Induced Defense In Phaeocystis Globosa

Phytoplankton plays key roles in climate change, primary production and marinebiochemistry. They contribute up to50%of the total primary productivity on the earth.In marine ecosystems, phytoplankton has to deal with a non-trivial risk of being eaten,and the ability to defend themselves against predators is therefore a main driving forcein their evolutionary histories. Constitutive and induced defences have also been foundin marine phytoplankton. Defences can be fixed or only be activated in times ofincreased grazing-mediated mortality risk. The evolution of inducible, as opposed toconstitutive, defences is favoured when the risk of herbivory is unpredictable, when theproduction of defences involves fitness costs, and when reliable cues to the risk ofherbivory are available. Induced defense provided protection to phytoplankton againstvarious grazers and contribute the success of harmful algae bloom species in marinesystems.The objective of this paper is to study the induced defense of P. globosa, a harmfulalgae species with worldwide distributions.We exposed P. globosa to grazingenvironment, where grazers（O. marina）or grazing chemical cues were present, andcontrol environment. Solitary cell abundance, colony numbers, colony size andpercentage of colonial cells to total cells between treatments and controls werecompared to investigated the response of growth and colony formation to the grazingSolitary cell and colonies in both grazing treatments and control increased withtime. O. marina consumed solitary P. globosa cells substantially and grew actively.The presence of grazing O. marina enhanced the colony formation in terms of colonydiameter. Enhancement of colony size in the grazing treatment was evident after10to15d. Colony enlargements can provide protection for the colonial cells due to thetough colony skin and size-mismatch between colonies with O. marina. Directphysical contact with O. marina was not required to initiate the response; instead, P.globosa colony size increase was also found to be stimulated by chemical signalsgenerated during grazing activities. P. globosa receiving grazing chemical cues associated with O. marina also formed greater colonies than those in controls.Exposure to grazing chemicals caused more cells were in colonial form. But induceddefense did not negatively affect the growth of P. globosa that had similar solitary cellabundance and colony numbers when compared to the controls. An alternativeinduced defense was also used by P. globosa that lost the ability to form regularcolonies. Aggregations were formed only in the presence of grazers, which clearlysuppressed the accumulation of solitary cells. These aggregations, however, had avery unique structure: cells without flagella closely clumped together to form anapproximately spherical structure. Microscopic observations showed that thesecolonies lacked the mucus envelop that is characteristic of Phaeocystis colonies, andthat naked cells were seen clearly on the colony surface. Aggregation size increasedwith time, and enhancement aggregation size provide refuge for P. globosa cells suchthat O. marina starved and decline since10day. Colony formation and enlargement,which suppressed grazing military, belongs to inducible defense strategy, and maycontribute to the the development of bloom dominated by Phaeocystis.