Foraminifera are crazy micro-messengers for the environment

Foraminifera abstract design.
This chandelier-looking thing is the benthic foraminifera Bulimina marginata. This, and other foraminifera-inspired designs are available as t-shirts in the scienceosaurus.com store!

Introduction:

Foraminifera (singular = foraminifer) are a typically-microscopic zooplankton that live in all marine environments.  They are protists, which means they are comprised of a single cell and have no organs.  However, despite these limitations in size and complexity, they manage to live amazingly complex lives.

Foraminifera are also testate organisms, which means that they live inside of a shell called a “test.”  Interestingly, the word testate also describes a human who has left or created a valid will.  This is fitting because the tests that foraminifera leave behind provide us with important environmental information.

But foraminifera aren’t just tiny, crusty doodads in the sea.  They have very specific environmental preferences and habits.  These behaviors provide scientists with fascinating and important information.  Where foraminifera live and the tests that they leave behind inform us on environmental conditions that would otherwise need to be inferred in less direct ways.

General foraminifer habits and life cycle:

There are two basic types of foraminifera distinguished by their modes of life: planktonic and benthic.  Planktonic foraminifera live in the water column and feed on suspended organic matter (particles that are floating around).  They collect their food using pseudopodia, which are protrusions of sticky ectoplasm (clear, viscus cytoplasm)—yep, ectoplasm is real and foraminifera are little slimers (Figure 1)!  This all means that they extend sticky snot-limbs into the ocean to catch their dinner.  Umm…briny snot gravy.

foraminifera with ectoplasm tendrils.
Figure 1: View of a live benthic foraminifera through a microscope. It is extending ectoplasm tendrils. Credit: Scott Fay, UC Berkley.  Licence: CC 2.5.

Benthic foraminifera live in, or on, the seafloor.  (“Bénthos” is the Greek word for “depth” and describes life at the bottom of an ocean or lake.)  Benthic foraminifera typically feed on detrital organic matter that settles on the seafloor.  They make their tests either by secreting calcium carbonate (like other seashells), or by cementing mineral grains together (agglutination).

Some benthic foraminifera are sessile (they can’t move), but many are vagile (they can move) and make complex burrows (Figure 2).  It’s kind of amazing that these little critters, who consist of a single cell, can maneuver at all.  However, even more fascinating is that they actively roam around in the seafloor to depths of over 5 cm!  They actually burrow around in the seafloor sediment to find food and the preferred chemical conditions for their specific species.  For more information on habitat preferences and distributions of Benthic foraminifera see Linke and Lutze (1993) and Thibault de Chanvalon et al. (2015).

cartoon of a foraminifer's burrow
Figure 2: A cartoon of a burrowing foraminifera. The human-sized equivalnt scale on the right depicts the depth of a foraminifer’s burrow in relation to its size. (As the foraminifera says, it is not drawn to scale.)

You might be thinking: “big woop.  What’s so special about burrowing past 5 cm?”  Well, if we consider that most benthic foraminifera are smaller than 200 μm it’s actually quite a feat.  That’s the equivalent of a human digging a hole over 500 m (half a kilometre) deep (Figure 2)!  And all with one cell!

Crazy foraminifer facts:

Foraminifera have other amazing abilities and fetishes besides their seemingly preternatural digging ability.  Some of these quirks are legitimate mind-exploders.

Some benthic foraminifera consistently arrange larger and spikier mineral grains towards the outside of their tests (Loeblich and Tappan, 1989).  How do they do this with one cell and no brain or sensory organs?  I don’t know.  Why do they do this?  Is it for comfort or protection?  It’s a mystery.

Other species of agglutinated benthic foraminifera have even stranger habits.  They will selectively collect specific types of mineral grains to construct their tests with (Allen et al., 1999).  Again, they manage this without sensory organs [mind explosion].  Some foraminifera have been shown to preferentially collect shocked micro-diamonds after meteorite impacts (Kaminski et al., 2008)!  How and why do they hunt for these geologically and geographically rare minerals?  How could it possibly benefit them?  No one knows [again: mind explosion].

Some calcareous, planktonic species of foraminifera have different morphotypes (different types of individuals within one species).  Often these morphotypes involve the test’s coiling direction.  For example, the species Neogloboquadrina pachyderma coils dextrally (clockwise or righthandedly) in warm waters and sinistrally (counter-clockwise or lefthandedly) in cold waters (Figure 3).  New research strongly suggests that this isn’t ecophenotypic (an adaptation to the environment); instead it is a genetic trait (Darling et al., 2006).  So, this foraminifer fact isn’t really neurologically explosive, but it is quite useful.

cartoon of foraminifera coiling direction
Figure 3: A cartoon of the coiling direction of Neogloboquadrina pachyderma and the relationship of this coiling to ocean temperature. Note that the coiling direction is best identified from the ventral side (the bottom) because of the aperture (opening). But the coiling direction is described from the dorsal (top) perspective.

Some general scientific applications of foraminifera:

As micro-messengers for the environmnet:

The tests that foraminifera leave behind provide extremely useful data for reconstructing the environment that they lived in.  The entire package of tests in the seafloor sediments can be interpreted as a signature for ancient climates and environments.

The study of modern or living foraminifera can provide information on the health of ecosystems.  For instance, certain foraminifera species are intolerant of changing conditions.  Thus fluctuation in their population can signal changes in the local or regional environment.

In addition to signalling modern changes, foraminiferal tests are often preserved or fossilized in the sediment record.  So, a sediment core taken from the seafloor will often contain enough foraminifera tests to compile a sort of fingerprint of the ancient environment.  The study of this extended foraminiferal record is called micropaleontology.

As mentioned above, the coiling direction of Neogloboquadrina pachyderma is thermo-correlative.  Thus, when the foraminiferal record is full of this particular species, we can be fairly certain that the ocean was cold at that location.  Indeed, we typically see Neogloboquadrina pachyderma (sinistral) in areas that have recently been deglaciated (e.g. Knudsen et al., 2004).  This is super handy when trying to figure out how big ice sheets used to be or how they behaved along their sensitive marine margins.

The tests left behind by the entire population of foraminifera species tell us details about other oceanic variables.  These variables include nutrient supply, salinity fluctuations, current velocity and water oxygenation.

As micro-messengers for the death of the dinosaurs:

The oddly obsessed foraminifera who used to scour the seafloor for ultra-rare micro-diamonds (mentioned above) even provide evidence for the global effects of a bolide impact that was very important.  This impact event was most likely the same event that formed the Chicxulub crater (Kaminski et al., 2008).  That bolide impact is the most likely cause for the demise of the (non-avian) dinosaurs (Schulte et al., 2010).  It’s really serendipitous, the impact-identifying micro-diamonds were so rare and well distributed that researchers couldn’t find them.  Without the work of some strange, greedy foraminifera to collect them millions of years ago, the scientists of today would be less certain of the global effects of the impact event.

Conclusion:

Foraminifera are strange, and sometimes eccentric, little protists.  They are everywhere in the Earth’s oceans, so chances are, you have been fairly covered by them at some point and not even known it.  Because they are ubiquitous and different species have different environmental preferences, they are quite valuable in recording our planet’s oceanic conditions.

References:

Allen, K., Roberts, S., & Murray, J.W. (1999). Marginal marine agglutinated foraminifera: affinities for mineral phases. Journal of Micropalaeontology, 18(2), 183-191.

Darling, K.F., Kucera, M., Kroon, D., & Wade, C.M. (2006). A resolution for the coiling direction paradox in. Neogloboquadrina pachyderma Paleoceanography, 21.

Kaminski, M.A., Armitage, D.A., Jones, A.P. & Coccioni, R. (2008). Shocked diamonds in agglutinated foraminifera from the Cretaceous/Paleogene Boundary, Italy-a preliminary report. Grzybowski Foundation Special Publication, 13, 57-61.

Knudsen, K.L., Eiríksson, J., Jansen, E., Jiang, H., Rytter, F., & Gudmundsdóttir, E.R. (2004). Palaeoceanographic changes off North Iceland through the last 1200 years: foraminifera, stable isotopes, diatoms and ice rafted debris. Quaternary Science Reviews, 23(20), 2231-2246.

Linke, P., & Lutze, G.F. (1993). Microhabitat preferences of benthic foraminifera—a static concept or a dynamic adaptation to optimize food acquisition?. Marine Micropaleontology, 20(3-4), 215-234.

Loeblich, A.R., & Tappan, H. (1989). Implications of wall composition and structure in agglutinated foraminifers. Journal of Paleontology, 63(06), 769-777.

Thibault De Chanvalon, A., Metzger, E., Mouret, A., Cesbron, F., Knoery, J., Rozuel, E., Launeau, P., Nardelli, M.P., Jorissen, F.J., & Geslin, E. (2015) Two-dimensional distribution of living benthic foraminifera in anoxic sediment layers of an estuarine mudflat (Loire estuary, France). Biogeosciences 12(20), 6219-6234.

Schulte, P., Alegret, L., Arenillas, I., Arz, J.A., Barton, P.J., Bown, P.R., Bralower, T.J., Christeson, G.L., Claeys, P., Cockell, C.S. and Collins, G.S. (2010). The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, 327(5970), pp.1214-1218.

Jared Peters

Jared Peters

Jared Peters, PhD, is a geoscientist who specialises in marine sedimentology, marine palaeoglaciology and climate change.
Jared Peters
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Jared Peters, PhD, is a geoscientist who specialises in marine sedimentology, marine palaeoglaciology and climate change.