Short-term response of pelagic planktonic communities after inoculation with the mass cultured dinoflagellate Alexandrium affine in a large-scale mesocosm experiment

Abstract

As a consequence of difficulties in mass culturing dinoflagellates, there have been few ecological studies and little information on the interactions of microbial planktonic communities with species that cause harmful algal blooms (HABs). To address this problem, we used a water circulation culture system that provided in-water air bubbles to achieve active growth and mass cultivation of the dinoflagellate Alexandrium affine. This facilitated assessment in a large-scale mesocosm (1000 L) of the short-term response of the marine pelagic planktonic community to inoculation with A. affine under diatom-dominant conditions. Members of the pelagic planktonic community (bacteria and phytoplankton) varied in their response to A. affine addition, even over short time periods. Following nutrient depletion, the population of the dominant diatom (Chaetoceros debilis) rapidly declined, while the proportion of A. affine increased to more than 80% in both the low and high concentration treatment groups. The bacterial community was influenced by the population dynamics of phytoplankton, but a similar effect was not evident for the mesozooplankton community. Rhodobacteraceae dominated during the diatom-dominant stage in the mesocosms, and their numbers were positively correlated with most of the diatoms, including C. debilis. In contrast, the bacterial SAR11 clade and C. debilis showed markedly opposite patterns, and their occurrences were strongly negatively correlated. At the end of the experiment, there were similar time-series changes evident between the major bacterial group, and phytoplankton of the SAR11 clade and A. affine, but these groups were not directly correlated in association network analysis, indicating that the SAR11 clade was more associated with the decline in diatom populations rather than having a stimulatory effect on dinoflagellates. The results indicate that when a large population of A. affine is introduced into an environment dominated by diatoms, it predominately contributes to changing the phytoplankton community by maintaining its own population under low nutrient conditions, rather than affecting the entire planktonic community.