No abstract

Physiological mechanisms of bleaching were studied on larger benthic foraminifera (LBF) hosting endosymbiotic diatoms. Amphistegina radiata, Heterostegina depressa, and Calcarina hispida were exposed to increasing temperatures in static temperature experiments (23uC to 33uC, 6 d). Photosynthetic activity (F v : F m , measured with a pulse-amplitude modulated fluorometer), chlorophyll a (a proxy for symbiont biomass), and motility (a proxy for overall fitness of the foraminifera) were reduced in specimens at 32uC to 33uC, and cytoplasm color changes associated with bleaching were observed. A 30-d flow-through experiment at three temperatures (26uC to 31uC) and three levels of inorganic nitrate concentration (0.5 to 1.4 mmol L 21 ) confirmed negative effects of temperature at 31uC for A. radiata (including growth) and H. depressa. Another Calcarina species, Calcarina mayorii, was not affected. This suggests that temperature effects are species-specific. However, elevated nutrient concentrations did not affect any of the parameters measured. Temperatures . 30uC stress the foram-diatom endosymbiosis in some LBF species, which may lead to subsequent bleaching of the host. Given that a 2-3uC increase led to rapid bleaching of most species, we propose that, similar to corals, these species are threatened by sea-surface temperature increase predicted for tropical reef waters in the near future.

We measured organic carbon uptake rates by deep-sea benthic foraminifera and studied differences among species, living depth, and seasons to investigate how these protists contribute to carbon consumption on the deep-sea floor. In situ feeding experiments using 13 C-labeled algae were carried out in the central part of Sagami Bay from 24 to 29 November 2001 and 1 to 12 April 2002. Our results indicate that carbon assimilation rates were higher in shallow infaunal species (Uvigerina akitaensis, Bulimina aculeata) and lower in intermediate (Textularia kattegatensis) and deep infaunal species (Chilostomella ovoidea). Some shallow and intermediate infaunal species showed higher carbon uptake in spring than in autumn. In total, benthic foraminifera assimilated C at 5.8 Ϯ 4.8 mg m Ϫ2 and 2.0 Ϯ 1.3 mg m Ϫ2 (in spring and in autumn, respectively) of labeled algae within 2 d, which was more than that by total metazoans (1.5 Ϯ 0.4 mg m Ϫ2 and 0.4 Ϯ 0.1 mg m Ϫ2 , respectively). Deep-sea benthic foraminifera rapidly ingest large amounts of carbon and may play an important role in carbon consumption on the deep-sea floor. Different responses to algal carbon among species may explain foraminiferal assemblages and shifts after environmental changes, such as seasonal pulses of organic matter supply.

Benthic foraminiferal food sources were examined in the central part of Sagami Bay, Japan (water depth 1450 m) based on an in situ feeding experiment with 13 C-labeled food materials. In this study, 3 different 13 C-labeled food materials were used: the unicellular marine algae Dunaliella tertiolecta, the marine diatom Chaetoceros sociale, and the marine bacterium Vibrio alginolyticus. The first two are representatives of phytodetritus and the third of organic matter produced in the sediments. Each type of food material was injected into a series of in situ culture cores and incubated for up to 21 d. We observed that some benthic foraminiferal species selectively ingested 13 C-labeled algae from the sedimentary organic matter. On the other hand, benthic foraminifera ingested 13 C-labeled bacteria unselectively from sedimentary organic matter. Total benthic foraminifera assimilated 8.8 mg C m-2 d-1 of sedimentary organic matter without phytodetritus assimilation. Based on the assimilation rates estimated in this experiment, we recognized 3 types of feeding strategy among deep-sea benthic foraminifera in Sagami Bay. There are those that ingest (1) fresh phytodetritus selectively (phytophagous species: Uvigerina akitaensis, Bolivina spissa, Bolivina pacifica); (2) fresh phytodetritus selectively but sedimentary organic matter as well when phytodetritus is absent (seasonal-phytophagous species: Bulimina aculeata, Textularia kattegatensis, Globobulimina affinis); and (3) sedimentary organic matter at random (deposit feeders: Cyclammina cancellata, Chilostomella ovoidea). These different types of carbon utilization should be considered not only for understanding modern ecosystems on the deep-sea floor but also for paleoceanographic reconstructions using the abundance and distribution, or isotopic composition, of benthic foraminifera.

No abstract