth:
Colours progressively disappear; blue is the last to go
Below approximately 100 metres, no natural sunlight reaches, divers need artificial light
Organisms living below 100 m must be adapted to permanent darkness
At the sea surface, you experience 1 atmosphere (atm) of pressure from the weight of the air above
For every 10 metres of water depth (about 33 feet), pressure increases by 1 additional atmosphere
10 m depth = 2 atm, 80 m depth = 9 atm, 100 m depth = 11 atm, 120 m depth = 13 atm
This is because 10 metres of water is equivalent in weight to about 1,000 km of air (water is far denser)
As pressure increases, gases compress; as pressure decreases, gases expand
Biological consequences of rapid pressure change:
If a diver ascends too quickly, expanding gas in the lungs can cause serious injury or death
Fish swim bladders: a gas-filled sac used to control buoyancy; if a fish is brought to the surface too quickly, the bladder can expand dramatically, pushing the stomach out through the mouth
Gas dissolved in blood and muscle tissue can form bubbles during rapid ascent, causing severe pain ("the bends" or decompression sickness)
Treatment: hyperbaric chamber re-pressurises the patient, allows bubbles to dissolve, then slowly returns to 1 atm
Pressure at depth: Total pressure (atm) = 1 + (depth in metres / 10)
Example: at 80 m depth, total pressure = 1 + 8 = 9 atm
Average ocean salinity: ~35 ppt (35 g salt per 1 kg seawater)
Light penetration limit: ~100 m for natural sunlight in clear ocean water
Dissolved oxygen range: 0 to 8 mL of O₂ per litre of seawater
CO₂ dominance: >80% of all dissolved gas in the ocean is carbon dioxide
Scuba divers must ascend slowly to avoid decompression sickness. The pressure concepts from this lecture explain why: gas dissolved in your blood and tissues at depth expands on the way up, and if it forms bubbles faster than your body can reabsorb them, the result is painful and potentially fatal.
Iron fertilisation experiments in the ocean test whether adding iron to iron-limited regions can boost phytoplankton growth and draw down atmospheric CO₂. The dissolved-gas and nutrient concepts from this lecture are the foundation for understanding why that might work.
"Osmosis and diffusion are the same thing." They are not. Osmosis is the movement of water; diffusion is the movement of ions (solutes). Both are passive, but the substance moving is different.
"Active transport is just faster diffusion." Wrong direction entirely. Active transport moves substances against their concentration gradient, which requires energy. Diffusion goes with the gradient and is free.
"The ocean is blue because it reflects the sky." Partly, but the main reason is that seawater is most transparent to blue wavelengths of light. Red and yellow light are absorbed at shallower depths.
"Pressure only matters for deep-sea creatures." Pressure changes are significant even at recreational diving depths (10 to 40 m). Rapid ascent from just 10 m can cause injury.
⚠️ Be able to distinguish osmosis (water moves) from diffusion (ions move) from active transport (energy required, against gradient)
⚠️ Know the pressure formula and be able to calculate total pressure at a given depth
⚠️ Understand why cold water holds more dissolved gas than warm water
⚠️ Explain why the ocean appears blue in terms of light absorption and wavelength
⚠️ Active transport consuming one third or more of a cell's energy budget is a commonly tested fact
⚠️ The swim bladder and decompression sickness examples are likely to appear in application questions
Fill in the blank: Average ocean salinity is approximately ___ ppt. (35 ppt)
True or False: Osmosis requires energy from the cell. (False: osmosis is passive.)
Fill in the blank: For every 10 metres of ocean depth, pressure increases by ___ atmosphere(s). (1 atmosphere)
True or False: Carbon dioxide makes up more than 80% of dissolved gas in the ocean. (True)
Fill in the blank: The ocean appears blue because seawater is most transparent to ___ light. (blue)
Q: What is salinity, how is it measured, and what is the average salinity of ocean water?
A: Salinity is the total amount of dissolved salt in seawater, expressed in parts per thousand (ppt) or practical salinity units (psu). It is measured by collecting water samples (using Niskin bottles or rosettes) and analysing them in the lab, or by electronic instruments in situ. Average ocean salinity is about 35 ppt.
Q: Compare and contrast osmosis, diffusion, and active transport in marine organisms.
A: Osmosis is the passive movement of water from low-salinity to high-salinity areas across a semi-permeable membrane. Diffusion is the passive movement of ions from high to low concentration. Neither requires energy. Active transport uses membrane proteins to pump substances against their concentration gradient and requires significant energy, sometimes more than one third of a cell's total energy budget.
Q: A diver is at 50 metres depth. What is the total pressure they experience? Show your working.
A: Total pressure = 1 atm (atmospheric) + (50 m / 10) = 1 + 5 = 6 atmospheres.
Q: Why does cold ocean water hold more dissolved gas than warm ocean water?
A: Gas molecules are less energetic in cold water and are more easily held in solution. As water warms, gas molecules gain energy and are more likely to escape from solution. This is why polar waters contain more dissolved oxygen and CO₂ than tropical waters.
Q: Explain why a fish brought rapidly to the surface from deep water may have its stomach pushed out of its mouth.
A: Fish have a swim bladder, a gas-filled sac used to control buoyancy. At depth, the gas is compressed by high pressure. If the fish is brought to the surface too quickly, the surrounding pressure drops rapidly and the gas in the swim bladder expands dramatically, inflating like a balloon and pushing the stomach out through the mouth.
Osmosis and active transport connect directly to later lectures on marine organism adaptations, including osmoregulation in fish, invertebrates, and marine mammals. The dissolved-gas content of seawater is foundational for understanding ocean productivity and the global carbon cycle. Pressure and light concepts come back in ocean zonation (photic zone, aphotic zone, mesopelagic, bathypelagic) and when studying deep-sea organisms and their adaptations to extreme environments.
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