Coral reef biology

Mesophotic reefs represent biodiverse ecosystems that may act as a refuge for
depth-generalist coral species threatened in shallow habitats. Despite the importance of
coral-algal symbioses, few studies focus on mesophotic zooxanthellae assemblages and
their influence on connectivity. This study compared zooxanthellae in Montastraea
cavernosa at shallow and mesophotic depths at Flower Garden Banks National Marine
Sanctuary and McGrail Bank. Mesophotic corals contained more zooxanthellae and more
chlorophyll a and c2 per unit area coral. Increased zooxanthellae within mesophotic
corals may represent an adaptive strategy to optimize light capture in low-light
environments. Genetic profiles for zooxanthellae assemblages from shallow and
mesophotic corals showed similar diversity across banks and between depths. The
dominant sequence making up assemblages was identified as Symbiodinium type C1.
Similar assemblage diversity suggests that zooxanthellae assemblages will not limit
connectivity potential between shallow and mesophotic corals at these reefs.

The unique coral reef at St. Lucie Reef (Stuart, FL) persists despite environmental variability from extensive freshwater discharges, summer upwelling, and thermal
instability. By examining the symbiotic zooxanthellae, or Symbiodinium, that reside
in corals, we can gain insight to coral physiology impacted by local stressors. Two
scleractinian corals, Montastraea cavernosa and Pseudodiploria clivosa were sampled
over 1.5 years, including both wet and dry seasons. Zooxanthellae were isolated and
quantitatively characterized using standard measurements and molecular techniques.
Both coral species varied in zooxanthellae biomass, where Pseudodiploria clivosa had
Higher cell densities and chlorophyll concentrations than Montastraea cavernosa. Over
time, these parameters varied, but were not significantly altered by fresh water discharge events. Symbiodinium diversity and abundance were identified by ITS2 region amplification and next-generation sequencing .Novel associations between Symbiodinium and each coral explained the observed physiological differences. The symbioses remained stable throughout and could indicate local adaptation for St. Lucie Reef corals.

A symbiosis between a gorgonian coral, Pseudoplexaura flagellosa (Houttuyn) and a filamentous green alga results in spongy nodules on the coral host. Morphological and histochemical differences between the normal and algal-infected coral were studied, using transmission electron microscopy and histochemistry. The algal filaments located within the host skeleton are covered with skeletal laminae. The infected skeleton is structurally abnormal and only remants of the mesogleal region adjacent to the skeleton remain. Acidic mucopoly-saccharide substances are associated with these areas. Host granulocytes are located in the mesoglea adjacent to the infected skeleton. These cells release vesicles that attach to algal filaments extending into the tissue zone. These host cells also stain positively for tyrosine and polyphenols which are known to be associated with the production of Pseudoplexaura skeleton. Granulocytes coat the algae and secrete skeletal-like material about them, therefore, participating in an encapsulation mechanism in defense of the host.

Nitrogen (N) has traditionally been regarded as the primary limiting nutrient to algal growth in marine coastal waters, but recent studies suggest that phosphorus (P) can be limiting in carbonate-rich environments. To better understand the importance of P. alkaline phosphatase activity (APA) was measured in reef macroalgae in seven counties of south Florida ; several significant trends emerged : 1) APA decreased geographically from the highest values in Dada>Monroe>Palm Beach>St. Lucie>Broward>Martin>Lee counties 2) APA varied temporally with increasing nutrient-rich runoff in the wet season 3) APA varied due to taxonomic division Phaeophyta>Rhodophyta>Chlorophyta 4) Nutrient enrichment experiments demonstrated that increased N-enrichment enhanced P-limitation while increased P decreased P-limitation. These results suggest that high APA observed in carbonate-rich waters of Dade County and low APA in Broward County resulted from high nutrient inputs associated with anthropogenic nutrient pollution.