Small Alga – Great Effect

In a new study, scientists from the ZMT have noted significant changes in
the calcareous skeleton of the alga Halimeda as a result of a more acidic
water environment. These may have an impact on the formation of tropical
beaches and islands, as the calcareous structures of Halimeda are an
important component of their sediments.

The acidification of the oceans is increasing inexorably. In particular,
marine organisms with calcareous skeletons such as sea shells, corals or
calcareous algae can suffer. In a new study, scientists from the Leibniz
Center for Tropical Marine Ecology (ZMT) in Bremen have noted significant
changes in the calcareous skeleton of the green alga Halimeda as a result
of a more acidic water environment. These changes are an indication of the
processes that occur at a lower pH level – that is, in more acidic water –
in the skeleton of the calcareous alga. Since sandy beaches in many
tropical regions largely consist of skeletal fragments of these algae,
these changes may have an impact on the composition and formation of
tropical beaches and islands.

In the seawater facility of the ZMT, the researchers exposed the Halimeda
algae to water with a lower pH level, as may be found in many regions of
the oceans 40 to 50 years from now. For the first time, the research group
focused on the structure of the algal skeleton. “While many studies on
calcareous algae or corals have thus far only compared the quantity of
calcareous skeleton produced at different pH values, we concentrated on
the microstructure of the skeleton. For this purpose, we used our scanning
electron microscope which can magnify the structures of animals and plants
up to 100,000 times“, said biologist Andre Wizemann, one of the authors of
the study.

As the researchers observed, the Halimeda alga forms a skeleton of fine
calcareous needles, which are formed during the day at the cellular
surface. At night these needles recrystallise – they partially dissolve
and fuse to a dense, compact skeleton armor. Thus, the alga protects
itself from predators and gains stability, so that it can survive at
higher water flow, for example at the edges of coral reefs.

However, such a massive skeleton can only form if the calcium saturation
in the surrounding water is high. At a lower pH level the content of
calcium carbonate in the sea decreases. “In the algae from the more acidic
water we mainly found just the fine needles which were formed by the algal
cells. While the calcium production of the alga was not hindered, it
lacked the solid support structure because the process of
recrystallisation was disturbed,” said Wizemann.

The small Halimeda algae may not seem very spectacular at first glance.
“In warmer coastal regions, however, the calcareous structures of the dead
Halimeda algae are an important component of sediments,” Wizemann added.
“On the Caribbean islands their skeletal parts can comprise up to 50% of
the beach sand.” If their calcareous skeleton is weak and brittle, this
can have far-reaching consequences. The fine calcareous needles dissolve
more easily in water than a compact skeleton does. Therefore, not much
remains of the algae after their death. This in turn could have a negative
effect on the formation of tropical beaches and coral reef islands, which
consist largely of calcareous sediments.

Publication:
Wizemann, A., Meyer, F.W., Hofmann, L.C., Wild, C., Westphal, H. (2015).
Ocean acidification alters the calcareous microstructure of the green
macro-alga Halimeda opuntia. Coral Reefs 34(3), pp. 941-954. DOI:
10.1007/s00338-015-1288-9.

Picture: Halimeda opuntia, A. Wizemann, ZMT