Foraminifera feeding on diatoms
by Heather AUSTIN (2006)
Benthic foraminifera are known to feed on (Murray, 1963; Lee et al., 1966; Lee, 1980; Anderson et al.,1991; Moodley et al., 2000; Ward et al., 2003) and/or sequester diatoms (Lopez, 1979; Cedhagen, 1991; Bernhard and Bowser, 1999; Correia and Lee, 2000, 2002) and their chloroplasts. Intracellular ingestion, also known as phagocytosis, is recognised among some foraminifera species e.g. Crithionina delacai (Gooday et al., 1995).
Foraminifera use their pseudopodia to collect diatoms often creating large feeding bundles which are initially concentrated around the aperture (Figure 1 and 2). However, these feeding bundles can encompass the whole test. When the foraminifera have consumed all they require you can often find empty feeding bundles are often observed in the general shape of the foraminifera.
The mechanism by which foraminifera feed on diatoms is not well understood. Previous studies have suggested a possible function for the surface “tooth-like” tubercles in modern benthic foraminifera (Arnold, 1964; Banner and Culver, 1978; Alexander and Banner, 1984; Bernhard and Bowser, 1999). Studies of H. germanica (Alexander and Banner, 1984; Banner and Culver, 1978) suggest that as food particles are transported over the surface of the foraminifera, they are channeled to areas of high tubercle density. In H. germanica this correlates with the major aperture and the latero-umbilical supplementary apertures (Alexander and Banner, 1984) (Figure 3). As the particles are moved past the tubercles, they are sorted by size and larger fragments become disaggregated in preparation for later ingestion (phagocytosis) at the apertures (Banner and Culver, 1978; Bernhard and Bowser, 1999). Alexander and Banner (1984) further suggested that the forces created during this active transportation over the tubercles were of sufficient magnitude to break open diatom frustules releasing the diatom contents, including the chloroplasts, for ingestion.
The study carried out by Austin et al (2005) provides the first direct observational link between the fracturing of diatom frustules and active feeding/sequestration mechanisms in benthic foraminifera. Chemical fixation and subsequent scanning electron microscopy (SEM) suggest the active transportation of diatoms and use of the tubercles in H. germanica to crack/fracture diatom frustules in a characteristic manner which may allow recognition of benthic foraminiferal feeding/sequestration activity.
The forces required to induce such a cracking effect are likely to be large. For example, a study carried out by Hamm et al. (2003), where the pennate planktonic diatom Fragilariopsis kerguelensis was studied, demonstrated cracking when an experimental force of 750 AN was applied. The processes controlling silicification in both planktonic and benthic diatoms are known to vary according to a number of factors (see Raven and Waite, 2004 for a review) and it can be speculated that similar or even greater forces are required to crack the large, benthic species P. angulatum. Interestingly, Hamm et al. (2003) speculated because of the very large forces required to break diatom frustules, that grazers are likely to have evolved specialised tools to break open diatoms.