Use of a pair of amplification primers for the detection and/or identification of an alga in a sample likely to contain it, by amplification of at least one variable region of the LSU gene coding for the large subunit of a ribosome, wherein said amplified variable region of the LSU gene of an alga has a size less than or equal to 220 base pairs, said variable region constituting a marker for the detection and/or identification of an alga, wherein: - a first pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 1 sequence of the LSU gene of an alga and a second oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 2 sequence of the LSU gene of an alga, or - a second pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the sequence of SEQ ID NO:5 of the LSU gene of an alga and the second oligonucleotide hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the SEQ ID NO:6 sequence of the LSU gene of an alga. wherein said amplified variable region of the LSU gene of an alga has a size less than or equal to 220 base pairs, said variable region constituting a marker for the detection and/or identification of an alga, wherein: - a first pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 1 sequence of the LSU gene of an alga and a second oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 2 sequence of the LSU gene of an alga, or - a second pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the sequence of SEQ ID NO:5 of the LSU gene of an alga and the second oligonucleotide hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the SEQ ID NO:6 sequence of the LSU gene of an alga. - a first pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 1 sequence of the LSU gene of an alga and a second oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides, chosen in the SEQ ID NO: 2 sequence of the LSU gene of an alga, or - a second pair of amplification primers consists of a first oligonucleotide that hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the sequence of SEQ ID NO:5 of the LSU gene of an alga and the second oligonucleotide hybridizes to a sequence from 15 to 30 contiguous nucleotides, preferably from 18 to 24 contiguous nucleotides chosen in the SEQ ID NO:6 sequence of the LSU gene of an alga.