Multicellular
Chlorophyceae. 1  Unicellular algae:-These algae are single cells.All the vital function of life are performed by the single cell (occasionally the term acellular-used). 2  The unicellular algae are all sizes and shapes. 3  They range from small spherical cells to large irregular shaped cells. 1 Unicellular algae:-These algae are single cells.All the vital function of life are performed by the single cell (occasionally the term acellular-used). 2  The unicellular algae are all sizes and shapes. Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. The particular shade depends upon the amount of fucoxanthin present in the alga. Second, all brown algae are multicellular. There are no known species that exist as single cells or as colonies of cells, and the brown algae are the only major group of seaweeds that does not include such forms. Characteristics of Algae. All algae are eukaryotic photoautotrophs that produce oxygen (except for the water molds, which are fungus-like chemoheteroptrophs). The thallus (body) of multicellular algae usually consists of a stipe, a holdfast, and blades. Algae reproduce asexually by cell division and fragmentation. Many algae reproduce sexually. Algae are classified according to their structures and pigments. Chlorophyta. Green algae have cellulose and chlorophyll a and b and store starch. Bacillariophyta. Diatoms are unicellular and have pectin and silica cell walls: some produce a neurotoxin, domoic acid, that causes a rare syndrome known as amnesic shellfish poisoning (ASP). 1 : small phylum of the kingdom protista, consisting of mostly unicellular aguatic algae. 2  Some euglenoids contain chloroplasts with the photosynthetic pigments; others are heterotrophic and can ingest or absorb their food. 3  Reproduction occurs by longitudinal cell division. 4  Most live in freshwater.  The closest relatives of the brown algae include unicellular and filamentous species, but no unicellular species of brown algae are known. However, most scientists assume that the Phaeophyceae evolved from unicellular ancestors. The particular shade depends upon the amount of fucoxanthin present in the alga. Second, all brown algae are multicellular. There are no known species that exist as single cells or as colonies of cells, and the brown algae are the only major group of seaweeds that does not include such forms. The largest forms are however only found in cool waters. Brown algae are always multicellular, never unicellular or colonial. They have a large surface area since this makes the process of absorbing nutrients from the water more efficient. The Phaeophyceae or brown algae (singular: alga), is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form. The particular shade depends upon the amount of fucoxanthin present in the alga. Second, all brown algae are multicellular. There are no known species that exist as single cells or as colonies of cells, and the brown algae are the only major group of seaweeds that does not include such forms. Green algae can be either unicellular or multicellular. They live mostly in fresh water, but some can live on land in moist soils. A few green algae are found in marine environments. These organisms often live symbiotically with aquatic and marine animals. 
http://www.peoi.org/Courses/Coursesen/bot/bot15.html https://en.wikipedia.org/wiki/Brown_algae http://classes.midlandstech.edu/carterp/Courses/bio225/chap12/lecture3.htm http://www.lenntech.com/eutrophication-water-bodies/algae.htm https://en.wikipedia.org/wiki/Brown_algae http://www.aquaticcommunity.com/algae-control/brown.php https://en.wikipedia.org/wiki/Brown_algae http://www.sparknotes.com/biology/microorganisms/protista/section1.rhtml
is brown algae unicellular or multicellular