I just called BSD and talked to Tim about wether to keep the cryopaste in the fridge or freezer He said that if you're going to use it up within a month, keep it in the fridge. If it's going to take longer to use it up, keep it in the freezer. He said that keeping in the fridge will allow the cells to disperse better.

OK, as I promised I will reply to Kory's message in a little more detail and explain why the advice offered by BSD is, IMO, poor. I am cc'ing this post to them, and will let you know what I hear back as well.

As I said originally, I would store the algae paste in the freezer no matter what BSD recommends. Algae paste is made by growing the phytoplankton, concentrating it (there are a variety of ways to do this, but I have not asked the Reeds which one they use) and then using a cryopreservative (the Reeds use food-grade propylene glycol) to prevent cellular damage (and keep the product from turning into a green ice cube in the freezer). Typically, cryopreservation is done by either flash-freezing (dropping the container into liquid nitrogen -- around -196C) or by slowly bringing the temperature down (1C/min) prior to storage at -20C for short-term (months) or -80 to -196C for long term (years to indefinite). The reason that the samples require freezing, is that protein denaturation continue to occur at temperatures above -80C even though they are slowed dramatically and some enzymatic processes can occur even at this temperature. Hence, for really long-term storage of important tissue samples, storage in liquid nitrogen (-196C) is considered the only option.

Now of course, we're not too worried about that sort of storage when discussing cryopreservation of algal pastes, but if you're paying money for the stuff, you would probably like it to have as much of the original nutritional value as possible when feeding it to your aquarium. Experiments on the concentration of cells from algal cultures showed that certain species (such as Thalassiosira pseudonana and Chaetoceros calcitrans) survived the concentration very well and the cells were easily resuspended and found to be intact if proper care was taken in the method of concentration. Other species, however (such as Pavlova lutheri and Isochrysis galbana) were prone to severe cellular damage during concentration, and the resulting paste of concentrated cells could declined in nutritional value very quickly. The rate of nutritional decline is dependant on the bacth and concentration method used, and since I do not know which technique the Reeds are using, I cannot guess at how their pastes would perform exactly. However, when researchers attempted to store concentrated paste at 4C in the experiment to which I am referring, in the best case, nutritional value of the paste was comparable to uncentrifuged cells after 12 weeks in storage, but in most all cases it had dropped significantly (animals did much more poorly when fed the paste) within 2 or 3 weeks!

Furthermore, research on the cryopreservation of algae has shown that different species handle the freezing process with differing levels of success. For example, in a test of the effectiveness of different cryoprservation techniques, researchers at the Virginia Institute of Marine Science found that for the most effective method tested, 78% (285 of 365 algal strains tested) of cultures had some cells survive the cryopreservation intact and if thawed immediately, cultures could be restarted from those frozen samples. The remaining 22% all died during the freezing process, either due to cell disruption or stress. Cells that simply died did not lose much in terms of nutritional value in the process, but cellular disruption leads to almost immediate declines in nutritional value, and free cellular contents leads to a cascade of further degradation of the algal pastes and is likely to be beneficial primarily to only marine bacteria at that point. Depending on the species, the concentration process and the freezing process used, there is a great deal of variability in the amount of nutritional value that is preserved in the algae by the freezing process.

I would add that in laboratory trials we have found that most of the Inland pastes perform as well as live cultures when we first recieve them, but tests of pastes stored for 1 year at -20C (the coldest setting on a regular upright refrigerator/freezer) we are finding that the nutrtional quality of the paste has declined significantly and the animals kept on fresh paste or live cultures thrive while those we tried to raise on the old pastes quickly died.

Given that the rate of nutritional decline is curvilinear (the decrease in nutritional value happens much more slowly the colder it gets), and we know from other research that several of the pastes produced by Inland are among the most nutritious, but also the most difficult to concentrate and fastest to decline in nutritional value, I would suggest that you not consider storing your algal pastes at any temperature above freezing. As I have suggested in the past, I personally keep mine in the back of the freezer and only remove them for long enough to collect the amount that I am about to feed, and replace the paste in the freezer as quickly as possible (i.e., stand in front of the freezer to do this, and don't leave the paste sitting around on the counter at all). The bottom line is that regardless of the species or the method of preservation, the colder they are, the longer they will retain something very close to their original nutritional value, and unless you go through as much of the paste as I do, you're unlikely to use your bottles up within the 2-3 weeks that was the most common period to record a significant decline in nutritional value among experimental pastes stored in a refrigerator at 4C...