Nocturnal dissolved organic matter release by turf algae and its role in the microbialization of reefs

Abstract

1. The increased release of dissolved organic matter (DOM) by algae has been associated with the fast but inefficient growth of opportunistic microbial pathogens and the ongoing degradation of coral reefs. Turf algae (consortia of microalgae and macroalgae commonly including cyanobacteria) dominate benthic communities on many reefs worldwide. Opposite to other reef algae that predominantly release DOM during the day, turf algae containing cyanobacteria may additionally release large amounts of DOM at night. However, this night-DOM release and its potential contribution to the microbialization of reefs remains to be investigated.2. We first tested the occurrence of hypoxic conditions at the turf algae-water interface, as a lack of oxygen will facilitate the production and release of fermentation intermediates as night-time DOM. Second, the dissolved organic carbon (DOC) release by turf algae was quantified during day time and nighttime, and the quality of day and night exudates as food for bacterioplankton was tested. Finally, DOC release rates of turf algae were combined with estimates of DOC release based on benthic community composition in 1973 and 2013 to explore how changes in benthic community composition affected the contribution of night-DOC to the reef-wide DOC production. 3. A rapid shift from supersaturated to hypoxic conditions at the turf algae-water interface occurred immediately after the onset of darkness, resulting in night-DOC release rates similar to those during daytime. Bioassays revealed major differences in the quality between day and night exudates: Night-DOC was utilized by bacterioplankton two times faster than day-DOC, but yielded a four times lower growth efficiency. Changes in benthic community composition were estimated to have resulted in a doubling of DOC release since 1973, due to an | 2105 Functional Ecology MUELLER et al.

Divide and conquer: Spatial and temporal resource partitioning structures benthic cyanobacterial mats

“…It is paramount to understand BCM structure and function to be able to assess how community dynamics are created and sustained through biogeochemical gradients. Benthic cyanobacterial mat communities create feedback loops that facilitate mat proliferation, even releasing copious amounts of carbon and nitrogen back into the environment (Brocke et al, 2015(Brocke et al, , 2018Mueller et al, 2022). Mechanisms of how these feedback loops are connected to broader biogeochemical cycles and for the uptake and release of other elements such as sulfur still remain unknown.…”

“…Direct comparison of mats in multiple different flow environments is necessary for our understanding of how diversity, complexity, and niche structure changes in response to flow speed. Although we know from Mueller et al (2022) that BCMs can release excess dissolved organic matter and create hypoxic conditions at their surface, we still lack an understanding of DBL thickness, longevity, and the effects an anoxic DBL will have on other mat community members, competitors, and consumers. Most publications have examined the effects of oxygen concentrations within DBLs (e.g., Dillon et al, 2009;van Erk et al, 2023).…”

The effects of dissolved organic matter supplements on the metabolism of corals under heat stress

“…The apparent higher DOC consumption may just be the result of lower release rates of DOC by corals during the night, shifting the balance between uptake and release toward net uptake. Indeed, in algae, but also corals, the release of photosynthetically fixed carbon in the form of DOC has been shown to be directly related to primary production, with a positive relationship between DOC release and light availability (e.g., Kurihara et al 2018; Mueller et al 2022). During the day, photosynthesis can meet more than 100% of the respiratory needs of hexacorals, which therefore release the excess carbon as DOC and mucus (Davies 1984).…”

Convergent photophysiology and prokaryotic assemblage structure in epilithic cyanobacterial tufts and algal turf communities

“…Mat‐building cyanobacteria can rapidly take up and utilize environmental nutrients relative to macroalgal competitors, likely exacerbating their contribution to reef productivity (den Haan et al., 2016). However, it was also recently demonstrated that benthic cyanobacterial mats, at least on Caribbean reefs, release a great deal of this fixed carbon back into their surrounding reef environment as Dissolved Organic Carbon (DOC) and thus act as net sources of DOC, especially during the night (Brocke et al., 2015; Mueller et al., 2022). This algal‐derived DOC is more readily bioavailable to microbes and sponges on reefs and fuels inefficient carbon metabolism compared to coral‐derived DOC, which can play a role in the expansion of opportunistic pathogens on reefs and contribute to overall reef microbialization (Haas et al., 2016; Silva et al., 2021).…”

“…Cyanobacteria, however, can also be predominant members of the prokaryotic assemblage within turf algal communities, alongside a dominance of Proteobacteria (predominantly Alphaproteobacteria), Bacterioidota, and Firmicutes (Barott et al., 2011; Hester et al., 2016). Indeed, shifts in turf composition toward higher cyanobacterial dominance can skew turf algal carbon cycling toward net DOC release (Mueller et al., 2022), which can contain cyanobacteria taxonomically similar to those known to be dominant mat‐building cyanobacteria (Fricke et al., 2011). Few papers, however, have simultaneously examined the prokaryotic assemblage structure in benthic cyanobacterial aggregations and turf algae to define overlap and resolve distinctions between the prokaryotic assemblage well—and namely the cyanobacterial component—of these two critical function groups well.…”