Poets and philosophers have long walked ocean shores and marveled at the meeting of solid land and rolling waves. Rarely, if ever, have they acknowledged the microbes beneath their feet that depend on this dynamic intersection of sand and sea.

As the tide and waves surge in, water soaks into the sand. Microbes living in the sand capture particles and dissolved components in the seawater, like organic matter and oxygen, that are necessary for them to grow and survive. As the tide flows out, it carries out their by-products.

Ahrens et al. investigated this phenomenon on the tidally influenced high-energy beach of a barrier island off northwestern Germany. They found that over the course of a year, changes in factors such as seawater composition and temperature altered the way microbes could process incoming organic matter.

In cold seasons, microbes are less active, and seawater can hold more dissolved oxygen. This allows oxygen to penetrate deep into the sand of the shore, and microbes use the oxygen in aerobic pathways to break down organic matter. Furthermore, in areas saturated with oxygen, the microbes convert ammonium (NH4+) to nitrate (NO3) and then use the nitrate to run other reactions to break down organic material. In warm seasons, microbes are more active, and these oxygen-rich zones don’t penetrate as deep—so active microbes use the oxygen close to the surface. This leaves more of the microbes in the sand to pursue anoxic pathways, converting manganese and iron minerals from a particle to a dissolved form. These dissolved elements may then partly be released into seawater, along with ammonium as a by-product.

Because of these seasonal changes, the shore of this beach is a sink for dissolved inorganic nitrogen forms in cold seasons, which reduces the coastal nitrogen stock. In warm seasons, the beach turns into a source for ammonium, supplying dissolved nitrogen to algae. Dissolved manganese, and probably iron, is released year-round, but in the warmer seasons their output is more significant.

These results come from one beach, but similar high-energy beaches exist throughout the world. As scientists make global estimates about ocean and especially coastal water changes, they must take into account these tiny microbes and their substantial role. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005399, 2020)

—Elizabeth Thompson, Science Writer

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