Diatoms like it turbulent. They grow fast as long as silicate is available and sink rapidly when inorganic nutrients get exhausted. This is what you read in textbooks. Well, Coscinodiscus doesn’t seem to care about what it says about diatoms in the textbooks.
The waters enclosed in our mesocosms are measurably less turbulent then the surrounding waters, simply because much of the turbulent energy in the open waters is absorbed by the enclosure walls. This didn’t keep Coscinodiscus from dominating the phytoplankton biomass during the spring bloom. It probably could do so because of its (for diatoms) extraordinary size. Whereas all its fellow diatom species were grazed down by the abundant zooplankton community in our mesocosms, Coscinodiscus is too big to be swallowed (see blog entry on huge burgers in glass boxes on April 6). And the strong convection in the enclosures during the first four weeks of the experiment may have contributed to keeping them in suspension. But then the inorganic nutrients became exhausted. Soon after that the convection stopped and the water column became stratified due to warming of the upper layer. Time for Coscinodiscus to behave according to the textbooks?
Not for a moment. They continued to party while others went down the drain – into Tim’s sediment traps or as zooplankton diet. That was in April. It’s 8 weeks later now, and Coscinodiscus is still the most abundant diatom in our mesocosms. At times Tim collected zillions of them in his traps, but many still remained. Big Coscinodiscus hanging out in the water column while the microbial loop community of tiny plankton is developing, that doesn’t fit the picture of a well-behaved diatom. But how do they do it in a nutrient depleted and well-stratified environment?
Well, if you can’t build proteins because the nutrients are long gone, you can still make lipids. If you make enough of them, you start getting neutrally buoyant even when housed in a glass box. And the ones that remained in our mesocosms are full of lipids by now. They not only look like giant burgers in glass boxes, they are also greasy as burgers. And they taught us that our textbook knowledge about what diatoms can and cannot do needs some refinement.