CO2
1ppm global CO2 is how many tonnes of carbon?
The mass of the atmosphere is 5.15e18 kg. We will assume mole fraction is proportional to volume fraction
Gas 
Mole Fraction 
At wt 
Mass*e12 kg 
N₂ Nitrogen 
0.7808 
28 
4054226 
O₂ Oxygen 
0.2095 
32 
1087808 
Ar Argon 
0.0093 
40 
48289 
CO₂ Carbon Dioxide 
0.0004 
48 
2077 
Dry air atomic wt 
1.0000 
28.9576 
5129400 



0.9960 
28.9576 
5129400 
H₂O Water 
0.0040 
18 
20600 
Wet air atomic wt 
1.0000 
28.9138 
5150000 
400ppm CO₂ is 2077e12 kg, of which 12/44 is carbon, or 566.45e12 kg. Hence 1ppm CO₂ is 1.42e12 kg of carbon atoms.
Wild Speculation Follows
Toray T1100G carbon fiber has a density of 1.79 (1790 kg/m³), a tensile modulus of 324 GPa, and a tensile strength of 7 GPa.
If 100ppm of excess CO₂ was magically converted into oxygen and 1.42e14 kg of Toray T1100 carbon fiber, that would 80 cubic kilometers of carbon fiber.
For comparison, high strength steel has a tensile strength of 600 MPa and a density of 8000 kg/m³. So, T1100G is 11.6 times stronger and 0.224 denser, hence a kilogram of T1100 might replace 50 times its weight of high strength steel. Annual global steel production is around 2 billion tonnes per year (2e12 kg), so 1.4e14 kg could supply 3500 years of structural needs at current global demand.
Even Wilder Speculation Follows
How much energy would it take to extract 1.42e14 kg of carbon from 5.2e14 kg of CO₂? CO₂ enthalpy of formation is 393.5 kJ/mol, and a mole of CO₂ is 0.044 kg, so at 100% efficiency that would be 8.94 MJ/kg, or 4.65e21 Joules to split 100ppm of CO₂ at 100% efficiency. If the process was 3% energy efficient, starting with sunlight, that is 1.55e23 Joules.
The Earth is 70% ocean, and the average surface insolation is around 500 W/m². The earth's disk averages 6371 km radius, about 1.27e14 square meters total, perhaps 8e13 of that being ocean, capturing perhaps 4e16 watts of sunlight. If 1% of that ocean surface (which is mostly lifeless now) was covered with 3% energyefficient "carbon fiber plankton", that would be 4e14 joules/second, 1.26e22 joules per year  converting all the atmospheric CO₂ excess into structural fiber in 12 years.
A long string of speculations with a multiplied probability approaching zero, but still ...
As the old joke goes, "If I had all the money I have spent on drink ... I would spend it ... on drink!". Chances are, if we could design magic plankton that could make carbon fiber from excess CO₂, we would instead design plankton to make motor fuel, and give everyone on the planet 800 horsepower muscle cars. And make even more steel. Humans are perverse.