Would marine algae be able to live below 100 meters with only chlorophyll a?

Probably yes.

Explanation:

Lets check some facts:

1) Chlorophyll a absorbs 2 wavelengths best: 430nm (blue) and 662nm (red) (see first figure)

2) Sea water absorbs some wavelengths of color spectrum as it gets deeper. A diver can experience this in first hand with the absence of colors. Red and violet is the first two that is cannot be seen anymore (see second figure) . But blue light can reach up to 200 meters but there is a catch: with higher turbidity this can be reduced. So; since chlorophyll a can absorb blue light marine algae can survive with only chlorophyll a up to 200 meters in oceans (see third figure).

did you fail the test? xd

1: most common garden plants prefer loam — soils with a balance of different-sized mineral particles (approximately 40% sand, 40% silt, and 20% clay) with ample organic matter and pore space. however, some plants grow better in sandy soils, while others are well-adapted to clay soils.

2: available water capacity is the maximum amount of plant available water a soil can provide. it is an indicator of a soil's ability to retain water and make it sufficiently available for plant use. available water capacity is the water held in soil between its field capacity and permanent wilting point.

3: it is the same for all living things (plants and animals). it also includes very tiny organisms that our eyes cannot see. they are known as microorganisms, and include viruses, bacteria and fungi.