Ecophysiological linkage of nitrogen enrichment to heavily silicified diatoms in winter

摘要

Over recent decades, increased anthropogenic nitrogen and reduced land-based loading of silica in many coastal waters have asymmetrically changed the nitrogen: silica ratios. These changes have contributed to shifts in phytoplankton assemblages from diatoms to non-diatoms, as well as increases in the frequency and magnitude of non-diatom harmful algal blooms. Here we show a subtle and counterintuitive change in diatom assemblage, i.e. heavily silicified diatoms significantly increased in winter after nitrogen enrichment, based on paleoecological and contemporary seasonal water column data from 2 eutrophic bays in the Yellow Sea. The heavily silicified diatom Paralia sulcata showed an increasing trend over time in parallel with nitrogen enrichment, which was associated with low temperature, and low dissolved inorganic nitrogen: phosphorus, high dissolved nitrogen: silica, and low ammonium: nitrate ratios on a seasonal basis. Applying recent insights regarding diatom nitrogen metabolism and its putative urea cycle, a physiological mechanism linking nitrogen, carbon, and silica metabolism is suggested to explain the phenomenon of increased silicification under winter conditions at the cellular level. Winter sequestration of silica in P. sulcata valves also has biogeochemical consequences, including a weakening of the silica pump and a slowing of biogenic silica dissolution, thereby reducing the availability of silica for further diatom growth in subsequent seasons and increasing the window of opportunity for summer growth of non-diatom harmful algal bloom species.