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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.

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Foraminifera are an important faunal element of the abyssal ecosystem and largely depend on deposited particulate organic matter from the photic zone to sustain their metabolism for growth and reproduction. However, their role in the carbon cycle in deep-sea sediments is insufficiently studied. We investigated benthic foraminifera at Station M (4000 m depth) in the Northeast Pacific and assessed the response of individual species to a simulated phytodetritus pulse during an in situ feeding experiment. Sediments were incubated for 4 d with 13 C-labeled diatoms (Thalassiosira weissflogii) applied to the sediment surface. The living foraminiferal community (> 0.063 mm) of the upper 3 cm contained >100 species and was strongly dominated by a few taxa of soft-walled saccamminids. Population density of the entire living foraminiferal community was highest at the sediment surface (mean ± SD = 279 ± 72 ind. 10 cm −3 in background and 13 C-incubated cores) and decreased gradually with depth. Large differences were observed in the uptake of the algal material among species and between depth levels. During the experiment, 0.82 mg C m −2 were ingested, mainly by calcareous (~60%) and agglutinated (~40%) foraminifera. Uptake was highest at the sediment surface and 3 to 5 times less in deeper sediment horizons. Despite clear signs of vitality and a strong representation in the foraminiferal community, none of the soft-walled species showed a noticeable response to the offered algal material. We conclude that soft-walled foraminifera may not be important to the short-term phytodetrital matter cycling at the abyssal sea floor.

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Food and oxygen concentrations have been identified as environmental constraints influencing the vertical distribution of benthic foraminifera. Field studies, however, have been largely inconclusive as to which degree each factor regulates the observed distribution pattern. For this reason, different experiments were performed to investigate the response of deep-sea benthic foraminifera to simulated phytodetritus pulses under laboratory conditions, where oxygen concentrations can be influenced separately. In the laboratory, deep-sea foraminifera developed a normal vertical distribution pattern, and the habitats of single species reflected the results obtained from field investigations. Therefore, conclusions from the data produced in the laboratory can be transferred to nature. A mainly epifaunal life style was shown for Adercotryma glomerata and Spiroplectinella earlandi, but also indicated for Uvigerina peregrina. Hippocrepina sp. was spread over the entire sediment column with a shallow infaunal maximum. Epistominella pusilla, Seabrookia earlandi and Alveolophragmium wiesneri showed an epifaunal to shallow infaunal distribution. Ceratobulimina arctica, Trochammina inflata and Melonis barleeanum preferred an infaunal habitat. No suspension feeders were observed in the experiments. The addition of algae as food material resulted in elevated population densities. Under the influence of high oxygen contents with no or only short-term fluctuations, no migration to the upper layers was recorded after the addition of food. However, more specimens were found in deeper layers, because more organic material was transported downward into the sediment after the food pulse. The experimental laboratory results support the theoretical scenarios outlined in the TROX-model.