You can do the two experiment set one maintaining the pH with buffers and another maintaining the salinity without the buffer.

Hi Fionnuala,

Usually I do my serial dilutions with sterile artificial sea water, this allows the salinity and the pH to remain stable as well as the trace elements and other minerals needed for the cells.

Hope this helps

I am assuming you are adding salt to distilled water to create ocean salinity.  Use sodium bicarbonate in your salt mix.  Adjust sodium chloride addition accordingly with respect to sodium.  Are you matching salinity only or are you trying match the ionic composition of  seawater also?

Hi. Thank you all for your suggestions! The only reason as to why I worry if diluting with sea water is if the dead cells within the sample alter the bacterial colonies formed in the plating process eg giving additional energy sources not normally present in such high quantities in the sea.

William A. Wurts your assumption is correct. I had not given thought to that earlier, I could attempt both the seawater and salt/sodium bicarbonate as a comparison. Thank you all very much for your suggestions.

Hi Fionnuala.

I don't think you need to worry about the dead cells altering your colony formation. Maybe I did not get what you want to do exactly but if you just want to plate your marine sample there should not be any problem. Considering the fact that your are not concentrating your sample, the dead cells already present in your sample will be diluted in the same time than your living cells so you won't have that much dead cells to influence the colony formation at the end. Usually marine bacteria are quite tricky to grow (except what we call the "weeds" which will grow very easily on any basic marine medium). It is the medium you use that will influence your colony formation, specifically if it is too rich or limited in one or more inorganic nutrient (e.g phosphate, iron etc). Also the fact that you are plating your sample will already select specific bacteria.