Effect Of Digestion Process Of Filter Feeders On Silicon Cycle In The Ocean

Since the late twentieth century,the importance of the silicon recycling in marine biogeochemistry began to be appreciated.Despite long-standing interest in siliceous organisms and silica sediments,processes controlling biogenic silica(BSi)production and dissolution is still poorly known.Crassostrea gigas is an oyster species cultivated in raft net cages or bottom seeding in coastal areas,and Urechis unicinctus is a burrowing worm in the intertidal zone and shallow water zone of the north coast of China.As filter feeders,they can dispose of suspended particles in the water column by consuming phytoplankton,microplankton and debris.From ingestion to digestion and defecation,particulate matter is transformed by these consumers through physical and biochemical gut processes,which has a significant impact on energy and material flux in the ocean.Herbivore digestion in aquatic ecosystems is usually considered a process of nutrient repackaging rather than recycling.Therefore,the dissolution of biogenic silicon,phosphorus and iron in fecal particles produced by oyster C.gigas and U.unicinctus in artificial seawater without bacteria was compared.Under the laboratory single bait training conditions,relative to raw food materials,the mass proportion in fecal pellets of BSi increased,while that of P and Fe decreased.During the 21 days of incubation,the relative dissolution loss rate of BSi of fecal pellets was the highest.The dissolution rate of BSi in oyster feces reached 2.65 mg/g when fed with Phaeodactylum tricornutum and 3.05 mg/g when fed with rice husk powder.The dissolution rates of BSi from fecal particles produced by U.unicinctus were 1.58mg/g and 1.18 mg/g,respectively.In sterile artificial seawater,the total dissolution rate of BSi was 13.9-35.9 times higher in fecal pellets than food items,followed by P(1.1-4.9 times)and Fe(1.3-6.5 times).Regarding dissolution of BSi of fecal pellets,a higher rate was observed in oyster C.gigas than Echiuran U.unicinctus,but no significant difference was found between fecal pellets in either species under naturally available diatom food(P.tricornutum)and introduced terrestrial food(rice husk powder),respectively.It has been suggested that animal digestion,with significant element and consumer specific differences,plays an important role in the regulation of nutrient regeneration and limitation.As a result,in the process of culture,the release of each element in the freeze-dried diet was similar,and the final release rate was almost the same.Digestion accelerated the regeneration of each nutrient element,but the dissolution and release promoted by digestion was the most obvious in the Si element.In addition,during the field experiment,the dissolution of P,Si,Al and Fe in the fecal particles of C.gigas fed on mineral particles was also compared.The rapid release of each element was basically in the early stage,and tended to be stable in the late stage.Compared with FWHM of the main peak of XRD,the FWHM of feces was slightly wider than that of POM,about 0.01°.It indicated that the crystal size in feces became smaller and the ingested mineral particles were digested.Some of the original crystal forms of suspended particulate matter in seawater were changed by the digestion of oysters,which were derived from stable minerals into unstable minerals.It showed that some mineral particles were digested.The dissolution rates of P elements in surface and bottom feces reached 3.13 mg/g and2.72 mg/g respectively,the Si dissolution rates were 16.10 mg/g and 10.23 mg/g,Al were 4.88 mg/g and 2.33 mg/g,and Fe were 2.62 mg/g and 1.79 mg/g.After digestion,P in the feces of the surface layer and bottom layer was 6.5 and 5.3 times that of the bait,DSi in the feces was 10.2-17.9 times that of the bait,Al was 2.5-4.7times,and Fe was 1.9-4.9 times.It indicated that,in addition to the traditional view that terrigenous input and biological sedimentation played an important role in the cycling of elements,mineral components could also participate in the regeneration of elements after biological digestion.Algal debris is an important direct nutritional supplement in the marine material cycle,but the fecal particles produced by herbivores can provide additional and indirect energy connection.Biological fecal particles can be used as "silicate pump" in marine ecosystem,which can make a certain contribution to marine silicon cycle,and also have a certain reference significance for mariculture.