Which alga can still catch light down here?

Lower a green, a brown and a red alga down a sea column; each depth filters the light colours, and each alga's pigment set (chl a+b / a+c+fucoxanthin / a+d+phycoerythrin) shows how much light it can still capture, with its stored food and flagella listed alongside.

Find the depth where only the red alga still captures more than half the available light.

Red algae are red because they absorb red light; brown algae carry chlorophyll b.

• Chlorophyceae (green algae) may be unicellular, colonial or filamentous. They are grass green because chlorophyll a and b dominate, held in definite chloroplasts whose shape varies: discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-like.
• Most green algae have one or more pyrenoids in their chloroplasts; these storage bodies hold protein as well as starch. Some store oil droplets. The rigid wall has cellulose inside and pectose outside.
• Green algae reproduce vegetatively by fragmentation, asexually by flagellated zoospores formed in zoosporangia, and sexually by isogamy, anisogamy or oogamy. Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.
• Phaeophyceae (brown algae) are mostly marine and range from simple branched filaments (Ectocarpus) to heavily branched kelps that can grow to 100 metres. Their pigments are chlorophyll a and c, carotenoids and xanthophylls; the amount of fucoxanthin sets the colour, from olive green to shades of brown.
• Brown algae store food as laminarin or mannitol. The cellulose wall usually has a gelatinous algin coat outside. The body is anchored by a holdfast and has a stalk (stipe) and a leaf-like photosynthetic frond.
• Brown algae make pear-shaped (pyriform) zoospores and gametes with two unequal flagella attached at the side. Sexual reproduction may be isogamous, anisogamous or oogamous; in oogamous species the gametes unite inside the oogonium. Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
• Rhodophyceae (red algae) owe their colour to r-phycoerythrin. Most are marine, more common in warmer seas, and grow both near the lit surface and at great depths where little light reaches.
• Red algae store floridean starch, which resembles amylopectin and glycogen in structure. They reproduce vegetatively by fragmentation, asexually by non-motile spores, and sexually by oogamy with non-motile gametes and elaborate changes after fertilisation. Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.
• Table 3.1 in brief. Green: chlorophyll a, b; starch; cellulose wall; 2–8 equal apical flagella; fresh, brackish and salt water. Brown: chlorophyll a, c and fucoxanthin; mannitol and laminarin; cellulose and algin; 2 unequal lateral flagella; mostly salt water, rarely fresh. Red: chlorophyll a, d and phycoerythrin; floridean starch; cellulose, pectin and polysulphate esters; no flagella; mostly salt water, some fresh.

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