Plant Kingdom - Algae

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic organisms (both fresh water and marine).

Vegetative reproduction occurs by fragmentation, where each fragment develops into a thallus.

Asexual reproduction is primarily by the production of different types of spores, the most common being the $Zoospores$ which are flagellated (motile).

Sexual reproduction takes place through the fusion of two gametes: $Isogamous$ (similar size), $Anisogamous$ (dissimilar size), or $Oogamous$ (large non-motile female and small motile male).

Chlorophyceae (Green Algae): Contain chlorophyll $a$ and $b$. Stored food is starch in $Pyrenoids$. Cell wall is made of an inner layer of cellulose and an outer layer of pectose.

Phaeophyceae (Brown Algae): Contain chlorophyll $a$, $c$, carotenoids, and xanthophylls ($Fucoxanthin$). Stored food is complex carbohydrates like $Laminarin$ or $Mannitol$.

Rhodophyceae (Red Algae): Contain chlorophyll $a$, $d$, and the red pigment $r-phycoerythrin$. Stored food is $Floridean$ $starch$, which is structurally similar to amylopectin and glycogen.

Economic Importance: At least half of the total $CO_2$ fixation on earth is carried out by algae through photosynthesis. Many species like $Porphyra$, $Laminaria$, and $Sargassum$ are used as food.