A close relationship between algicidal bacteria and termination of Heterosigma akashiwo (Raphidophyceae) blooms in Hiroshima Bay, Japan

Abstract

ABSTRACT. Blooms of the noxious red tide phytoplankton Heterosigma akashiwo (Raphidophyceae) occurred in Hiroshima Bay. Japan. in 1994 and 1995. During these blooms we monitored rnicroorganisms which killed H. akashiwo by use of the microplate MPN (most probable number) method using an axenic culture of H. akashiwo as a susceptible host organism. At every sampling site abundance of algicidal microorganisms in seawater samples filtered through 0 8 pm nuclepore filters increased rapidly during the termination period of each bloom. However, the number of algicidal microorganisms In seawater samples filtered through 0 2 pm nuclepore filters was less abundant and correlated poorly with the extinction of H, akashiwo blooms. The latter samples were assumed to indicate viral activity. Thus, it is possible that H. akashiwo-killing bacteria (HAKB) played a more dominant role in the terrninatlon of the blooms in 1994 and 1995 in Hiroshima Bay than viruses. The number of algicidal bacteria targeting Chattonella antiqua (Raphidophyceae), which was not detected during the investigation period, was 1 or 2 orders of magnitude lower than that of HAKB. We isolated some HAKB strains capable of causing mortality in H. akashiwo. These results suggest that the population dynamics of algicidal bacteria has a close relationship to the blooms of the phytoplankton, and that, in marine ecosystems, algicidal bacteria targeting specific phytoplankton may be one of the agents which regulate the change of species structure of phytoplankton communities.

Population structure of algicidal marine bacteria targeting the red tide forming alga Heterosigma akashiwo (Raphidophyceae), determined by restriction fragment length polymorphism analysis of the bacterial 16S ribosomal RNA genes

“…In 1995, the HAKB members of 2B, 2C and 2D accounted for 52 strains and corresponded to 89.7 % of all 1 6 s rDNA RFLPs analyzed (data not shown). The HAKB strains of ribotype 2C and 2D were especially dominant in the HAKB populations isolated a t Stn V on June 30 (19 strains of 2C and 8 strains of 2D were observed in 29 strains of HAKB), when the MPN of HAKB dramatically increased and was followed by disintegration of the Heterosigma akashiwo bloom (Kim et al 1998).…”

“…These results strongly support the fact that the ribotyping by the 5 endonucleases used in this study was sufficiently powerful to resolve the population structure and dynamics of the HAKB. At Stn V in Hiroshima Bay in 1994 and 1995, Heterosigma akashiwo blooms occurred at the beginning of June and at the end of June, respectively (Kim et al 1998). In the both cases, the bloom at the surface dramatically disintegrated within a few days after the climax.…”

“…We recently developed a most probable number (MPN) method for detecting and enumerating algicidal bacteria in seawater and found a close relationship 0 Inter-Research 1998 between the dynamics of algicidal bacterial populadurlng decay periods of the H a k a s h~w o blooms In tions and of marine phytoplankton communities Hiroshima Bay in 1994 and 1995 (Kim et al 1998). In (Yoshinaga et al 1995b, Imai et al 1998 this study, several stralns of HAKB were isolated during 1998, In this issue) An Increase or decrease of the the H. akashiwo blooms, and the 16s [RNA genes of the marine bacteria w h~c h inh~blt the growth of Grmno-HAKB isolates were analyzed to determine the genetic dinium mikimotoi (Dinophyceae) correlated negatively diversity and structure of the HAKB populations.…”

“…1998). tom) at 2 stations (Stns V and M) in Hiroshima Bay in Several researchers have isolated the bacteria algici-1994 and 1995 (Kim et al 1998). Using the MPN procedal to marine microalgae from nearshore seawater, dure for counting HAKBs in seawater, strains of HAKB sediment, an aquaculture pond for prawns and a mass were lsolated from the microplate well in which Hetculture of marine diatoms (Baker & Herson 1978, erosigma akashiwo cells were killed or lysed after Sakata 1990, Imai et al 1991 inoculation of a seawater sample.…”

Bacterivory by co-occurring red-tide algae, heterotrophic nanoflagellates, and ciliates

“…If red-tide algae ingest bacteria in a significantly large number, they may compete with HNFs and ciliates for bacterial prey, and the grazing impact by the algae on bacteria may make a considerable contribution to the total grazing impact of protistan predators on bacteria. However, there have been few studies on the feeding of red-tide algae on marine bacteria (Nygaard & Tobiesen 1993, Jeong et al 2005a, although there have been many studies on the interactions between red-tide algae and algicidal bacteria (Kim et al 1998, Doucette et al 1999, Imai et al 2001, Skerratt et al 2002, Mayali & Azam 2003. Nygaard & Tobiesen (1993) reported that some prymnesiophyte species, the raphidophyte Heterosigma akashiwo and the dinoflagellates Karlodinium micrum (previously Gyrodinium galatheanum) and Alexandrium tamarense were able to ingest bacteria, but H. akashiwo and K. micrum did not ingest bacteria when the phosphate concentrations were high (batch cultures, 0.33 to 1.56 µM).…”