Red algae-bearing benthic foraminifera adapted to extremely warm temperatures and high salinity in the Persian Gulf

The Persian Gulf hosts corals reefs under the most extreme conditions in the world, where summer maxima reach >36°C in combination with high salinities >44 PSU. While high bleaching thresholds characterize corals on these reefs, knowledge of adaptation of other calcifiers to local conditions is lacking. Benthic foraminifera are important calcifiers for coral reefs ecosystems as they build calcium carbonate tests. To map the environmental envelopes and the physiological limits of dispersal of benthic foraminfera, we exposed adult and juvenile foraminifera to a range of temperature and salinity conditions. Samples were collected from two reefs in the southern Gulf of Abu Dhabi, UAE. The dominant symbiont-bearing foraminifera was Peneroplis planatus hosting the endosymbiotic red algae Porphyridium purpureum. This was a surprising finding of sampling in these extreme reefs, as other symbiont-bearing benthic foraminifera are normally more abundant, but were completely absent in the reefs investigated. In the laboratory, we exposed P. planatus to 27°C (control), 35°C local summer maxima, and 39°C, +4°C above summer maxima, each with and without sediment substrates. The ecophysiological parameters growth, survivorship and photophysiological performance were measured. Photosynthetic rates declined after one week of exposure to 35°C and symbionts were photoinhibited at 39°C. Conditions were clearly more hostile for the symbionts, as host survival was high and growth rates unaffected by temperature. We hypothesize that to sustain growth the holobionts gained energy through heterotrophy under these conditions. In a second experiment, we exposed asexually reproduced offspring to an orthogonal temperature and salinity stress treatment (27-39°C, x 34-42 PSU) for four weeks. Asexual reproduction occurred in several treatments but was reduced under high salinity and temperature and combination of both parameters. The higher rate of asexual reproduction at control conditions indicates that stressful conditions do not trigger asexual reproduction in P. planatus, but rather suppress it. The results indicate that P. planatus can resist temperatures above current summer maxima for short periods of time, but that reproduction is impaired. Such heat-adapted populations may be considered a refuge for colonizing an increasingly warming Indian Ocean. Reproductive declines in the local population with increased warming threatens the long-term viability of this uniquely adapted organism.