This is a broad question and the answer really depends on what you want to do with the marine green algae and what facilities you have. Let us say you want a seaweed. The simple answer is to collect from the field and put in containers of seawater (real or synthetic) and have lights on it and air bubbling through, and this will work for a while. Some marine labs have tanks with seawater being continuously pumped through which might work. Algae from the field will be contaminated with other organisms including other algae, diatoms, fungi, bacteria and viruses of course. For scientific work you might want the cultures to be free of other organisms. You might be able to get a gift of a clean green alga from another lab or buy from a culture collection. If you want to start a new culture that is free of other organisms, you will have a lot of work to do. Start by washing the alga with tap water. Put in to glass containers with air bubbling through and with lights and use liquid medium that has been sterilized. f/2 is a seawater-based medium to which some nutrients are added and that can be autoclaved (search for Provasoli media) or use ASP-12 (fully defined medium) that can be filtered-sterilized. The medium can contain GeO2 (10 mg/L) to eliminate diatoms. Bacteria and fungi are more of a problem, and you will need the help of a microbiology lab. We have used a test to determine which antibiotics can be used for whatever microorganisms you have. If you talk to a microbiologist, they have a kit used to test sensitivity of bacteria to many antibiotics. Small pieces of your alga are spread on agar and different paper disks containing different antibiotics are placed on the agar plate. Look for a clear zone around a paper disk where the bacteria are killed but your green alga is not affected. This tells you which antibiotic to add to your culture medium. Our technique was published previously as “One step antibiotic disk method for obtaining axenic cultures of multicellular marine algae.” Bradley et al. 1988. Plant cell, tissue and organ culture 12: 55-60. Available on Peter Bradley’s ResearchGate page. You will end up with bottles containing pieces of algae in sterile medium with air bubbling through and with lights on the flasks. Inside a temperature-controlled incubator is best. Good luck with your work.

Complementing what Peter M Bradley mentioned, you can start your cultures as "crude" ones with a source of N and P. Using an enriched media (like f/2) with field-collected samples (even if you have washed them well) might cause an overgrowth of bacteria, fungi or other algal epiphytes. In our laboratory, we maintain "crude" Ulva cultures in flasks using only ammonium and phosphate.

Best

Dear Mst Shamim Ara Supty ,

You can follow the protocol in chapter 9 of the book "Protocols for Macroalgal Research," where the authors describe a method for getting axenic Ulva cultures from gametes based on their positive phototaxis.

Best regards

Jose