Toxic Effects And Mechanisms Of Mixotrophic Dinoflagellate Karlodinium Veneficum On Co-Occurring Phytoplankton

In recent decades,the mixotrophic dinoflagellate Karlodinium veneficum formed harmful algal blooms in the coastal waters of China frequently,inducing huge economic losses.Our previous studies showed that K.veneficum JX24 and K.veneficum JX27 strains have different degrees of inhibition on the growth of co-culture phytoplankton,but the mechanisms of toxicity are still unclear.In this study,the toxic effects of the K.veneficum on phytoplankton species were studied.Moreover,the regulation of different nutrient conditions on toxicity of K.veneficum and the mechanism of toxins action were further explored.The main findings are as follows:K.veneficum inhibited the growth of Rodomonas salina: the strains JX27 and JX24 have the ability of inhibiting the growth of R.salina,and the inhibition effect of JX24 was stronger,while the KVND-1 had no obvious inhibition effect on target species.The growth of JX24 was promoted when co-cultured with R.salina,and the highest growth rate was 3.4 times of that in the control.The sonicate fraction of K.veneficum inhibited the growth of Chaetoceros sp.and Nitzschia sp.in natural phytoplankton community,while the species of Skeletonema costatum was promoted.The toxicity of K.veneficum is regulated by nutrients.With increase of nutrient concentration,the toxic effects of K.veneficum were enhanced.K.veneficum barely inhibited R.salina without cell-contact,which means the toxins release of K.veneficum relies on cell-contact with target algae.The results of the study showed that dinoflagellate K.veneficum can use toxins to inhibit the growth of co-occurring phytoplankton and promote its own growth through this process.The toxins of the K.veneficum can affect the phytoplankton community composition in natural waters and change the direction of phytoplankton succession.The regulation of nutrients on toxins is consistent with the situation of field investigation in blooms,which suggest toxins of K.veneficum may play an important role in blooms forming.Our results suggested,K.veneficum toxins aimed at co-occurring phytoplankton species and may be closely associated to mixotrophy.As an important nutritional strategy,heterotrophy can make up for its growth deficiency,and may play an important role in the formation of K.veneficum blooms.