You do not say at what pH you are working and this is vital information. Inorganics are often positively charged at neutral pH (for example sulfides in froth flotation) whereas natural/organic materials are often negatively charged at neutral pH. These are very broad generalizations and again, 'often' there's a point of zero charge or the isoelectric point depending on pH and concentration of polyvalent ions etc.

sorry for the slow reply and thanks for your answer.

i am incubating the magnetic cysts in seawater at pH ~8.1.

after hatching they still be attracted by a magnet. now if they are attracted by a magnet means they are negatively charged? or the other way around?

Thanks

If they are attracted by a magnet, it means that they're magnetic - nothing more.  Long range magnetic forces always overcome short range forces including charge stabilized systems.  Note that van der Waals forces are always attractive.

so if i add a cationic molecules in solution, they will be attracted or rejected by the cysts?

Thanks

In all likelihood they are negatively charged - natural materials in slightly alkaline pH, but you need to measure the zeta potential to be sure.  Be aware that magnetism and zeta potential are totally different things - the long range magnetic forces will overcome any charge repulsion effects remembering that van der Waals forces are always attractive .

Hi Alan,

that is so helpful thank a lot.

do you have any reference to suggest in this regards?

thanks again

Danilo