Fragilariopsis kerguelensis – a key species for the global silicate cycle
Fig. 2. The micrograph taken using a scanning electron microscope impressively shows the solid silicate frustule, consisting of a lower half and a larger matching cover-like half. The lower frustule section can be seen from the outside, the upper section from the inside. The thick supports inside are clearly visible. (Photo: F. Hinz)
The unicellular algae of the phytoplankton form the basis for food chains in the ocean. An especially important group is that of the diatoms or diatomaceae, which differ from all other algae in that they are encased in a siliceous cell wall. This so-called frustule is built up through the intake of silicic acid (silicate) dissolved in the water. Diatoms dominate phytoplankton communities in oceanic regions in which sufficient silicic acid is available. Their population development leads to expansive bloom in spring and summer. The different species involved in this phenomenon differ greatly from each other in terms of their characteristics (size and shape: smooth or spiny, thin-walled or thick-walled, etc.) as well as their impacts on the ecosystem.
The diatom species presented here, Fragilariopsis kerguelensis from the Southern Ocean, has an unusually large need for silicic acid and thus suppresses other diatom species. Its local dominance has consequences for the capability of the ecosystem to bind CO2. The silicate frustule of diatoms protects them against many an algae eater in the water, which is why phytoplankton bloom in the ocean is generally dominated by this group of algae. As a result, a large number of diatoms sink to the seabed after an algal bloom and thus large quantities of organically bonded carbon dioxide are embedded in the sediment as “diatom ooze”.
Diatoms thus make a decisive contribution to the function of the oceans as CO2 sinks. This is impressively demonstrated by the results of the German-Indian iron fertilisation experiment LOHAFEX, which was conducted at the beginning of last year. Other algae groups that do not have effective protection are kept in check by zooplankton, which is why there is no significant carbon sedimentation. However, large areas of the ocean are rich in plant nutrients like nitrogen and phosphorus, but low in silicic acid. One of these regions is the northern belt of the Antarctic circumpolar current, where the LOHAFEX experiment was conducted on an area measuring 300 km2 [1]. The reason why this area is low in silicic acid is because of the substantial needs of the species Fragilariopsis kerguelensis, which builds extraordinarily rugged, thick-walled frustules (Fig. 1).
Fig. 1. Plankton sample with three different Fragilariopsis species from the Southern Ocean. F. kerguelensis displays the longest cell chain and differs perceptibly from the other species by virtue of its rugged silicate frustules. Under favourable conditions over 100 cells can be linked together [2]. (Photo: U. Freier)
Measurements by scientists of the Alfred Wegener Institute have shown that both frustule architecture and cell wall thickness (Fig. 2) are the reasons why the cells of this species are exceptionally resistant and it therefore occurs frequently [2]. The ocean floor below the circumpolar current – the largest reservoir of biogenic silicate – is thus primarily composed of the frustules of F. kerguelensis. As a consequence, this silica alga removes a considerable portion of the silicic acid in the ocean from the cycle, which is mainly fed by the influx from rivers. It plays a genuinely global role in this context. Around 70% of the marine biogenic silicate sediments in the Southern Ocean [3], and 80% of the diatom ooze there is accounted for by F. kerguelensis (see [4])! As a thought experiment, one could draw the following conclusion: If Fragilariopsis kerguelensis did not exist, there would be correspondingly more silicic acid available for other, thin-walled diatom species. This means there would also be a greater variety of opportunities for utilising the conditions that fluctuate during the year for photosynthesis and thus for binding CO2. Consequently F. kerguelensis not only represents a key species for the global silicate cycle, but also indirectly controls the global carbon cycle via the storage of silicic acid.
[1] M. G. Mazzocchi et al., Globec International Newsletter October 2009, 3-6.
[2] V. Smetacek, P. Assmy, J. Henjes, Antarctic Science 16, 541 (2004).
[3] P. Tréguer et al., Science 268, 375 (1995).
[4] P. Assmy et al., J. Phycol. 42, 1002 (2006).
Dr. Christine Klaas, Dr. Philipp Assmy, Alfred-Wegener-Institut, Bremerhaven