A Thousand-year-old, Exoskeleton-building Arctic Alga

The exoskeleton of a thousand-year-old Arctic alga has allowed scientists to reconstruct historical data on Arctic temperatures and sea-ice cover.  Not surprisingly, according to the  abstract of a new paper from University of Toronto professor Jochen Halfar, “algae show that … the 20th century exhibited the lowest sea-ice cover in the past 646 years.”

Perhaps more suprising than the recent drastic loss of Arctic sea ice is the fact that there exists a thousand-year-old, exoskeleton-building, Arctic-dwelling alga in the first place.  It’s called Clathromorphum compactum.  And it’s pink.  Let’s take a closer look and see how C. compactum has allowed scientists to study ancient sea ice.

A master’s thesis from a former student of Prof. Halfar’s [pdf] provides some details about Clathromorphum compactum and its exoskeleton-building ability:

C. compactum has been found in marine habitats of the Northern Hemisphere, including the North Atlantic, North Pacific, and the Arctic oceans. This long-lived crustose coralline alga builds its skeleton by depositing annual layers of high-magnesium calcite.

Morphologically C. compactum displays cell differentiation with small and heavily calcified cells formed during the cold months, separated by large and poorly calcified cells built during the warm periods. This abrupt change is marked by a growth line.

In other words, take a cross-section of one of these skeletons and you’ll see rings just like the ones that provide evidence of a tree’s age and health record.

The same master’s thesis explains a little bit about how C. compactum and its relatives build their skeletons:

Crustose coralline algae build their calcified skeletons by precipitating calcite crystals directly in the vegetative cell-wall.  While the external factors influencing crustose coralline algae calcification such as temperature, salinity, pH, light, and food availability have been investigated, the physiological processes are less understood.

The effects of temperature and light on algal skeleton-building are particularly significant to anyone who wants to extract information from the skeletons’ growth lines.

As to temperature, the warmer the weather, the more magnesium the algae’s skeletons contain.  This means that the amount of magnesium in a particular ring of a skeleton’s cross-section can show us how warm or cold the water was when the skeleton was built.

And as to light, sea ice blocks light and inhibits the alga from growing at all.

The takeaway is this: when there’s warm water and not much sea ice, C. compactum builds thick, magnesium rich layers of exoskeleton.  And according to Prof. Halfar’s abstract, that’s what’s been happening for the last hundred-plus years:

The 646-y multisite record from the Canadian Arctic indicates that during the Little Ice Age, sea ice was extensive but highly variable on subdecadal time scales and coincided with an expansion of ice-dependent Thule/Labrador Inuit sea mammal hunters in the region. The past 150 y instead have been characterized by sea ice exhibiting multidecadal variability with a long-term decline distinctly steeper than at any time since the 14th century.

So please take a moment to thank C. compactum and its peculiarities for serving science and confirming that we’re busily destroying our planet.

P.S. Apparently, while the alga Heterosigma akashiwo is not a plant, the alga Clathromorphum compactum is a plant — a plant that builds exoskeletons.  Weird.

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