Abstract Two peculiar morphological features, observed in the marine alga T suecica are described in this report. An ultrastructural study of actively swimming cells showed an unusual abundance of starch in the stroma and the pyrenoid and this uncommon storage was discussed in relation to growth factors. Numerous osmiophilic vesicles, infrequent in this species, were also found; they contained lipids. After 5 days exposure to Cu or Ag (50 μg 1 −1 and 20 μg 1 −1 respectively), the algae responded by storing these elements in the pre-existing osmiophilic vesicles. The analysis by electron probe microanalysis (EPMA) revealed a coprecipitation of Cu, Ag, Pb, Ca, P and S in these structures and led us to relate these vesicles to polyphosphate-bodies, already described in algae. Osmiophilic vesicles were devoid of phosphate in control samples. Following Cu exposure, this storage could be explained as an active detoxification process for most algae whose other structures were well preserved. After Ag exposure, the storage occurred in a few algae presenting some morphological damage. In both cases (Cu or Ag exposure), the metabolism was modified as starch was absent after Cu exposure and stored only in the pyrenoid after Ag exposure.