Pilot sampling and a filtering dilemma

Let’s talk about the triangle of filtering. Just to make sure we’re all on the same page, when we talk about ‘filtering’, we are talking about pushing litres of seawater through a very fine mesh. And what’s key here is that, at least in my case, what I’m interested in is what stays on the filter. The water produced just runs back into the sea, but what doesn’t fit through the pores is what we’re here for: tiny, microscopic phytoplankton that live in seawater (about a million in a teaspoon!).

Which all sounds like fun and games, but can be a bit more complex than first meets the eye. What’s really important is that you need to balance the volume you can filter, with the time you’re filtering it in, and the pressure you’re filtering it at. Don’t filter enough? You won’t be able to extract sufficient biomass. Filter it too slowly and all your RNA will have degraded by the time you preserve your sample (RNA is incredibly volatile – it’s only designed to last in a cell for long enough to be read like a set of instructions to produce DNA). Filter at too high a pressure and you break open your phytoplankton, losing all that precious RNA along the way.

My peristaltic pump filtering set up, pump courtesy of Woods Hole Oceanographic Institution.

It’s generally accepted that, in oligotrophic (low phytoplankton biomass) regions like we’re in here, you want somewhere around 10 litres of water to get enough plankton to sequence their RNA. And, as a rule, you want to keep the time from water coming on deck to samples in the freezer under an hour. So really, the only metric you can play with is the pressure you’re filtering at. We (as a scientific community) have also come up with a number for that, which is under 200 mmHg (or 7.8 inHg, for our imperial unit-using friends), as a good point of reference. Therefore, the question is less, ‘how much should I filter, how fast and for how long’, and more ‘can I filter this much, this fast, under this pressure?’.

The fourth and final factor that we can control (at least now I’m on board with the equipment I have available), is the type of filtration rig used. Here you have two main options; essentially what this comes down to is whether you’re pulling water or pushing water. The most commonly used filtration rig in oceanography uses vacuum – pulling water through a filter using negative pressure. The other option, which I’ve used prior to now, is a peristaltic pump. This uses a peristaltic ‘head’ to push water through lines of tubing, forcing it out through a filter at the other end.

BATS vacuum filtering rig aboard the R/V Atlantic Explorer, Bermuda Institute of Ocean Science.

Pros of peristalsis: it can in theory filter more water, quicker, which gets us a tick on both volume and time. Cons are that the rig set up is fiddly – lots of individual tubes running through individual heads of the machine which can easily become twisted and dislodged (and do, regularly). Pros of vacuum: the rig, particularly the one the BATS people are using out here, is more sophisticated, has less moving parts, and, importantly, you can read the pressure you’re filtering at, which gets you a tick on the pressure front.

So, we shall see. My first CTD is on deck at 0200 tomorrow morning, so it’s time to do some soul searching and come up with an answer before then!