Effects of concentrated viral communities on photosynthesis and community composition of co-occurring benthic microalgae and phytoplankton

Marine viruses have been shown to affect phytoplankton productivity; howeve r, there are no reports on the effect of viruses on benthic microalgae (mic rophytobenthos). Hence, this study investigated the effects of elevated con centrations of virus-like particles on the photosynthetic physiology and co mmunity composition of benthic microalgae and phytoplankton. Virus populati ons were collected near the sediment surface and concentrated by tangential flow ultrafiltration, and the concentrate was added to benthic and water c olumn samples that were obtained along a eutrophication gradient in the Bri sbane River/Moreton Bay estuary, Australia. Photosynthetic and community re sponses of benthic microalgae, phytoplankton and bacteria were monitored ov er 7 d in aquaria and in situ. Benthic microalgal communities responded to viral enrichment in both eutrophic and oligotrophic sediments. In eutrophic sediments, Euglenophytes (Euglena sp.) and bacteria decreased in abundance by 20 to 60 and 26 to 66%, respectively, from seawater controls. In oligot rophic sediments, bacteria decreased in abundance by 30 to 42% from seawate r controls but the dinoflagellate Gymnodinium sp. increased in abundance by 270 to 3600% from seawater controls, The increased abundance of Gymnodiniu m sp. may be related to increased availability of dissolved organic matter released from lysed bacteria. Increased (140 to 190% from seawater controls ) initial chlorophyll a fluorescence measured with a pulse-amplitude modula ted fluorometer was observed in eutrophic benthic microalgal incubations fo llowing virus enrichment, consistent with photosystem II damage. Virus enri chment in oligotrophic water significantly stimulated carbon fixation rates , perhaps due to increased nutrient availability by bacterial lysis. The in terpretation of data from virus amendment experiments is difficult due to p otential interaction with unidentified bioactive compounds within seawater concentrates. However, these results show that viruses are capable of influ encing microbial dynamics in sediments.