How do diatoms die …

by Karine LEBLANC (Mediterranean Institute of Oceanography, CNRS/Aix-Marseille University)

During the KEOPS I and II programs, we studied the growth conditions of the phytoplankton bloom in spring and summer off the Kerguelen plateau. In particular, we studied the effects of the natural iron fertilization from the island on diatoms, a siliceous cell wall-bearing unicellular algal group, which is an important marine primary producer and the MIO’s « Team silica »’s favourite study organisms. Southern Ocean waters, naturally enriched in dissolved silicon (Si), allow this group to build up important biomasses and a large diversity of species. The scheduling of the MOBYDICK cruise at the very end of summer, is allowing us to witness the demise of the diatom bloom, an event that is much less described than the spring bloom, but that nonetheless plays a crucial role in the carbon (C) biological pump, since it is at that period that the modes of mortality of organisms are going to determine the amount of C exported to depth.

Example of diatoms and radiolarians observed during MOBYDICK.
The rosette, the ‘Bottle Net’, and the phytoplankton net.

To study the mortality mechanisms impacting diatoms, we deployed at each site several collecting devices. First a phytoplankton net, which collects plankton over the first 100 m of the surface layer. We then used a CTD equipped with Niskin bottles to collect water at selected depths intervals to sample for diversity, stocks and processes (activity, productivity, grazing). Last, we use a new device, the bottlenet, which is a « mini-net » fixed inside a Niskin bottle, to collect particles sedimenting in deep waters, which will allow us to characterize the nature of the vertical Si and C flux.

First observations: Promising!

The first on-board observations using microscopes are already very exciting! We are indeed witnessing the demise of the diatom bloom, as we hoped we would, and we have already identified several life stages and mortality mechanisms which are distinct for different species. For instance, we are seeing some winter forms of diatoms, that are beginning to appear in certain species such as Eucampia antarctica, Proboscia alata or Chaetoceros atlanticus. The frustule (cell wall) morphology changes, and several stages are co-exiting in the surface layer. Next, we have measured a clear drop in dissolved Si in surface waters, which likely induced the production of resting spores in other species such as Odontella weissflogii, Thalassiosira sp. and Chaetoceros sp. These resting stages produce considerably transformed frustule morphologies, with heavily silicified, rounded and robust cells, packed with lipid storage products.

Above: Resting spores of ‘Otontella weissflogii (a), Thalassiosira sp. (b), Chaetoceros sp. (c). Left: Winter forms of Chaetoceros atlanticus (a-c), Eucampia antarctica (d), Proboscia alata (d).

Parasitized diatoms …

Besides deploying survival strategies to counteract the increasing nutrient and light limitation, diatoms are of course eaten by a wide variety of organisms: parasites, bacteria, micro- and macro-zooplanktonic grazers. Our microscopic observations revealed that certain species are preferential preys compared to other species. For instance, the long chains of Corethron inerme are starting to be infected by parasites, which will likely result in a swift death. Our colleagues from LOG Wimereux have succeeded in isolating these cells and plan on sequencing the host-parasites DNA, which will give us the culprit organism’s name.

Chains of Corethron inerme, cell infected by parasites (small black dots).

To eat and to be eaten …

Other Southern Ocean key species such as Fragilariopsis kerguelensis encounter a different fate These heavily silicified cells, thought to be more resistant to grazing, are under heavy attack by different grazers. On the one hand, numerous broken or ‘crunched’ frustules evidence the action of zooplanktonic crustacean’s jaws such as copepods, on the other hand, whole cells have been consistently ingested by small but abundant radiolarians such as Protocystis tridens, Protocystis bicornuta and Protocystis harstoni. We have not been able to catch them in action, and hence we yet ignore whether these radiolarians feed on F. kerguelensis directly by catching them with their rhizopoda or if they feed on fecal pellets and organic aggregates containing remains of F. kerguelensis. In any case, their fate is no more enviable, as they will in turn be eaten by larger predators, as evidenced by their crunched siliceous skeleton remains

« To eat and to be eaten », illustrated. a-b. Radiolarians (Protocystis) ingesting whole cells of diatoms (Fragilariopsis kerguelensis, white arrows), c-d. eaten skeleton remains of Protocystis.
Intact empty diatom frustules (a-c) or eaten by zooplankton (b-d) of Thalassiosira lentiginosa (left) et Fragilariopsis kerguelensis (right).

Finally, our first samples collected with the bottlenet in deep waters revealed that the vast majority of the particulate flux is composed of broken diatom debris or whole cells, a minority of which are still alive, whereas fecal pellets and organic aggregates are relatively less abundant. Within fecal pellets, some, completely brown and opaque, still seem to contain a lot of undigested organic material which will enhance the C flux, while others, completely transparent, are exclusively composed of pulverized diatom remains rather enhancing the Si flux.

Different fecal pellet types. a. richer in organic matter, b. richer in biogenic silica. White arrows indicate the presence of Fragilariopsis kerguelensis in both cases.

These preliminary observations confirm our initial hypothesis, which is that the C biological pump is modulated by the planktonic community structure, and that all diatoms do not play an equal part in this mechanism. Rendez-vous in 2 years approximately for a full synthesis of our results under scientific publications form !

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